Half-hours with the Telescope
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Title: Half-hours with the Telescope
Being a Popular Guide to the Use of the Telescope as a
Means of Amusement and Instruction.
Author: Richard A. Proctor
Release Date: September 28, 2005 [EBook #16767]
Language: English
Character set encoding: ASCII
*** START OF THIS PROJECT GUTENBERG EBOOK HALF-HOURS WITH THE TELESCOPE ***
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[Illustration: PLATE I. Maps I.-IV.]
HALF-HOURS
WITH
THE TELESCOPE;
BEING A POPULAR GUIDE TO THE USE OF THE TELESCOPE
AS A MEANS OF AMUSEMENT AND INSTRUCTION.
BY
RICHARD A. PROCTOR, B.A., F.R.A.S.,
AUTHOR OF "SATURN AND ITS SYSTEM," ETC.
WITH ILLUSTRATIONS ON STONE AND WOOD.
* * * * *
An undevout astronomer is mad:
True, all things speak a God; but, in the small
Men trace out Him: in great He seizes man.
YOUNG.
* * * * *
NEW YORK:
G.P. PUTNAM'S SONS.
1873.
LONDON:
PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET
AND CHARING CROSS.
PREFACE.
The object which the Author and Publisher of this little work have
proposed to themselves, has been the production, at a moderate price, of
a useful and reliable guide to the amateur telescopist.
Among the celestial phenomena described or figured in this treatise, by
far the larger number may be profitably examined with small telescopes,
and there are none which are beyond the range of a good 3-inch
achromatic.
The work also treats of the construction of telescopes, the nature and
use of star-maps, and other subjects connected with the requirements of
amateur observers.
R.A.P.
_January_, 1868.
CONTENTS.
CHAPTER I. PAGE
A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE 1
CHAPTER II.
A HALF-HOUR WITH ORION, LEPUS, TAURUS, ETC. 33
CHAPTER III.
A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC. 47
CHAPTER IV.
A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC. 56
CHAPTER V.
A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC. 66
CHAPTER VI.
HALF-HOURS WITH THE PLANETS 74
CHAPTER VII.
HALF-HOURS WITH THE SUN AND MOON 93
DESCRIPTION OF PLATES.
PLATE I.--_Frontispiece._
This plate presents the aspect of the heavens at the four seasons, dealt
with in Chapters II., III., IV., and V. In each map of this plate the
central point represents the point vertically over the observer's head,
and the circumference represents his horizon. The plan of each map is
such that the direction of a star or constellation, as respects the
compass-points, and its elevation, also, above the horizon, at the given
season, can be at once determined. Two illustrations of the use of the
maps will serve to explain their nature better than any detailed
description. Suppose first, that--at one of the hours named under Map
I.--the observer wishes to find Castor and Pollux:--Turning to Map I. he
sees that these stars lie in the lower left-hand quadrant, and very
nearly towards the point marked S.E.; that is, they are to be looked for
on the sky towards the south-east. Also, it is seen that the two stars
lie about one-fourth of the way from the centre towards the
circumference. Hence, on the sky, the stars will be found about
one-fourth of the way from the zenith towards the horizon: Castor will
be seen immediately above Pollux. Next, suppose that at one of the hours
named the observer wishes to learn what stars are visible towards the
west and north-west:--Turning the map until the portion of the
circumference marked W ... N.W. is lowermost, he sees that in the
direction named the square of Pegasus lies not very high above the
horizon, one diagonal of the square being vertical, the other nearly
horizontal. Above the square is Andromeda, to the right of which lies
Cassiopeia, the stars [beta] and [epsilon] of this constellation lying
directly towards the north-west, while the star [alpha] lies almost
exactly midway between the zenith and the horizon. Above Andromeda, a
little towards the left, lies Perseus, Algol being almost exactly
towards the west and one-third of the way from the zenith towards the
horizon (because one-third of the way from the centre towards the
circumference of the map). Almost exactly in the zenith is the star
[delta] Aurigae.
The four maps are miniatures of Maps I., IV., VII., and X. of my
'Constellation Seasons,' fourth-magnitude stars, however, being omitted.
PLATES II., III., IV., and V., illustrating Chapters II., III., IV., and V.
Plates II. and IV. contain four star-maps. They not only serve to
indicate the configuration of certain important star-groups, but they
illustrate the construction of maps, such as the observer should make
for himself when he wishes to obtain an accurate knowledge of particular
regions of the sky. They are all made to one scale, and on the conical
projection--the simplest and best of all projections for maps of this
sort. The way in which the meridians and parallels for this projection
are laid down is described in my 'Handbook of the Stars.' With a little
practice a few minutes will suffice for sweeping out the equidistant
circular arcs which mark the parallels and ruling in the straight
meridians.
The dotted line across three of the maps represents a portion of the
horizontal circle midway between the zenith and the horizon at the hour
at which the map is supposed to be used. At other hours, of course, this
line would be differently situated.
Plates III. and V. represent fifty-two of the objects mentioned in the
above-named chapters. As reference is made to these figures in the text,
little comment is here required. It is to be remarked, however, that the
circles, and especially the small circles, do not represent the whole
of the telescope's field of view, only a small portion of it. The object
of these figures is to enable the observer to know what to expect when
he turns his telescope towards a difficult double star. Many of the
objects depicted are very easy doubles: these are given as objects of
reference. The observer having seen the correspondence between an easy
double and its picture, as respects the relation between the line
joining the components and the apparent path of the double across the
telescope's field of view, will know how to interpret the picture of a
difficult double in this respect. And as all the small figures are drawn
to one scale, he will also know how far apart he may expect to find the
components of a difficult double. Thus he will have an exact conception
of the sort of duplicity he is to look for, and this is--_crede
experto_--a great step towards the detection of the star's duplicity.
PLATES VI. and VII., illustrating Chapters VI. and VII.
The views of Mercury, Venus, and Mars in these plates (except the
smaller view of Jupiter in Plate VII.) are supposed to be seen with the
same "power."
The observer must not expect to see the details presented in the views
of Mars with anything like the distinctness I have here given to them.
If he place the plate at a distance of six or seven yards he will see
the views more nearly as Mars is likely to appear in a good three-inch
aperture.
The chart of Mars is a reduction of one I have constructed from views by
Mr. Dawes. I believe that nearly all the features included in the chart
are permanent, though not always visible. I take this opportunity of
noting that the eighteen orthographic pictures of Mars presented with my
shilling chart are to be looked on rather as maps than as representing
telescopic views. They illustrate usefully the varying presentation of
Mars towards the earth. The observer can obtain other such illustrations
for himself by filling in outlines, traced from those given at the foot
of Plate VI., with details from the chart. It is to be noted that Mars
varies in presentation, not only as respects the greater or less opening
out of his equator towards the north or south, but as respects the
apparent slope of his polar axis to the right or left. The four
projections as shown, or inverted, or seen from the back of the plate
(held up to the light) give presentations of Mars towards the sun at
twelve periods of the Martial year,--viz., at the autumnal and vernal
equinoxes, at the two solstices, and at intermediate periods
corresponding to our terrestrial months.
In fact, by means of these projections one might readily form a series
of sun-views of Mars resembling my 'Sun-views of the Earth.'
In the first view of Jupiter it is to be remarked that the three
satellites outside the disc are supposed to be moving in directions
appreciably parallel to the belts on the disc--the upper satellites from
right to left, the lower one from left to right. In general the
satellites, when so near to the disc, are not seen in a straight line,
as the three shown in the figure happen to be. Of the three spots on the
disc, the faintest is a satellite, the neighbouring dark spot its
shadow, the other dark spot the shadow of the satellite close to the
planet's disc.
HALF-HOURS WITH THE TELESCOPE.
CHAPTER I.
A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE.
There are few instruments which yield more pleasure and instruction than
the Telescope. Even a small telescope--only an inch and a half or two
inches, perhaps, in aperture--will serve to supply profitable amusement
to those who know how to apply its powers. I have often seen with
pleasure the surprise with which the performance even of an opera-glass,
well steadied, and directed towards certain parts of the heavens, has
been witnessed by those who have supposed that nothing but an expensive
and colossal telescope could afford any views of interest. But a
well-constructed achromatic of two or three inches in aperture will not
merely supply amusement and instruction,--it may be made to do useful
work.
The student of astronomy is often deterred from telescopic observation
by the thought that in a field wherein so many have laboured, with
abilities and means perhaps far surpassing those he may possess, he is
little likely to reap results of any utility. He argues that, since the
planets, stars, and nebulae have been scanned by Herschel and Rosse, with
their gigantic mirrors, and at Pulkova and Greenwich with refractors
whose construction has taxed to the utmost the ingenuity of the
optician and mechanic, it must be utterly useless for an unpractised
observer to direct a telescope of moderate power to the examination of
these objects.
Now, passing over the consideration that a small telescope may afford
its possessor much pleasure of an intellectual and elevated character,
even if he is never able by its means to effect original discoveries,
two arguments may be urged in favour of independent telescopic
observation. In the first place, the student who wishes to appreciate
the facts and theories of astronomy should familiarize himself with the
nature of that instrument to which astronomers have been most largely
indebted. In the second place, some of the most important discoveries in
astronomy have been effected by means of telescopes of moderate power
used skilfully and systematically. One instance may suffice to show what
can be done in this way. The well-known telescopist Goldschmidt (who
commenced astronomical observation at the age of forty-eight, in 1850)
added fourteen asteroids to the solar system, not to speak of important
discoveries of nebulae and variable stars, by means of a telescope only
five feet in focal length, mounted on a movable tripod stand.
The feeling experienced by those who look through a telescope for the
first time,--especially if it is directed upon a planet or nebula--is
commonly one of disappointment. They have been told that such and such
powers will exhibit Jupiter's belts, Saturn's rings, and the
continent-outlines on Mars; yet, though perhaps a higher power is
applied, they fail to detect these appearances, and can hardly believe
that they are perfectly distinct to the practised eye.
The expectations of the beginner are especially liable to
disappointment in one particular. He forms an estimate of the view he is
to obtain of a planet by multiplying the apparent diameter of the planet
by the magnifying power of his telescope, and comparing the result with
the apparent diameter of the sun or moon. Let us suppose, for instance,
that on the day of observation Jupiter's apparent diameter is 45", and
that the telescopic power applied is 40, then in the telescope Jupiter
should appear to have a diameter of 1800", or half a degree, which is
about the same as the moon's apparent diameter. But when the observer
looks through the telescope he obtains a view--interesting, indeed, and
instructive--but very different from what the above calculation would
lead him to expect. He sees a disc apparently much smaller than the
moon's, and not nearly so well-defined in outline; in a line with the
disc's centre there appear three or four minute dots of light, the
satellites of the planet; and, perhaps, if the weather is favourable and
the observer watchful, he will be able to detect faint traces of belts
across the planet's disc.
Yet in such a case the telescope is not in fault. The planet really
appears of the estimated size. In fact, it is often possible to prove
this in a very simple manner. If the observer wait until the planet and
the moon are pretty near together, he will find that it is possible to
view the planet with one eye through the telescope and the moon with the
unaided eye, in such a manner that the two discs may coincide, and thus
their relative apparent dimensions be at once recognised. Nor should the
indistinctness and incompleteness of the view be attributed to
imperfection of the telescope; they are partly due to the nature of the
observation and the low power employed, and partly to the inexperience
of the beginner.
It is to such a beginner that the following pages are specially
addressed, with the hope of affording him aid and encouragement in the
use of one of the most enchanting of scientific instruments,--an
instrument that has created for astronomers a new sense, so to speak, by
which, in the words of the ancient poet:
Subjecere oculis distantia sidera nostris,
AEtheraque ingenio supposuere suo.
In the first place, it is necessary that the beginner should rightly
know what is the nature of the instrument he is to use. And this is the
more necessary because, while it is perfectly easy to obtain such
knowledge without any profound acquaintance with the science of optics,
yet in many popular works on this subject the really important points
are omitted, and even in scientific works such points are too often left
to be gathered from a formula. When the observer has learnt what it is
that his instrument is actually to do for him, he will know how to
estimate its performance, and how to vary the application of its
powers--whether illuminating or magnifying--according to the nature of
the object to be observed.
Let us consider what it is that limits the range of _natural_ vision
applied to distant objects. What causes an object to become invisible as
its distance increases? Two things are necessary that an object should
be visible. It must be _large_ enough to be appreciated by the eye, and
it must _send light_ enough. Thus increase of distance may render an
object invisible, either through diminution of its apparent size, or
through diminution in the quantity of light it sends to the eye, or
through both these causes combined. A telescope, therefore, or (as its
name implies) an instrument to render distant objects visible, must be
both a magnifying and an illuminating instrument.
[Illustration: _Fig. 1._]
Let EF, fig. 1, be an object, not near to AB as in the figure, but so
far off that the bounding lines from A and B would meet at the point
corresponding to the point P. Then if a large convex glass AB (called an
_object-glass_) be interposed between the object and the eye, all those
rays which, proceeding from P, fall on AB, will be caused to converge
nearly to a point _p_. The same is true for every point of the object
EMF, and thus a small image, _emf_, will be formed. This image will not
lie exactly on a flat surface, but will be curved about the point midway
between A and B as a centre. Now if the lens AB is removed, and an eye
is placed at _m_ to view the distant object EMF, those rays only from
each point of the object which fall on the pupil of the eye (whose
diameter is about equal to _mp_ suppose) will serve to render the object
visible. On the other hand, every point of the image _emf_ has received
the whole of the light gathered up by the large glass AB. If then we can
only make this light _available_, it is clear that we shall have
acquired a large increase of _light_ from the distant object. Now it
will be noticed that the light which has converged to _p_, diverges from
_p_ so that an eye, placed that this diverging pencil of rays may fall
upon it, would be too small to receive the whole of the pencil. Or, if
it did receive the whole of this pencil, it clearly could not receive
the whole of the pencils proceeding from other parts of the image _emf_.
_Something_ would be gained, though, even in this case, since it is
clear that an eye thus placed at a distance of ten inches from _emf_
(which is about the average distance of distinct vision) would not only
receive much more light from the image _emf_, than it would from the
object EMF, but see the image much larger than the object. It is in this
way that a simple object-glass forms a telescope, a circumstance we
shall presently have to notice more at length. But we want to gain the
full benefit of the light which has been gathered up for us by our
object-glass. We therefore interpose a small convex glass _ab_ (called
an eye-glass) between the image and the eye, at such a distance from the
image that the divergent pencil of rays is converted into a pencil of
parallel or nearly parallel rays. Call this an emergent pencil. Then all
the emergent pencils now converge to a point on the axial line _m_M
(produced beyond _m_), and an eye suitably placed can take in all of
them at once. Thus the whole, or a large part, of the image is seen at
once. But the image is seen inverted as shown. This is the Telescope, as
it was first discovered, and such an arrangement would now be called a
_simple astronomical Telescope_.
Let us clearly understand what each part of the astronomical telescope
does for us:--
The object-glass AB gives us an illuminated image, the amount of
illumination depending on the size of the object-glass. The eye-glass
enables us to examine the image microscopically.
We may apply eye-glasses of different focal length. It is clear that the
shorter the focal length of _ab_, the nearer must _ab_ be placed to the
image, and the smaller will the emergent pencils be, but the greater the
magnifying power of the eye-glass. If the emergent pencils are severally
larger than the pupil of the eye, light is wasted at the expense of
magnifying power. Therefore the eye-glass should never be of greater
focal length than that which makes the emergent pencils about equal in
diameter to the pupil of the eye. On the other hand, the eye-glass must
not be of such small focal length that the image appears indistinct and
contorted, or dull for want of light.
[Illustration: _Fig. 2._]
Let us compare with the arrangement exhibited in fig. 1 that adopted by
Galileo. Surprise is sometimes expressed that this instrument, which in
the hands of the great Florentine astronomer effected so much, should
now be known as the _non-astronomical Telescope_. I think this will be
readily understood when we compare the two arrangements.
In the Galilean Telescope a small concave eye-glass, _ab_ (fig. 2), is
placed between the object-glass and the image. In fact, no image is
allowed to be formed in this arrangement, but the convergent pencils are
intercepted by the concave eye-glass, and converted into parallel
emergent pencils. Now in fig. 2 the concave eye-glass is so placed as to
receive only a part of the convergent pencil A _p_ B, and this is the
arrangement usually adopted. By using a concave glass of shorter focus,
which would therefore be placed nearer to _m p_, the whole of the
convergent pencil might be received in this as in the former case. But
then the axis of the emergent pencil, instead of returning (as we see it
in fig. 1) _towards_ the axis of the telescope, would depart as much
_from_ that axis. Thus there would be no point on the axis at which the
eye could be so placed as to receive emergent pencils showing any
considerable part of the object. The difference may be compared to that
between looking through the small end of a cone-shaped roll of paper and
looking through the large end; in the former case the eye sees at once
all that is to be seen through the roll (supposed fixed in position), in
the latter the eye may be moved about so as to command the same range of
view, but _at any instant_ sees over a much smaller range.
To return to the arrangement actually employed, which is illustrated by
the common opera-glass. We see that the full illuminating power of the
telescope is not brought into play. But this is not the only objection
to the Galilean Telescope. It is obvious that if the part C D of the
object-glass were covered, the point P would not be visible, whereas, in
the astronomical arrangement no other effect is produced on the
visibility of an object, by covering part of the object-glass, than a
small loss of illumination. In other words, the dimensions of the field
of view of a Galilean Telescope depend on the size of the object-glass,
whereas in the astronomical Telescope the field of view is independent
of the size of the object-glass. The difference may be readily tested.
If we direct an opera-glass upon any object, we shall find that any
covering placed over a part of the object-glass _becomes visible_ when
we look through the instrument, interfering therefore _pro tanto_ with
the range of view. A covering similarly placed on any part of the
object-glass of an astronomical telescope does not become visible when
we look through the instrument. The distinction has a very important
bearing on the theory of telescopic vision.
In considering the application of the telescope to practical
observation, the circumstance that in the Galilean Telescope no real
image is formed, is yet more important. A real image admits of
measurement, linear or angular, while to a _virtual_ image (such an
image, for instance, as is formed by a common looking-glass) no such
process can be applied. In simple observation the only noticeable effect
of this difference is that, whereas in the astronomical Telescope a
_stop_ or diaphragm can be inserted in the tube so as to cut off what is
called the _ragged edge_ of the field of view (which includes all the
part not reached by _full pencils of light_ from the object-glass),
there is no means of remedying the corresponding defect in the Galilean
Telescope. It would be a very annoying defect in a telescope intended
for astronomical observation, since in general the edge of the field of
view is not perceptible at night. The unpleasant nature of the defect
may be seen by looking through an opera-glass, and noticing the gradual
fading away of light round the circumference of the field of view.
The properties of reflection as well as of refraction have been enlisted
into the service of the astronomical observer. The formation of an image
by means of a concave mirror is exhibited in fig. 3. As the observer's
head would be placed between the object and the mirror, if the image,
formed as in fig. 3, were to be microscopically examined, various
devices are employed in the construction of reflecting telescopes to
avoid the loss of light which would result--a loss which would be
important even with the largest mirrors yet constructed. Thus, in
Gregory's Telescope, a small mirror, having its concavity towards the
great one, is placed in the axis of the tube and forms an image which is
viewed through an aperture in the middle of the great mirror. A similar
plan is adopted in Cassegrain's Telescope, a small convex mirror
replacing the concave one. In Newton's Telescope a small inclined-plane
reflector is used, which sends the pencil of light off at right-angles
to the axis of the tube. In Herschel's Telescope the great mirror is
inclined so that the image is formed at a slight distance from the axis
of the telescope. In the two first cases the object is viewed in the
usual or direct way, the image being erect in Gregory's and inverted in
Cassegrain's. In the third the observer looks through the side of the
telescope, seeing an inverted image of the object. In the last the
observer sees the object inverted, but not altered as respects right and
left. The last-mentioned method of viewing objects is the only one in
which the observer's back is turned towards the object, yet this method
is called the _front view_--apparently _quasi lucus a non lucendo_.
[Illustration: _Fig. 3._]
It appears, then, that in all astronomical Telescopes, reflecting or
refracting, a _real image_ of an object is submitted to microscopical
examination.
Of this fact the possessor of a telescope may easily assure himself;
for if the eye-glass be removed, and a small screen be placed at the
focus of the object-glass, there will appear upon the screen a small
picture of any object towards which the tube is turned. But the image
may be viewed in another way which requires to be noticed. If the eye,
placed at a distance of five or six inches from the image, be directed
down the tube, the image will be seen as before; in fact, just as a
single convex lens of short focus is the simplest microscope, so a
simple convex lens of long focus is the simplest telescope.[1] But a
singular circumstance will immediately attract the observer's notice. A
real picture, or the image formed on the screen as in the former case,
can be viewed at varying distances; but when we view the image directly,
it will be found that for distinct vision the eye must be placed almost
exactly at a fixed distance from the image. This peculiarity is more
important than it might be thought at first sight. In fact, it is
essential that the observer who would rightly apply the powers of his
telescope, or fairly test its performance, should understand in what
respect an image formed by an object-glass or object-mirror differs from
a real object.
The peculiarities to be noted are the _curvature_, _indistinctness_, and
_false colouring_ of the image.
The curvature of the image is the least important of the three defects
named--a fortunate circumstance, since this defect admits neither of
remedy nor modification. The image of a distant object, instead of lying
in a plane, that is, forming what is technically called a _flat field_,
forms part of a spherical surface whose centre is at the centre of the
object-glass. Hence the centre of the field of view is somewhat nearer
to the eye than are the outer parts of the field. The amount of
curvature clearly depends on the extent of the field of view, and
therefore is not great in powerful telescopes. Thus, if we suppose that
the angular extent of the field is about half a degree (a large or
low-power field), the centre is nearer than the boundary of the field by
about 1-320th part only of the field's diameter.
The indistinctness of the image is partly due to the obliquity of the
pencils which form parts of the image, and partly to what is termed
_spherical aberration_. The first cause cannot be modified by the
optician's skill, and is not important when the field of view is small.
Spherical aberration causes those parts of a pencil which fall near the
boundary of a convex lens to converge to a nearer (_i.e._ shorter) focus
than those which fall near the centre. This may be corrected by a proper
selection of the forms of the two lenses which replace, in all modern
telescopes, the single lens hitherto considered.
The false colouring of the image is due to _chromatic aberration_. The
pencil of light proceeding from a point, converges, not to one point,
but to a short line of varying colour. Thus a series of coloured images
is formed, at different distances from the object-glass. So that, if a
screen were placed to receive the mean image _in focus_, a coloured
fringe due to the other images (_out of focus, and therefore too large_)
would surround the mean image.
Newton supposed that it was impossible to get rid of this defect, and
therefore turned his attention to the construction of reflectors. But
the discovery that the _dispersive_ powers of different glasses are not
proportional to their reflective powers, supplied opticians with the
means of remedying the defect. Let us clearly understand what is the
discovery referred to. If with a glass prism of a certain form we
produce a spectrum of the sun, this spectrum will be thrown a certain
distance away from the point on which the sun's rays would fall if not
interfered with. This distance depends on the _refractive_ power of the
glass. The spectrum will have a certain length, depending on the
_dispersive_ power of the glass. Now, if we change our prism for another
of exactly the same shape, but made of a different kind of glass, we
shall find the spectrum thrown to a different spot. If it appeared that
the length of the new spectrum was increased or diminished in exactly
the same proportion as its distance from the line of the sun's direct
light, it would have been hopeless to attempt to remedy chromatic
aberration. Newton took it for granted that this was so. But the
experiments of Hall and the Dollonds showed that there is no such strict
proportionality between the dispersive and refractive powers of
different kinds of glass. It accordingly becomes possible to correct the
chromatic aberration of one glass by superadding that of another.
[Illustration: _Fig. 4._]
This is effected by combining, as shown in fig. 4, a convex lens of
_crown_ glass with a concave lens of _flint_ glass, the convex lens
being placed nearest to the object. A little colour still remains, but
not enough to interfere seriously with the distinctness of the image.
But even if the image formed by the object-glass were perfect, yet this
image, viewed through a single convex lens of short focus placed as in
fig. 1, would appear curved, indistinct, coloured, and also _distorted_,
because viewed by pencils of light which do not pass through the centre
of the eye-glass. These effects can be diminished (but not entirely
removed _together_) by using an _eye-piece_ consisting of two lenses
instead of a single eye-glass. The two forms of eye-piece most commonly
employed are exhibited in figs. 5 and 6. Fig. 5 is Huyghens' eye-piece,
called also the _negative_ eye-piece, because a real image is formed
_behind_ the _field-glass_ (the lens which lies nearest to the
object-glass). Fig. 6 represents Ramsden's eye-piece, called also the
_positive_ eye-piece, because the real image formed by the object-glass
lies _in front of_ the field-glass.
[Illustration: _Fig. 5._]
[Illustration: _Fig. 6._]
The course of a slightly oblique pencil through either eye-piece is
exhibited in the figures. The lenses are usually plano-convex, the
convexities being turned towards the object-glass in the negative
eye-piece, and towards each other in the positive eye-piece. Coddington
has shown, however, that the best forms for the lenses of the negative
eye-piece are those shown in fig. 5.
The negative eye-piece, being achromatic, is commonly employed in all
observations requiring distinct vision only. But as it is clearly unfit
for observations requiring micrometrical measurement, or reference to
fixed lines at the focus of the object-glass, the positive eye-piece is
used for these purposes.
For observing objects at great elevations the diagonal eye-tube is
often convenient. Its construction is shown in fig. 7. ABC is a totally
reflecting prism of glass. The rays from the object-glass fall on the
face AB, are totally reflected on the face BC, and emerge through the
face AC. In using this eye-piece, it must be remembered that it
lengthens the sliding eye-tube, which must therefore be thrust further
in, or the object will not be seen in focus. There is an arrangement by
which the change of direction is made to take place between the two
glasses of the eye-piece. With this arrangement (known as the _diagonal
eye-piece_) no adjustment of the eye-tube is required. However, for
amateurs' telescopes the more convenient arrangement is the diagonal
eye-tube, since it enables the observer to apply any eye-piece he
chooses, just as with the simple sliding eye-tube.
[Illustration: _Fig. 7._]
We come next to the important question of the _mounting_ of our
telescope.
The best known, and, in some respects, the simplest method of
mounting a telescope for general observation is that known as the
_altitude-and-azimuth_ mounting. In this method the telescope is
pointed towards an object by two motions,--one giving the tube the
required _altitude_ (or elevation), the other giving it the required
_azimuth_ (or direction as respects the compass points).
For small alt-azimuths the ordinary pillar-and-claw stand is
sufficiently steady. For larger instruments other arrangements are
needed, both to give the telescope steadiness, and to supply slow
movements in altitude and azimuth. The student will find no difficulty
in understanding the arrangement of sliding-tubes and rack-work commonly
adopted. This arrangement seems to me to be in many respects defective,
however. The slow movement in altitude is not uniform, but varies in
effect according to the elevation of the object observed. It is also
limited in range; and quite a little series of operations has to be gone
through when it is required to direct the telescope towards a new
quarter of the heavens. However expert the observer may become by
practice in effecting these operations, they necessarily take up some
time (performed as they must be in the dark, or by the light of a small
lantern), and during this time it often happens that a favourable
opportunity for observation is lost.
These disadvantages are obviated when the telescope is mounted in such a
manner as is exhibited in fig. 8, which represents a telescope of my own
construction. The slow movement in altitude is given by rotating the rod
_he_, the endless screw in which turns the small wheel at _b_, whose
axle in turn bears a pinion-wheel working in the teeth of the quadrant
_a_. The slow movement in azimuth is given in like manner by rotating
the rod _h'e'_, the lantern-wheel at the end of which turns a
crown-wheel on whose axle is a pinion-wheel working in the teeth of the
circle _c_. The casings at _e_ and _e'_, in which the rods _he_ and
_h'e'_ respectively work, are so fastened by elastic cords that an
upward pressure on the handle _h_, or a downward pressure on the handle
_h'_, at once releases the endless screw or the crown-wheel
respectively, so that the telescope can be swept at once through any
desired angle in altitude or azimuth. This method of mounting has other
advantages; the handles are conveniently situated and constant in
position; also, as they do not work directly on the telescope, they can
be turned without setting the tube in vibration.
[Illustration: _Fig. 8._]
I do not recommend the mounting to be exactly as shown in fig. 8. That
method is much too expensive for an alt-azimuth. But a simple
arrangement of belted wheels in place of the toothed wheels _a_ and _c_
might very readily be prepared by the ingenious amateur telescopist; and
I feel certain that the comfort and convenience of the arrangement would
amply repay him for the labour it would cost him. My own
telescope--though the large toothed-wheel and the quadrant were made
inconveniently heavy (through a mistake of the workman who constructed
the instrument)--worked as easily and almost as conveniently as an
equatorial.
Still, it is well for the observer who wishes systematically to survey
the heavens--and who can afford the expense--to obtain a well-mounted
_equatorial_. In this method of mounting, the main axis is directed to
the pole of the heavens; the other axis, at right angles to the first,
carries the telescope-tube. One of the many methods adopted for mounting
equatorials is that exhibited--with the omission of some minor
details--in fig. 9. _a_ is the polar axis, _b_ is the axis (called the
declination axis) which bears the telescope. The circles _c_ and _d_
serve to indicate, by means of verniers revolving with the axes, the
motion of the telescope in right ascension and declination,
respectively. The weight _w_ serves to counterpoise the telescope, and
the screws _s_, _s_, _s_, _s_, serve to adjust the instrument so that
the polar axis shall be in its proper position. The advantage gained by
the equatorial method of mounting is that only one motion is required to
follow a star. Owing to the diurnal rotation of the earth, the stars
appear to move uniformly in circles parallel to the celestial equator;
and it is clear that a star so moving will be kept in the field of
view, if the telescope, once directed to the star, be made to revolve
uniformly and at a proper rate round the polar axis.
[Illustration: _Fig. 9._]
The equatorial can be directed by means of the circles _c_ and _d_ to
any celestial object whose right ascension and declination are known. On
the other hand, to bring an object into the field of view of an
alt-azimuth, it is necessary, either that the object itself should be
visible to the naked eye, or else that the position of the object should
be pretty accurately learned from star-maps, so that it may be picked up
by the alt-azimuth after a little searching. A small telescope called a
_finder_ is usually attached to all powerful telescopes intended for
general observation. The finder has a large field of view, and is
adjusted so as to have its axis parallel to that of the large telescope.
Thus a star brought to the centre of the large field of the finder
(indicated by the intersection of two lines placed at the focus of the
eye-glass) is at, or very near, the centre of the small field of the
large telescope.
If a telescope has no finder, it will be easy for the student to
construct one for himself, and will be a useful exercise in optics. Two
convex lenses not very different in size from those shown in fig. 1, and
placed as there shown--the distance between them being the sum of the
focal lengths of the two glasses--in a small tube of card, wood, or tin,
will serve the purpose of a finder for a small telescope. It can be
attached by wires to the telescope-tube, and adjusted each night before
commencing observation. The adjustment is thus managed:--a low power
being applied to the telescope, the tube is turned towards a bright
star; this is easily effected with a low power; then the finder is to be
fixed, by means of its wires, in such a position that the star shall be
in the centre of the field of the finder when also in the centre of the
telescope's field. When this has been done, the finder will greatly help
the observations of the evening; since with high powers much time would
be wasted in bringing an object into the field of view of the telescope
without the aid of a finder. Yet more time would be wasted in the case
of an object not visible to the naked eye, but whose position with
reference to several visible stars is known; since, while it is easy to
bring the point required to the centre of the _finder's_ field, in which
the guiding stars are visible, it is very difficult to direct the
_telescope's_ tube on a point of this sort. A card tube with wire
fastenings, such as we have described, may appear a very insignificant
contrivance to the regular observer, with his well-mounted equatorial
and carefully-adjusted finder. But to the first attempts of the amateur
observer it affords no insignificant assistance, as I can aver from my
own experience. Without it--a superior finder being wanting--our
"half-hours" would soon be wasted away in that most wearisome and
annoying of all employments, trying to "pick up" celestial objects.
It behoves me at this point to speak of star-maps. Such maps are of many
different kinds. There are the Observatory maps, in which the places of
thousands of stars are recorded with an amazing accuracy. Our beginner
is not likely to make use of, or to want, such maps as these. Then there
are maps merely intended to give a good general idea of the appearance
of the heavens at different hours and seasons. Plate I. presents four
maps of this sort; but a more complete series of eight maps has been
published by Messrs. Walton and Maberly in an octavo work; and my own
'Constellation-Seasons' give, at the same price, twelve quarto maps (of
four of which those in Plate I. are miniatures), showing the appearance
of the sky at any hour from month to month, or on any night, at
successive intervals of two hours. But maps intermediate in character to
these and to Observatory maps are required by the amateur observer.
Such are the Society's six gnomonic maps, the set of six gnomonic maps
in Johnstone's 'Atlas of Astronomy,' and my own set of twelve gnomonic
maps. The Society's maps are a remarkably good set, containing on the
scale of a ten-inch globe all the stars in the Catalogue of the
Astronomical Society (down to the fifth magnitude). The distortion,
however, is necessarily enormous when the celestial sphere is presented
in only six gnomonic maps. In my maps all the stars of the British
Association Catalogue down to the fifth magnitude are included on the
scale of a six-inch globe. The distortion is scarcely a fourth of that
in the Society's maps. The maps are so arranged that the relative
positions of all the stars in each hemisphere can be readily gathered
from a single view; and black duplicate-maps serve to show the
appearance of the constellations.
It is often convenient to make small maps of a part of the heavens we
may wish to study closely. My 'Handbook of the Stars' has been prepared
to aid the student in the construction of such maps.
In selecting maps it is well to be able to recognise the amount of
distortion and scale-variation. This may be done by examining the spaces
included between successive parallels and meridians, near the edges and
angles of the maps, and comparing these either with those in the centre
of the map, or with the known figures and dimensions of the
corresponding spaces on a globe.
We may now proceed to discuss the different tests which the intending
purchaser of a telescope should apply to the instrument.
The excellence of an object-glass can be satisfactorily determined only
by testing the performance of the telescope in the manner presently to
be described. But it is well to examine the quality of the glass as
respects transparency and uniformity of texture. Bubbles, scratches, and
other such defects, are not very important, since they do not affect the
distinctness of the field as they would in a Galilean Telescope,--a
little light is lost, and that is all. The same remark applies to dust
upon the glass. The glass should be kept as free as possible from dirt,
damp, or dust, but it is not advisable to remove every speck which,
despite such precaution, may accidentally fall upon the object-glass.
When it becomes necessary to clean the glass, it is to be noted that the
substance used should be soft, perfectly dry, and free from dust. Silk
is often recommended, but some silk is exceedingly objectionable in
texture,--old silk, perfectly soft to the touch, is perhaps as good as
anything. If the dust which has fallen on the glass is at all gritty,
the glass will suffer by the method of cleaning commonly adopted, in
which the dust is _gathered up_ by pressure. The proper method is to
clean a small space near the edge of the glass, and to _sweep_ from that
space as centre. In this way the dust is _pushed before_ the silk or
wash-leather, and does not cut the glass. It is well always to suspect
the presence of gritty dust, and adopt this cautious method of cleaning.
The two glasses should on no account be separated.
In examining an eye-piece, the quality of the glass should be noted, and
care taken that both glasses (but especially the field-glass) are free
from the least speck, scratch, or blemish of any kind, for these defects
will be exhibited in a magnified state in the field of view. Hence the
eye-pieces require to be as carefully preserved from damp and dust as
the object-glass, and to be more frequently cleaned.
The tube of the telescope should be light, but strong, and free from
vibration. Its quality in the last respect can be tested by lightly
striking it when mounted; the sound given out should be dead or
non-resonant. The inside of the tube must absorb extraneous light, and
should therefore be coloured a dull black; and stops of varying radius
should be placed along its length with the same object. Sliding tubes,
rack-work, etc., should work closely, yet easily.
The telescope should be well balanced for vision with the small
astronomical eye-pieces. But as there is often occasion to use
appliances which disturb the balance, it is well to have the means of at
once restoring equilibrium. A cord ring running round the tube (pretty
tightly, so as to rest still when the tube is inclined), and bearing a
small weight, will be all that is required for this purpose; it must be
slipped along the tube until the tube is found to be perfectly balanced.
Nothing is more annoying than, after getting a star well in the field,
to see the tube shift its position through defective balance, and thus
to have to search again for the star. Even with such an arrangement as
is shown in fig. 8, though the tube cannot readily shift its position,
it is better to have it well balanced.
The quality of the stand has a very important influence on the
performance of a telescope. In fact, a moderately good telescope,
mounted on a steady stand, working easily and conveniently, will not
only enable the observer to pass his time much more pleasantly, but will
absolutely exhibit more difficult objects than a finer instrument on a
rickety, ill-arranged stand. A good observing-chair is also a matter of
some importance, the least constraint or awkwardness of position
detracting considerably from the power of distinct vision. Such, at
least, is my own experience.
But the mere examination of the glasses, tube, mounting, &c., is only
the first step in the series of tests which should be applied to a
telescope, since the excellence of the instrument depends, not on its
size, the beauty of its mounting, or any extraneous circumstances, but
on its performance.
The observer should first determine whether the chromatic aberration is
corrected. To ascertain this the telescope should be directed to the
moon, or (better) to Jupiter, and accurately focussed for distinct
vision. If, then, on moving the eye-piece towards the object-glass, a
ring of purple appears round the margin of the object, and on moving the
eye-glass in the contrary direction a ring of green, the chromatic
aberration is corrected, since these are the colours of the secondary
spectrum.
To determine whether the spherical aberration is corrected, the
telescope should be directed towards a star of the third or fourth
magnitude, and focussed for distinct vision. A cap with an aperture of
about one-half its diameter should then be placed over the object-glass.
If no new adjustment is required for distinct vision, the spherical
aberration is corrected, since the mean focal length and the focal
length of the central rays are equal. If, when the cap is on, the
eye-piece has to be pulled out for distinct vision, the spherical
aberration has not been fully corrected; if the eye-piece has to be
pushed in, the aberration has been over-corrected. As a further test, we
may cut off the central rays, by means of a circular card covering the
middle of the object-glass, and compare the focal length for distinct
vision with the focal length when the cap is applied. The extent of the
spherical aberration may be thus determined; but if the first experiment
gives a satisfactory result, no other is required.
A star of the first magnitude should next be brought into the field of
view. If an irradiation from one side is perceived, part of the
object-glass has not the same refractive power as the rest; and the
part which is defective can be determined by applying in different
positions a cap which hides half the object-glass. If the irradiation is
double, it will probably be found that the object-glass has been too
tightly screwed, and the defect will disappear when the glass is freed
from such undue pressure.
If the object-glass is not quite at right angles to the axis of the
tube, or if the eye-tube is at all inclined, a like irradiation will
appear when a bright star is in the field. The former defect is not
easily detected or remedied; nor is it commonly met with in the work of
a careful optician. The latter defect may be detected by cutting out
three circular cards of suitable size with a small aperture at the
centre of each, and inserting one at each end of the eye-tube, and one
over the object-glass. If the tube is rightly placed the apertures will
of course lie in a right line, so that it will be possible to look
through all three at once. If not, it will be easy to determine towards
what part of the object-glass the eye-tube is directed, and to correct
the position of the tube accordingly.
The best tests for determining the defining power of a telescope are
close double or multiple stars, the components of which are not very
unequal. The illuminating power should be tested by directing the
telescope towards double or multiple stars having one or more minute
components. Many of the nebulae serve as tests both for illumination and
defining power. As we proceed we shall meet with proper objects for
testing different telescopes. For the present, let the following list
suffice. It is selected from Admiral Smyth's tests, obtained by
diminishing the aperture of a 6-in. telescope having a focal length of
8-1/2 feet:
A two-inch aperture, with powers of from 60 to 100, should exhibit
[alpha] Piscium (3".5). | [delta] Cassiopeiae (9".5),
| mag. (4 and 7-1/2)
[gamma] Leonis (3".2). | Polaris (18".6), mag. (2-1/2
| and 9-1/2)
A four-inch, powers 80 to 120, should exhibit
[xi] Ursae Majoris (2".4). | [sigma] Cassiopeiae (3".1),
| mag. (6 and 8).
[gamma] Ceti (2".6). | [delta] Geminorum (7".1),
| mag. (4 and 9).
The tests in the first column are for definition, those in the second
for illumination. It will be noticed that, though in the case of Polaris
the smaller aperture may be expected to show the small star of less than
the 9th magnitude, a larger aperture is required to show the 8th
magnitude component of [sigma] Cassiopeiae, on account of the greater
closeness of this double.
In favourable weather the following is a good general test of the
performance of a telescope:--A star of the 3rd or 4th magnitude at a
considerable elevation above the horizon should exhibit a small well
defined disc, surrounded by two or three fine rings of light.
A telescope should not be mounted within doors, if it can be
conveniently erected on solid ground, as every movement in the house
will cause the instrument to vibrate unpleasantly. Further, if the
telescope is placed in a warm room, currents of cold air from without
will render observed objects hazy and indistinct. In fact, Sir W.
Herschel considered that a telescope should not even be erected near a
house or elevation of any kind round which currents of air are likely to
be produced. If a telescope is used in a room, the temperature of the
room should be made as nearly equal as possible to that of the outer
air.
When a telescope is used out of doors a 'dew-cap,' that is, a tube of
tin or pasteboard, some ten or twelve inches long, should be placed on
the end of the instrument, so as to project beyond the object-glass. For
glass is a good radiator of heat, so that dew falls heavily upon it,
unless the radiation is in some way checked. The dew-cap does this
effectually. It should be blackened within, especially if made of metal.
"After use," says old Kitchener, "the telescope should be kept in a warm
place long enough for any moisture on the object-glass to evaporate." If
damp gets between the glasses it produces a fog (which opticians call a
sweat) or even a seaweed-like vegetation, by which a valuable glass may
be completely ruined.
The observer should not leave to the precious hours of the night the
study of the bearing and position of the objects he proposes to examine.
This should be done by day--an arrangement which has a twofold
advantage,--the time available for observation is lengthened, and the
eyes are spared sudden changes from darkness to light, and _vice versa_.
Besides, the eye is ill-fitted to examine difficult objects, after
searching by candle-light amongst the minute details recorded in maps or
globes. Of the effect of rest to the eye we have an instance in Sir J.
Herschel's rediscovery of the satellites of Uranus, which he effected
after keeping his eyes in darkness for a quarter of an hour. Kitchener,
indeed, goes so far as to recommend (with a _crede experto_) an
_interval of sleep_ in the darkness of the observing-room before
commencing operations. I have never tried the experiment, but I should
expect it to have a bad rather than a good effect on the eyesight, as
one commonly sees the eyes of a person who has been sleeping in his
day-clothes look heavy and bloodshot.
The object or the part of an object to be observed should be brought as
nearly as possible to the centre of the field of view. When there is no
apparatus for keeping the telescope pointed upon an object, the best
plan is so to direct the telescope by means of the finder, that the
object shall be just out of the field of view, and be brought (by the
earth's motion) across the centre of the field. Thus the vibrations
which always follow the adjustment of the tube will have subsided before
the object appears. The object should then be intently watched during
the whole interval of its passage across the field of view.
It is important that the student should recognise the fact that the
highest powers do not necessarily give the best views of celestial
objects. High powers in all cases increase the difficulty of
observation, since they diminish the field of view and the illumination
of the object, increase the motion with which (owing to the earth's
motion) the image moves across the field, and magnify all defects due to
instability of the stand, imperfection of the object-glass, or
undulation of the atmosphere. A good object-glass of three inches
aperture will in very favourable weather bear a power of about 300, when
applied to the observation of close double or multiple stars, but for
all other observations much lower powers should be used. Nothing but
failure and annoyance can follow the attempt to employ the highest
powers on unsuitable objects or in unfavourable weather.
The greatest care should be taken in focussing the telescope. When high
powers are used this is a matter of some delicacy. It would be well if
the eye-pieces intended for a telescope were so constructed that when
the telescope is focussed for one, this might be replaced by any other
without necessitating any use of the focussing rack-work. This could be
readily effected by suitably placing the shoulder which limits the
insertion of the eye-piece.
It will be found that, even in the worst weather for observation, there
are instants of distinct vision (with moderate powers) during which the
careful observer may catch sight of important details; and, similarly,
in the best observing weather, there are moments of unusually distinct
vision well worth patient waiting for, since in such weather alone the
full powers of the telescope can be employed.
The telescopist should not be deterred from observation by the presence
of fog or haze, since with a hazy sky definition is often singularly
good.
The observer must not expect distinct vision of objects near the
horizon. Objects near the eastern horizon during the time of morning
twilight are especially confused by atmospheric undulations; in fact,
early morning is a very unfavourable time for the observation of all
objects.
The same rules which we have been applying to refractors, serve for
reflectors. The performance of a reflector will be found to differ in
some respects, however, from that of a refractor. Mr. Dawes is, we
believe, now engaged in testing reflectors, and his unequalled
experience of refractors will enable him to pronounce decisively on the
relative merits of the two classes of telescopes.
We have little to say respecting the construction of telescopes. Whether
it is advisable or not for an amateur observer to attempt the
construction of his own telescope is a question depending entirely on
his mechanical ability and ingenuity. My own experience of telescope
construction is confined to the conversion of a 3-feet into a 5-1/2-feet
telescope. This operation involved some difficulties, since the aperture
had to be increased by about an inch. I found a tubing made of alternate
layers of card and calico well pasted together, to be both light and
strong. But for the full length of tube I think a core of metal is
wanted. A learned and ingenious friend, Mr. Sharp, Fellow of St. John's
College, informs me that a tube of tin, covered with layers of brown
paper, well pasted and thicker near the middle of the tube, forms a
light and strong telescope-tube, almost wholly free from vibration.
Suffer no inexperienced person to deal with your object-glass. I knew a
valuable glass ruined by the proceedings of a workman who had been told
to attach three pieces of brass round the cell of the double lens. What
he had done remained unknown, but ever after a wretched glare of light
surrounded all objects of any brilliancy.
One word about the inversion of objects by the astronomical telescope.
It is singular that any difficulty should be felt about so simple a
matter, yet I have seen in the writings of more than one distinguished
astronomer, wholly incorrect views as to the nature of the inversion.
One tells us that to obtain the correct presentation from a picture
taken with a telescope, the view should be inverted, held up to the
light, and looked at from the back of the paper. Another tells us to
invert the picture and hold it opposite a looking-glass. Neither method
is correct. The simple correction wanted is to hold the picture upside
down--the same change which brings the top to the bottom brings the
right to the left, _i.e._, fully corrects the inversion.
In the case, however, of a picture taken by an Herschelian reflector,
the inversion not being complete, a different method must be adopted. In
fact, either of the above-named processes, incorrect for the ordinary
astronomical, would be correct for the Herschelian Telescope. The latter
inverts but does not reverse right and left; therefore after inverting
our picture we must interchange right and left because they have been
reversed by the inversion. This is effected either by looking at the
picture from behind, or by holding it up to a mirror.
[Illustration: PLATE II.]
CHAPTER II.
A HALF-HOUR WITH ORION, LEPUS TAURUS, ETC.
Any of the half-hours here assigned to the constellation-seasons may be
taken first, and the rest in seasonal or cyclic order. The following
introductory remarks are applicable to each:--
If we stand on an open space, on any clear night, we see above us the
celestial dome spangled with stars, apparently fixed in position. But
after a little time it becomes clear that these orbs are slowly shifting
their position. Those near the eastern horizon are rising, those near
the western setting. Careful and continuous observation would show that
the stars are all moving in the same way, precisely, as they would if
they were fixed to the concave surface of a vast hollow sphere, and this
sphere rotated about an axis. This axis, in our latitude, is inclined
about 51-1/2 deg. to the horizon. Of course only one end of this imaginary
axis can be above our horizon. This end lies very near a star which it
will be well for us to become acquainted with at the beginning of our
operations. It lies almost exactly towards the north, and is raised from
50 deg. to 53 deg. (according to the season and hour) above the horizon. There
is an easy method of finding it.
We must first find the Greater Bear. It will be seen from Plate 1, that
on a spring evening the seven conspicuous stars of this constellation
are to be looked for towards the north-east, about half way between the
horizon and the point overhead (or _zenith_), the length of the set of
stars being vertical. On a summer's evening the Great Bear is nearly
overhead. On an autumn evening he is towards the north-west, the length
of the set of seven being somewhat inclined to the horizon. Finally, on
a winter's evening, he is low down towards the north, the length of the
set of seven stars being nearly in a horizontal direction.
Having found the seven stars, we make use of the pointers [alpha] and
[beta] (shown in Plate 1) to indicate the place of the Pole-star, whose
distance from the pointer [alpha] is rather more than three times the
distance of [alpha] from [beta].
Now stand facing the Pole-star. Then all the stars are travelling round
that star _in a direction contrary to that in which the hands of a watch
move_. Thus the stars below the pole are moving _towards the right_,
those above the pole _towards the left_, those to the right of the pole
_upwards_, those to the left of the pole _downwards_.
Next face the south. Then all the stars on our left, that is, towards
the east, are rising slantingly towards the south; those due south are
moving horizontally to the right, that is, towards the west; and those
on our right are passing slantingly downwards towards the west.
It is important to familiarise ourselves with these motions, because it
is through them that objects pass out of the field of view of the
telescope, and by moving the tube in a proper direction we can easily
pick up an object that has thus passed away, whereas if we are not
familiar with the varying motions in different parts of the celestial
sphere, we may fail in the attempt to immediately recover an object, and
waste time in the search for it.
The consideration of the celestial motions shows how advantageous it is,
when using an alt-azimuth, to observe objects as nearly as possible due
south. Of course in many cases this is impracticable, because a
phenomenon we wish to watch may occur when an object is not situated
near the meridian. But in examining double stars there is in general no
reason for selecting objects inconveniently situated. We can wait till
they come round to the meridian, and then observe them more comfortably.
Besides, most objects are higher, and therefore better seen, when due
south.
Northern objects, and especially those within the circle of perpetual
apparition, often culminate (that is, cross the meridian, or north and
south line) at too great a height for comfortable vision. In this case
we should observe them towards the east or west, and remember that in
the first case they are rising, and in the latter they are setting, and
that in both cases they have also a motion from left to right.
If we allow an object to pass right across the field of view (the
telescope being fixed), the apparent direction of its motion is the
exact reverse of the true direction of the star's motion. This will
serve as a guide in shifting the alt-azimuth after a star has passed out
of the field of view.
The following technical terms must be explained. That part of the field
of view towards which the star appears to move is called the _preceding_
part of the field, the opposite being termed the _following_ part. The
motion for all stars, except those lying in an oval space extending from
the zenith to the pole of the heavens, is more or less from right to
left (in the inverted field). Now, if we suppose a star to move along a
diameter of the field so as to divide the field into two semicircles,
then in all cases in which this motion takes places from right to left,
that semicircle which contains the lowest point (apparently) of the
field is the _northern_ half, the other is the _southern_ half. Over the
oval space just mentioned the reverse holds.
Thus the field is divided into four quadrants, and these are termed
_north following_ (_n.f._) and _south following_ (_s.f._); _north
preceding_ (_n.p._), and _south preceding_ (_s.p._). The student can
have no difficulty in interpreting these terms, since he knows which is
the following and which the preceding _semicircle_, which the northern
and which the southern. In the figures of plates 3 and 5, the letters
_n.f._, _n.p._, &c., are affixed to the proper quadrants. It is to be
remembered that the quadrants thus indicated are measured either way
from the point and feather of the diametral arrows.
Next, of the apparent annual motion of the stars. This takes place in
exactly the same manner as the daily motion. If we view the sky at eight
o'clock on any day, and again at the same hour one month later, we shall
find that at the latter observation (as compared with the former) the
heavens appear to have rotated by the _twelfth part_ of a complete
circumference, and the appearance presented is precisely the same as we
should have observed had we waited for two hours (the _twelfth part_ of
a day) on the day of the first observation.
* * * * *
Our survey of the heavens is supposed to be commenced during the first
quarter of the year, at ten o'clock on the 20th of January, or at nine
on the 5th of February, or at eight on the 19th of February, or at seven
on the 6th of March, or at hours intermediate to these on intermediate
days.
We look first for the Great Bear towards the north-east, as already
described, and thence find the Pole-star; turning towards which we see,
towards the right and downwards, the two guardians of the pole ([beta]
and [gamma] Ursae Minoris). Immediately under the Pole-star is the
Dragon's Head, a conspicuous diamond of stars. Just on the horizon is
Vega, scintillating brilliantly. Overhead is the brilliant Capella, near
which the Milky Way is seen passing down to the horizon on either side
towards the quarters S.S.E. and N.N.W.
For the present our business is with the southern heavens, however.
Facing the south, we see a brilliant array of stars, Sirius
unmistakeably overshining the rest. Orion is shining in full glory, his
leading brilliant, Betelgeuse[2] being almost exactly on the meridian,
and also almost exactly half way between the horizon and the zenith. In
Plate 2 is given a map of this constellation and its neighbourhood.
Let us first turn the tube on Sirius. It is easy to get him in the field
without the aid of a finder. The search will serve to illustrate a
method often useful when a telescope has no finder. Having taking out
the eye-piece--a low-power one, suppose--direct the tube nearly towards
Sirius. On looking through it, a glare of light will be seen within the
tube. Now, if the tube be slightly moved about, the light will be seen
to wax and wane, according as the tube is more or less accurately
directed. Following these indications, it will be found easy to direct
the tube, so that the object-glass shall appear _full of light_. When
this is done, insert the eye-piece, and the star will be seen in the
field.
But the telescope is out of focus, therefore we must turn the small
focussing screw. Observe the charming chromatic changes--green, and
red, and blue light, purer than the hues of the rainbow, scintillating
and coruscating with wonderful brilliancy. As we get the focus, the
excursions of these light flashes diminish until--if the weather is
favourable--the star is seen, still scintillating, and much brighter
than to the naked eye, but reduced to a small disc of light, surrounded
(in the case of so bright a star as Sirius) with a slight glare. If
after obtaining the focus the focussing rack work be still turned, we
see a coruscating image as before. In the case of a very brilliant star
these coruscations are so charming that we may be excused for calling
the observer's attention to them. The subject is not without interest
and difficulty as an optical one. But the astronomer's object is to get
rid of all these flames and sprays of coloured light, so that he has
very little sympathy with the admiration which Wordsworth is said to
have expressed for out-of-focus views of the stars.
We pass to more legitimate observations, noticing in passing that Sirius
is a double star, the companion being of the tenth magnitude, and
distant about ten seconds from the primary. But our beginner is not
likely to see the companion, which is a very difficult object, vowing to
the overpowering brilliancy of the primary.
Orion affords the observer a splendid field of research. It is a
constellation rich in double and multiple stars, clusters, and nebulae.
We will begin with an easy object.
The star [delta] (Plate 3), or _Mintaka_, the uppermost of the three
stars forming the belt, is a wide double. The primary is of the second
magnitude, the secondary of the seventh, both being white.
The star [alpha] (_Betelgeuse_) is an interesting object, on account of
its colour and brilliance, and as one of the most remarkable variables
in the heavens. It was first observed to be variable by Sir John
Herschel in 1836. At this period its variations were "most marked and
striking." This continued until 1840, when the changes became "much less
conspicuous. In January, 1849, they had recommenced, and on December
5th, 1852, Mr. Fletcher observed [alpha] Orionis brighter than Capella,
and actually the largest star in the northern hemisphere." That a star
so conspicuous, and presumably so large, should present such remarkable
variations, is a circumstance which adds an additional interest to the
results which have rewarded the spectrum-analysis of this star by Mr.
Huggins and Professor Miller. It appears that there is decisive evidence
of the presence in this luminary of many elements known to exist in our
own sun; amongst others are found sodium, magnesium, calcium, iron, and
bismuth. Hydrogen appears to be absent, or, more correctly, there are no
lines in the star's spectrum corresponding to those of hydrogen in the
solar spectrum. Secchi considers that there is evidence of an actual
change in the spectrum of the star, an opinion in which Mr. Huggins does
not coincide. In the telescope Betelgeuse appears as "a rich and
brilliant gem," says Lassell, "a rich topaz, in hue and brilliancy
differing from any that I have seen."
Turn next to [beta] (Rigel), the brightest star below the belt. This is
a very noted double, and will severely test our observer's telescope, if
small. The components are well separated (see Plate 3), compared with
many easier doubles; the secondary is also of the seventh magnitude, so
that neither as respects closeness nor smallness of the secondary, is
Rigel a difficult object. It is the combination of the two features
which makes it a test-object. Kitchener says a 1-3/4-inch object-glass
should show Rigel double; in earlier editions of his work he gave
2-3/4-inches as the necessary aperture. Smyth mentions Rigel as a test
for a 4-inch aperture, with powers of from 80 to 120. A 3-inch aperture,
however, will certainly show the companion. Rigel is an orange star, the
companion blue.
Turn next to [lambda] the northernmost of the set of three stars in the
head of Orion. This is a triple star, though an aperture of 3 inches
will show it only as a double. The components are 5" apart, the colours
pale white and violet. With the full powers of a 3-1/2-inch glass a
faint companion may be seen above [lambda].
The star [zeta], the lowest in the belt, may be tried with a 3-1/2-inch
glass. It is a close double, the components being nearly equal, and
about 2-1/2" apart (see Plate 3).
For a change we will now try our telescope on a nebula, selecting the
great nebula in the Sword. The place of this object is indicated in
Plate 2. There can be no difficulty in finding it since it is clearly
visible to the naked eye on a moonless night--the only sort of night on
which an observer would care to look at nebulae. A low power should be
employed.
The nebula is shown in Plate 3 as I have seen it with a 3-inch aperture.
We see nothing of those complex streams of light which are portrayed in
the drawings of Herschel, Bond, and Lassell, but enough to excite our
interest and wonder. What is this marvellous light-cloud? One could
almost imagine that there was a strange prophetic meaning in the words
which have been translated "Canst thou loose the bands of Orion?"
Telescope after telescope had been turned on this wonderful object with
the hope of resolving its light into stars. But it proved intractable to
Herschel's great reflector, to Lassell's 2-feet reflector, to Lord
Rosse's 3-feet reflector, and even partially to the great 6-feet
reflector. Then we hear of its supposed resolution into stars, Lord
Rosse himself writing to Professor Nichol, in 1846, "I may safely say
there can be little, if any, doubt as to the resolvability of the
nebula;--all about the trapezium is a mass of stars, the rest of the
nebula also abounding with stars, and exhibiting the characteristics of
resolvability strongly marked."
It was decided, therefore, that assuredly the great nebula is a
congeries of stars, and not a mass of nebulous matter as had been
surmised by Sir W. Herschel. And therefore astronomers were not a little
surprised when it was proved by Mr. Huggins' spectrum-analysis that the
nebula consists of gaseous matter. How widely extended this gaseous
universe may be we cannot say. The general opinion is that the nebulae
are removed far beyond the fixed stars. If this were so, the dimensions
of the Orion nebula would be indeed enormous, far larger probably than
those of the whole system whereof our sun is a member. I believe this
view is founded on insufficient evidence, but this would not be the
place to discuss the subject. I shall merely point out that the nebula
occurs in a region rich in stars, and if it is not, like the great
nebula in Argo, clustered around a remarkable star, it is found
associated in a manner which I cannot look upon as accidental with a set
of small-magnitude stars, and notably with the trapezium which surrounds
that very remarkable black gap within the nebula. The fact that the
nebula shares the proper motion of the trapezium appears inexplicable if
the nebula is really far out in space beyond the trapezium. A very small
proper motion of the trapezium (alone) would long since have destroyed
the remarkable agreement in the position of the dark gap and the
trapezium which has been noticed for so many years.
But whether belonging to our system or far beyond it, the great nebula
must have enormous dimensions. A vast gaseous system it is, sustained by
what arrangements or forces we cannot tell, nor can we know what
purposes it subserves. Mr. Huggins' discovery that comets have gaseous
nuclei, (so far as the two he has yet examined show) may suggest the
speculation that in the Orion nebula we see a vast system of comets
travelling in extensive orbits around nuclear stars, and so slowly as to
exhibit for long intervals of time an unchanged figure. "But of such
speculations" we may say with Sir J. Herschel "there is no end."
To return to our telescopic observations:--The trapezium affords a
useful test for the light-gathering power of the telescope. Large
instruments exhibit nine stars. But our observer may be well satisfied
with his instrument and his eye-sight if he can see five with a
3-1/2-inch aperture.[3] A good 3-inch glass shows four distinctly. But
with smaller apertures only three are visible.
The whole neighbourhood of the great nebula will well repay research.
The observer may sweep over it carefully on any dark night with profit.
Above the nebula is the star-cluster 362 H. The star [iota] (double as
shown in Plate 3) below the nebula is involved in a strong nebulosity.
And in searching over this region we meet with delicate double, triple,
and multiple stars, which make the survey interesting with almost any
power that may be applied.
Above the nebula is the star [sigma], a multiple star. To an observer
with a good 3-1/2-inch glass [sigma] appears as an octuple star. It is
well seen, however, as a fine multiple star with a smaller aperture.
Some of the stars of this group appear to be variable.
The star [rho] Orionis is an unequal, easy double, the components being
separated by nearly seven seconds. The primary is orange, the smaller
star smalt-blue (see Plate 3).
The middle star of the belt ([epsilon]) has a distant blue companion.
This star, like [iota], is nebulous. In fact, the whole region within
the triangle formed by stars [gamma], [kappa] and [beta] is full of
nebulous double and multiple stars, whose aggregation in this region I
do not consider wholly accidental.
We have not explored half the wealth of Orion, but leave much for future
observation. We must turn, however, to other constellations.
Below Orion is Lepus, the Hare, a small constellation containing some
remarkable doubles. Among these we may note [xi], a white star with a
scarlet companion; [gamma], a yellow and garnet double; and [iota], a
double star, white and pale violet, with a distant red companion. The
star [kappa] Leporis is a rather close double, white with a small green
companion. The intensely red star R Leporis (a variable) will be found
in the position indicated in the map.
Still keeping within the boundary of our map, we may next turn to the
fine cluster 2 H (vii.) in Monoceros. This cluster is visible to the
naked eye, and will be easily found. The nebula 2 H (iv.) is a
remarkable one with a powerful telescope.
The star 11 Monocerotis is a fine triple star described by the elder
Herschel as one of the finest sights in the heavens. Our observer,
however, will see it as a double (see Plate 3). [delta] Monocerotis is
an easy double, yellow and lavender.
We may now leave the region covered by the map and take a survey of the
heavens for some objects well seen at this season.
Towards the south-east, high up above the horizon, we see the twin-stars
Castor and Pollux. The upper is Castor, the finest double star visible
in the northern heavens. The components are nearly equal and rather more
than 5" apart (see Plate 3). Both are white according to the best
observers, but the smaller is thought by some to be slightly greenish.
Pollux is a coarse but fine triple star (in large instruments multiple).
The components orange, grey, and lilac.
There are many other fine objects in Gemini, but we pass to Cancer.
The fine cluster Praesepe in Cancer may easily be found as it is
distinctly visible to the naked eye in the position shown in Plate 1,
Map I. In the telescope it is seen as shown in Plate 3.
The star [iota] Cancri is a wide double, the colours orange and blue.
Procyon, the first-magnitude star between Praesepe and Sirius, is finely
coloured--yellow with a distant orange companion, which appears to be
variable.
Below the Twins, almost in a line with them, is the star [alpha] Hydrae,
called Al Fard, or "the Solitary One." It is a 2nd magnitude variable. I
mention it, however, not on its own account, but as a guide to the fine
double [epsilon] Hydrae. This star is the middle one of a group of three,
lying between Pollux and Al Fard rather nearer the latter. The
components of [epsilon] Hydrae are separated by about 3-1/2" (see Plate
3). The primary is of the fourth, the companion of the eighth magnitude;
the former is yellow, the latter a ruddy purple. The period of [epsilon]
Hydrae is about 450 years.
The constellation Leo Minor, now due east and about midway between the
horizon and the zenith, is well worth sweeping over. It contains several
fine fields.
Let us next turn to the western heavens. Here there are some noteworthy
objects.
To begin with, there are the Pleiades, showing to the naked eye only six
or seven stars. In the telescope the Pleiades appear as shown in Plate
3.
The Hyades also show some fine fields with low powers.
Aldebaran, the principal star of the Hyades, as also of the
constellation Taurus, is a noted red star. It is chiefly remarkable for
the close spectroscopic analysis to which it has been subjected by
Messrs. Huggins and Miller. Unlike Betelgeuse, the spectrum of Aldebaran
exhibits the lines corresponding to hydrogen, and no less than eight
metals--sodium, magnesium, calcium, iron, bismuth, tellurium, antimony,
and mercury, are proved to exist in the constitution of this brilliant
red star.
On the right of Aldebaran, in the position indicated in Plate 1, Map I.,
are the stars [zeta] and [beta] Tauri. If with a low power the observer
sweep from [zeta] towards [beta], he will soon find--not far from [zeta]
(at a distance of about one-sixth of the distance separating [beta] from
[zeta]), the celebrated Crab nebula, known as 1 M. This was the first
nebula discovered by Messier, and its discovery led to the formation of
his catalogue of 103 nebulae. In a small telescope this object appears as
a nebulous light of oval form, no traces being seen of the wisps and
sprays of light presented in Lord Rosse's well known picture of the
nebula.
Here I shall conclude the labours of our first half-hour among the
stars, noticing that the examination of Plate 1 will show what other
constellations besides those here considered are well situated for
observation at this season. It will be remarked that many constellations
well seen in the third half-hour (Chapter IV.) are favourably seen in
the first also, and _vice versa_. For instance, the constellation Ursa
Major well-placed towards the north-east in the first quarter of the
year, is equally well-placed towards the north-west in the third, and
similarly of the constellation Cassiopeia. The same relation connects
the second and fourth quarters of the year.
[Illustration: PLATE III.]
CHAPTER III.
A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC.
The observations now to be commenced are supposed to take place during
the second quarter of the year,--at ten o'clock on the 20th of April, or
at nine on the 5th of May, or at eight on the 21st of May, or at seven
on the 5th of June, or at hours intermediate to these on intermediate
days.
We again look first for the Great Bear, now near the zenith, and thence
find the Pole-star. Turning towards the north, we see Cassiopeia between
the Pole-star and the horizon. Towards the north-west is the brilliant
Capella, and towards the north-east the equally brilliant Vega, beneath
which, and somewhat northerly, is the cross in Cygnus. The Milky Way
passes from the eastern horizon towards the north (low down), and so
round to the western horizon.
In selecting a region for special observation, we shall adopt a
different plan from that used in the preceding "half-hour." The region
on the equator and towards the south is indeed particularly interesting,
since it includes the nebular region in Virgo. Within this space nebulae
are clustered more closely than over any corresponding space in the
heavens, save only the greater Magellanic cloud. But to the observer
with telescopes of moderate power these nebulae present few features of
special interest; and there are regions of the sky now well situated for
observation, which, at most other epochs are either low down towards
the horizon or inconveniently near to the zenith. We shall therefore
select one of these, the region included in the second map of Plate 2,
and the neighbouring part of the celestial sphere.
At any of the hours above named, the constellation Hercules lies towards
the east. A quadrant taken from the zenith to the eastern horizon passes
close to the last star ([eta]) of the Great Bear's tail, through [beta],
a star in Bootes' head, near [beta] Herculis, between the two "Alphas"
which mark the heads of Hercules and Ophiuchus, and so past [beta]
Ophiuchi, a third-magnitude star near the horizon. And here we may turn
aside for a moment to notice the remarkable vertical row of six
conspicuous stars towards the east-south-east; these are, counting them
in order from the horizon, [zeta], [epsilon], and [delta] Ophiuchi,
[epsilon], [alpha], and [delta] Serpentis.
Let the telescope first be directed towards Vega. This orb presents a
brilliant appearance in the telescope. Its colour is a bluish-white. In
an ordinary telescope Vega appears as a single star, but with a large
object-glass two distant small companions are seen. A nine-inch glass
shows also two small companions within a few seconds of Vega. In the
great Harvard refractor Vega is seen with no less than thirty-five
companions. I imagine that all these stars, and others which can be seen
in neighbouring fields, indicate the association of Vega with the
neighbouring stream of the Milky Way.
Let our observer now direct his telescope to the star [epsilon] Lyrae. Or
rather, let him first closely examine this star with the naked eye. The
star is easily identified, since it lies to the left of Vega, forming
with [zeta] a small equilateral triangle. A careful scrutiny suffices to
indicate a peculiarity in this star. If our observer possesses very
good eye-sight, he will distinctly recognise it as a "naked-eye double";
but more probably he will only notice that it appears lengthened in a
north and south direction.[4] In the finder the star is easily divided.
Applying a low power to the telescope itself, we see [epsilon] Lyrae as a
wide double, the line joining the components lying nearly north and
south. The southernmost component (the upper in the figure) is called
[epsilon]^{1}, the other [epsilon]^{2}. Seen as a double, both
components appear white.
Now, if the observer's telescope is sufficiently powerful, each of the
components may be seen to be itself double. First try [epsilon]^{1}, the
northern pair. The line joining the components is directed as shown in
Plate 3. The distance between them is 3".2, their magnitudes 5 and
6-1/2, and their colours yellow and ruddy. If the observer succeeds in
seeing [epsilon]^{1} fairly divided, he will probably not fail in
detecting the duplicity of [epsilon]^{2}, though this is a rather closer
pair, the distance between the components being only 2".6. The
magnitudes are 5 and 5-1/2, both being white. Between [epsilon]^{1} and
[epsilon]^{2} are three faint stars, possibly forming with the quadruple
a single system.
Let us next turn to the third star of the equilateral triangle mentioned
above--viz. to the star [zeta] Lyrae. This is a splendid but easy double.
It is figured in Plate 3, but it must be noticed that the figure of
[zeta] and the other nine small figures are not drawn on the same scale
as [epsilon] Lyrae. The actual distance between the components of [zeta]
Lyra is 44", or about one-fourth of the distance separating
[epsilon]^{1} from [epsilon]^{2}. The components of [zeta] are very
nearly equal in magnitude, in colour topaz and green, the topaz
component being estimated as of the fifth magnitude, the green component
intermediate between the fifth and sixth magnitudes.
We may now turn to a star not figured in the map, but readily found. It
will be noticed that the stars [epsilon], [alpha], [beta], and [gamma]
form, with two small stars towards the left, a somewhat regular
hexagonal figure--a hexagon, in fact, having three equal long sides and
three nearly equal short sides alternating with the others. The star
[eta] Lyrae forms the angle next to [epsilon]. It is a wide and unequal
double, as figured in Plate 3. The larger component is azure blue; the
smaller is violet, and, being only of the ninth magnitude, is somewhat
difficult to catch with apertures under 3 inches.
The star [delta]^{2} Lyrae is orange, [delta]^{1} blue. In the same field
with these are seen many other stars.
The stars [gamma]^{1} and [gamma]^{2} may also be seen in a single
field, the distance between them being about half the moon's mean
diameter. The former is quadruple, the components being yellow, bluish,
pale blue, and blue.
Turn next to the stars [beta] and [gamma] Lyrae, the former a multiple,
the latter an unequal double star. It is not, however, in these respects
that these stars are chiefly interesting, but for their variability. The
variability of [gamma] has not indeed been fully established, though it
is certain that, having once been less bright, [gamma] is now
considerably brighter than [beta]. The change, however, may be due to
the variation of [beta] alone. This star is one of the most remarkable
variables known. Its period is 12d. 21h. 53m. 10s. In this time it
passes from a maximum brilliancy--that of a star of the 3.4
magnitude--to a minimum lustre equal to that of a star of the 4.3
magnitude, thence to the same maximum brilliancy as before, thence to
another minimum of lustre--that of a star of the 4.5 magnitude--and so
to its maximum lustre again, when the cycle of changes recommences.
These remarkable changes seem to point to the existence of two unequal
dark satellites, whose dimensions bear a much greater proportion to
those of the bright components of [beta] Lyrae than the dimensions of the
members of the Solar System bear to those of the sun. In this case, at
any rate, the conjecture hazarded about Algol, that the star revolves
around a dark central orb, would be insufficient to account for the
observed variation.
Nearly midway between [beta] and [gamma] lies the wonderful ring-nebula
57 M, of which an imperfect idea will be conveyed by the last figure of
Plate 3. This nebula was discovered in 1772, by Darquier, at Toulouse.
It is seen as a ring of light with very moderate telescopic power. In a
good 3-1/2-inch telescope the nebula exhibits a mottled appearance and a
sparkling light. Larger instruments exhibit a faint light within the
ring; and in Lord Rosse's great Telescope "wisps of stars" are seen
within, and faint streaks of light stream from the outer border of the
ring. This nebula has been subjected to spectrum-analysis by Mr.
Huggins. It turns out to be a gaseous nebula! In fact, ring-nebulae--of
which only seven have been detected--seem to belong to the same class as
the planetary nebulae, all of which exhibit the line-spectrum indicative
of gaseity. The brightest of the three lines seen in the spectrum of the
ring-nebula in Lyra presents a rather peculiar appearance, "since it
consists," says Mr. Huggins, "of two bright dots, corresponding to
sections of the ring, and between these there is not darkness, but an
excessively faint line joining them. This observation makes it probable
that the faint nebulous matter occupying the central portion is similar
in constitution to that of the ring."
The constellation Hercules also contains many very interesting objects.
Let us first inspect a nebula presenting a remarkable contrast with that
just described. I refer to the nebula 13 M, known as Halley's nebula
(Plate 3). This nebula is visible to the naked eye, and in a good
telescope it is a most wonderful object: "perhaps no one ever saw it for
the first time without uttering a shout of wonder." It requires a very
powerful telescope completely to resolve this fine nebula, but the
outlying streamers may be resolved with a good 3-inch telescope. Sir W.
Herschel considered that the number of the stars composing this
wonderful object was at least 14,000. The accepted views respecting
nebulae would place this and other clusters far beyond the limits of our
sidereal system, and would make the component stars not very unequal (on
the average) to our own sun. It seems to me far more probable, on the
contrary, that the cluster belongs to our own system, and that its
components are very much smaller than the average of separate stars.
Perhaps the whole mass of the cluster does not exceed that of an average
first-magnitude star.
The nebulae 92 M and 50 H may be found, after a little searching, from
the positions indicated in the map. They are both well worthy of study,
the former being a very bright globular cluster, the latter a bright and
large round nebula. The spectra of these, as of the great cluster,
resemble the solar spectrum, being continuous, though, of course, very
much fainter.
The star [delta] Herculis (seen at the bottom of the map) is a wide and
easy double--a beautiful object. The components, situated as shown in
Plate 3, are of the fourth and eighth magnitude, and coloured
respectively greenish-white and grape-red.
The star [kappa] Herculis is not shown in the map, but may be very
readily found, lying between the two gammas, [gamma] Herculis and
[gamma] Serpentis (_see_ Frontispiece, Map 2), rather nearer the latter.
It is a wide double, the components of fifth and seventh magnitude, the
larger yellowish-white, the smaller ruddy yellow.[5]
Ras Algethi, or [alpha] Herculis, is also beyond the limits of the map,
but may be easily found by means of Map 2, Frontispiece. It is, properly
speaking, a multiple star. Considered as a double, the arrangement of
the components is that shown in Plate 3. The larger is of magnitude
3-1/2, the smaller of magnitude 5-1/2; the former orange, the latter
emerald. The companion stars are small, and require a good telescope to
be well seen. Ras Algethi is a variable, changing from magnitude 3 to
magnitude 3-1/2 in a period of 66-1/3 days.
The star [rho] Herculis is a closer double. The components are 3".7
apart, and situated as shown in Plate 3. The larger is of magnitude 4,
the smaller 5-1/2; the former bluish-white, the latter pale emerald.
There are other objects within the range of our map which are well
worthy of study. Such are [mu] Draconis, a beautiful miniature of
Castor; [gamma]^{1} and [gamma]^{2} Draconis, a wide double, the
distance between the components being nearly 62" (both grey); and
[gamma]^{1} and [gamma]^{2} Coronae, a naked-eye double, the components
being 6' apart, and each double with a good 3-inch telescope.
We turn, however, to another region of the sky. Low down, towards the
south is seen the small constellation Corvus, recognised by its
irregular quadrilateral of stars. Of the two upper stars, the left-hand
one is Algorab, a wide double, the components placed as in Plate 3,
23".5 apart, the larger of magnitude 3, the smaller 8-1/2, the colours
pale yellow and purple.
There is a red star in this neighbourhood which is well worth looking
for. To the right of Corvus is the constellation Crater, easily
recognised as forming a tolerably well-marked small group. The star
Alkes, or [alpha] Crateris, must first be found. It is far from being
the brightest star in the constellation, and may be assumed to have
diminished considerably in brilliancy since it was entitled [alpha] by
Bayer. It will easily be found, however, by means of the observer's star
maps. If now the telescope be directed to Alkes, there will be found,
following him at a distance of 42.5 s, and about one minute southerly, a
small red star, R. Crateris. Like most red stars, this one is a
variable. A somewhat smaller blue star may be seen in the same field.
There is another red star which may be found pretty easily at this
season. First find the stars [eta] and [omicron] Leonis, the former
forming with Regulus (now lying towards the south-west, and almost
exactly midway between the zenith and the horizon) the handle of the
Sickle in Leo, the other farther off from Regulus towards the right, but
lower down. Now sweep from [omicron] towards [eta] with a low power.[6]
There will be found a sixth-magnitude star about one-fourth of the way
from [omicron] to [eta]. South, following this, will be found a group of
four stars, of which one is crimson. This is the star R Leonis. Like R
Crateris and R Leporis it is variable.
Next, let the observer turn towards the south again. Above Corvus, in
the position shown in the Frontispiece, there are to be seen five stars,
forming a sort of wide V with somewhat bowed legs. At the angle is the
star [gamma] Virginis, a noted double. In 1756 the components were 6-1/2
seconds apart. They gradually approached till, in 1836, they could not
be separated by the largest telescopes. Since then they have been
separating, and they are now 4-1/2 seconds apart, situated as shown in
Plate 3. They are nearly equal in magnitude (4), and both pale yellow.
The star [gamma] Leonis is a closer and more beautiful double. It will
be found above Regulus, and is the brightest star on the blade of the
Sickle. The components are separated by about 3-1/5 seconds, the larger
of the second, the smaller of the fourth magnitude; the former
yellow-orange, the latter greenish-yellow.
Lastly, the star [iota] Leonis may be tried. It will be a pretty severe
test for our observer's telescope, the components being only 2".4 apart,
and the smaller scarcely exceeding the eighth magnitude. The brighter
(fourth magnitude) is pale yellow, the other light blue.
CHAPTER IV.
A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC.
We now commence a series of observations suited to the third quarter of
the year, and to the following hours:--Ten o'clock on the 22nd of July;
nine on the 8th of August; eight on the 23rd of August; seven on the 8th
of October; and intermediate hours on days intermediate to these.
We look first for the Great Bear towards the north-west, and thence find
the Pole-star. Turning towards the north we see Capella and [beta]
Aurigae low down and slightly towards the left of the exact north point.
The Milky Way crosses the horizon towards the north-north-east and
passes to the opposite point of the compass, attaining its highest point
above the horizon towards east-south-east. This part of the Milky Way is
well worth observing, being marked by singular variations of brilliancy.
Near Arided (the principal star of Cygnus, and now lying due east--some
twenty-five degrees from the zenith) there is seen a straight dark rift,
and near this space is another larger cavity, which has been termed the
northern Coal-sack. The space between [gamma], [delta], and [beta] Cygni
is covered by a large oval mass, exceedingly rich and brilliant. The
neighbouring branch, extending from [epsilon] Cygni, is far less
conspicuous here, but near Sagitta becomes brighter than the other,
which in this neighbourhood suddenly loses its brilliancy and fading
gradually beyond this point becomes invisible near [beta] Ophiuchi.
The continuous stream becomes patchy--in parts very brilliant--where it
crosses Aquila and Clypeus. In this neighbourhood the other stream
reappears, passing over a region very rich in stars. We see now the
greatest extent of the Milky Way, towards this part of its length, ever
visible in our latitudes--just as in spring we see its greatest extent
towards Monoceros and Argo.
[Illustration: PLATE IV.]
I may note here in passing that Sir John Herschel's delineation of the
northern portion of the Milky Way, though a great improvement on the
views given in former works, seems to require revision, and especially
as respects the very remarkable patches and streaks which characterise
the portion extending over Cepheus and Cygnus. It seems to me, also,
that the evidence on which it has been urged that the stars composing
the Milky Way are (on an average) comparable in magnitude to our own
sun, or to stars of the leading magnitudes, is imperfect. I believe, for
instance, that the brilliant oval of milky light in Cygnus comes from
stars intimately associated with the leading stars in that
constellation, and not far removed in space (proportionately) beyond
them. Of course, if this be the case, the stars, whose combined light
forms the patch of milky light, must be far smaller than the leading
brilliants of Cygnus. However, this is not the place to enter on
speculations of this sort; I return therefore to the business we have
more immediately in hand.
Towards the east is the square of Pegasus low down towards the horizon.
Towards the south is Scorpio, distinguished by the red and brilliant
Antares, and by a train of conspicuous stars. Towards the west is
Bootes, his leading brilliant--the ruddy Arcturus--lying somewhat nearer
the horizon than the zenith, and slightly south of west. Bootes as a
constellation is easily found if we remember that he is delineated as
chasing away the Greater Bear. Thus at present he is seen in a slightly
inclined position, his head (marked by the third-magnitude star [beta])
lying due west, some thirty degrees from the zenith. It has always
appeared to me, by the way, that Bootes originally had nobler
proportions than astronomers now assign to him. It is known that Canes
Venatici now occupy the place of an upraised arm of Bootes, and I
imagine that Corona Borealis, though undoubtedly a very ancient
constellation, occupies the place of his other arm. Giving to the
constellation the extent thus implied, it exhibits (better than most
constellations) the character assigned to it. One can readily picture to
oneself the figure of a Herdsman with upraised arms driving Ursa Major
before him. This view is confirmed, I think, by the fact that the Arabs
called this constellation the Vociferator.
Bootes contains many beautiful objects. Partly on this account, and
partly because this is a constellation with which the observer should be
specially familiar, a map of it is given in Plate 4.
Arcturus has a distant pale lilac companion, and is in other respects a
remarkable and interesting object. It is of a ruddy yellow colour.
Schmidt, indeed, considers that the star has changed colour of late
years, and that whereas it was once very red it is now a yellow star.
This opinion does not seem well grounded, however. The star _may_ have
been more ruddy once than now, though no other observer has noticed such
a peculiarity; but it is certainly not a pure yellow star at present (at
any rate as seen in our latitude). Owing probably to the difference of
colour between Vega, Capella and Arcturus, photometricians have not been
perfectly agreed as to the relative brilliancy of these objects. Some
consider Vega the most brilliant star in the northern heavens, while
others assign the superiority to Capella. The majority, however,
consider Arcturus the leading northern brilliant, and in the whole
heavens place three only before him, viz., Sirius, Canopus, and [alpha]
Centauri. Arcturus is remarkable in other respects. His proper motion is
very considerable, so great in fact that since the time of Ptolemy the
southerly motion (alone) of Arcturus has carried him over a space nearly
half as great again as the moon's apparent diameter. One might expect
that so brilliant a star, apparently travelling at a rate so great
compared with the average proper motions of the stars, must be
comparatively near to us. This, however, has not been found to be the
case. Arcturus is, indeed, one of the stars whose distance it has been
found possible to estimate roughly. But he is found to be some three
times as far from us as the small star 61 Cygni, and more than seven
times as far from us as [alpha] Centauri.
The star [delta] Bootis is a wide and unequal double, the smaller
component being only of the ninth magnitude.
Above Alkaid the last star in the tail of the Greater Bear, there will
be noticed three small stars. These are [theta], [iota], and [kappa]
Bootis, and are usually placed in star-maps near the upraised hand of
the Herdsman. The two which lie next to Alkaid, [iota] and [kappa], are
interesting doubles. The former is a wide double (see Plate 5), the
magnitudes of components 4 and 8, their colours yellow and white. The
larger star of this pair is itself double. The star [kappa] Bootis is
not so wide a double (see Plate 5), the magnitudes of the components 5
and 8, their colours white and faint blue--a beautiful object.
The star [xi] Bootis is an exceedingly interesting object. It is
double, the colours of the components being orange-yellow and ruddy
purple, their magnitudes 3-1/2 and 6-1/2. When this star was first
observed by Herschel in 1780 the position of the components was quite
different from that presented in Plate 5. They were also much closer,
being separated by a distance of less than 3-1/2 seconds. Since that
time the smaller component has traversed nearly a full quadrant, its
distance from its primary first increasing, till in 1831 the stars were
nearly 7-1/2 seconds apart, and thence slowly diminishing, so that at
present the stars are less than 5 seconds apart. The period usually
assigned to the revolution of this binary system is 117 years, and the
period of peri-astral passage is said to be 1779. It appears to me,
however, that the period should be about 108 years, the epoch of last
peri-astral passage 1777 and of next peri-astral passage, therefore,
1885. The angular motion of the secondary round the primary is now
rapidly increasing, and the distance between the components is rapidly
diminishing, so that in a few years a powerful telescope will be
required to separate the pair.
Not far from [xi] is [pi] Bootis, represented in Plate 5 as a somewhat
closer double, but in reality--now at any rate--a slightly wider pair,
since the distance between the components of [xi] has greatly diminished
of late. Both the components of [pi] are white, and their magnitudes are
3-1/2 and 6.
We shall next turn to an exceedingly beautiful and delicate object, the
double star [epsilon] Bootis, known also as Mirac and, on account of its
extreme beauty, called Pulcherrima by Admiral Smyth. The components of
this beautiful double are of the third and seventh magnitude, the
primary orange, the secondary sea-green. The distance separating the
components is about 3 seconds, perhaps more; it appears to have been
slowly increasing during the past ten or twelve years. Smyth assigns to
this system a period of revolution of 980 years, but there can be little
doubt that the true period is largely in excess of this estimate.
Observers in southern latitudes consider that the colours of the
components are yellow and blue, not orange and green as most of our
northern observers have estimated them.
A little beyond the lower left-hand corner of the map is the star
[delta] Serpentis, in the position shown in the Frontispiece, Map 3.
This is the star which at the hour and season depicted in Map 2 formed
the uppermost of a vertical row of stars, which has now assumed an
almost horizontal position. The components of [delta] Serpentis are
about 3-1/2 seconds apart, their magnitudes 3 and 5, both white.
The stars [theta]^{1} and [theta]^{2} Serpentis form a wide double, the
distance between the components being 21-1/2 seconds. They are nearly
equal in magnitude, the primary being 4-1/2, the secondary 5. Both are
yellow, the primary being of a paler yellow colour than the smaller
star. But the observer may not know where to look for [theta] Serpentis,
since it falls in a part of the constellation quite separated from that
part in which [delta] Serpentis lies. In fact [theta] lies on the
extreme easterly verge of the eastern half of the constellation. It is
to be looked for at about the same elevation as the brilliant Altair,
and (as to azimuth) about midway between Altair and the south.
The stars [alpha]^{1} and [alpha]^{2} Librae form a wide double, perhaps
just separable by the naked eye in very favourable weather. The larger
component is of the third, the smaller of the sixth magnitude, the
former yellow the latter light grey.
The star [beta] Librae is a beautiful light-green star to the naked eye;
in the telescope a wide double, pale emerald and light blue.
In Scorpio there are several very beautiful objects:--
The star Antares or Cor Scorpionis is one of the most beautiful of the
red stars. It has been termed the Sirius of red stars, a term better
merited perhaps by Aldebaran, save for this that, in our latitude,
Antares is, like Sirius, always seen as a brilliant "scintillator"
(because always low down), whereas Aldebaran rises high above the
horizon. Antares is a double star, its companion being a minute green
star. In southern latitudes the companion of Antares may be seen with a
good 4-inch, but in our latitudes a larger opening is wanted. Mr. Dawes
once saw the companion of Antares shining alone for seven seconds, the
primary being hidden by the moon. He found that the colour of the
secondary is not merely the effect of contrast, but that this small star
is really a green sun.
The star [beta] Scorpionis is a fine double, the components 13".1 apart,
their magnitudes 2 and 5-1/2, colours white and lilac. It has been
supposed that this pair is only an optical double, but a long time must
elapse before a decisive opinion can be pronounced on such a point.
The star [sigma] Scorpionis is a wider but much more difficult double,
the smaller component being below the 9th magnitude. The colour of the
primary (4) is white, that of the secondary maroon.
The star [xi] Scorpionis is a neat double, the components 7".2 apart,
their magnitudes 4-1/2 and 7-1/2, their colours white and grey. This
star is really triple, a fifth-magnitude star lying close to the
primary.
In Ophiuchus, a constellation covering a wide space immediately above
Scorpio, there are several fine doubles. Among others--
39 Ophiuchi, distance between components 12".1, their magnitudes 5-1/2
and 7-1/2, their colours orange and blue.
The star 70 Ophiuchi, a fourth-magnitude star on the right shoulder of
Ophiuchus, is a noted double. The distance between the components about
5-1/2", their magnitudes 4-1/2 and 7, the colours yellow and red. The
pair form a system whose period of revolution is about 95 years.
36 Ophiuchi (variable), distance 5".2, magnitudes 4-1/2 and 6-1/2,
colours red and yellow.
[rho] Opiuchi, distance 4", colours yellow and blue, magnitudes 5 and 7.
Between [alpha] and [beta] Scorpionis the fine nebula 80 M may be looked
for. (Or more closely thus:--below [beta] is the wide Double [omega]^{1}
and [omega]^{2} Scorpionis; about as far to the right of Antares is the
star [sigma] Scorpionis, and immediately above this is the
fifth-magnitude star 19.) The nebula we seek lies between 19 and
[omega], nearer to 19 (about two-fifths of the way towards [omega]).
This nebula is described by Sir W. Herschel as "the richest and most
condensed mass of stars which the firmament offers to the contemplation
of astronomers."
There are two other objects conveniently situated for observation, which
the observer may now turn to. The first is the great cluster in the
sword-hand of Perseus (see Plate 4), now lying about 28 deg. above the
horizon between N.E. and N.N.E. The stars [gamma] and [delta] Cassiopeiae
(see Map 3 of Frontispiece) point towards this cluster, which is rather
farther from [delta] than [delta] from [gamma], and a little south of
the produced line from these stars. The cluster is well seen with the
naked eye, even in nearly full moonlight. In a telescope of moderate
power this cluster is a magnificent object, and no telescope has yet
revealed its full glory. The view in Plate 5 gives but the faintest
conception of the glories of [chi] Persei. Sir W. Herschel tried in
vain to gauge the depths of this cluster with his most powerful
telescope. He spoke of the most distant parts as sending light to us
which must have started 4000 or 5000 years ago. But it appears
improbable that the cluster has in reality so enormous a longitudinal
extension compared with its transverse section as this view would imply.
On the contrary, I think we may gather from the appearance of this
cluster, that stars are far less uniform in size than has been commonly
supposed, and that the mere irresolvability of a cluster is no proof of
excessive distance. It is unlikely that the faintest component of the
cluster is farther off than the brightest (a seventh-magnitude star) in
the proportion of more than about 20 to 19, while the ordinary estimate
of star magnitudes, applied by Herschel, gave a proportion of 20 or 30
to 1 at least. I can no more believe that the components of this cluster
are stars greatly varying in distance, but accidentally seen in nearly
the same direction, (or that they form an _enormously long system_
turned by accident directly towards the earth), than I could look on the
association of several thousand persons in the form of a procession as a
fortuitous arrangement.
Next there is the great nebula in Andromeda--known as "the
transcendantly beautiful queen of the nebulae." It will not be difficult
to find this object. The stars [epsilon] and [delta] Cassiopeiae (Map 3,
Frontispiece) point to the star [beta] Andromedae. Almost in a vertical
line above this star are two fourth-magnitude stars [mu] and [gamma],
and close above [nu], a little to the right, is the object we
seek--visible to the naked eye as a faint misty spot. To tell the truth,
the transcendantly beautiful queen of the nebulae is rather a
disappointing object in an ordinary telescope. There is seen a long
oval or lenticular spot of light, very bright near the centre,
especially with low powers. But there is a want of the interest
attaching to the strange figure of the Great Orion nebula. The Andromeda
nebula has been partially resolved by Lord Rosse's great reflector, and
(it is said) more satisfactorily by the great refractor of Harvard
College. In the spectroscope, Mr. Huggins informs us, the spectrum is
peculiar. Continuous from the blue to the orange, the light there
"appears to cease very abruptly;" there is no indication of gaseity.
Lastly, the observer may turn to the pair Mizar and Alcor, the former
the middle star in the Great Bear's tail, the latter 15' off. It seems
quite clear, by the way, that Alcor has increased in brilliancy of late,
since among the Arabians it was considered an evidence of very good
eyesight to detect Alcor, whereas this star may now be easily seen even
in nearly full moonlight. Mizar is a double star, and a fourth star is
seen in the same field of view with the others (see Plate 5). The
distance between Mizar and its companion is 14".4; the magnitude of
Mizar 3, of the companion 5; their colours white and pale green,
respectively.
CHAPTER V.
A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC.
Our last half-hour with the double stars, &c., must be a short one, as
we have already nearly filled the space allotted to these objects. The
observations now to be made are supposed to take place during the fourth
quarter of the year,--at ten o'clock on October 23rd; or at nine on
November 7th; or at eight on November 22nd; or at seven on December 6th;
or at hours intermediate to these on intermediate days.
We look first, as in former cases, for the Great Bear, now lying low
down towards the north. Towards the north-east, a few degrees easterly,
are the twin-stars Castor and Pollux, in a vertical position, Castor
uppermost. Above these, a little towards the right, we see the brilliant
Capella; and between Capella and the zenith is seen the festoon of
Perseus. Cassiopeia lies near the zenith, towards the north, and the
Milky Way extends from the eastern horizon across the zenith to the
western horizon. Low down in the east is Orion, half risen above
horizon. Turning to the south, we see high up above the horizon the
square of Pegasus. Low down towards the south-south-west is Fomalhaut,
pointed to by [beta] and [alpha] Pegasi. Towards the west, about
half-way between the zenith and the horizon, is the noble cross in
Cygnus; below which, towards the left, we see Altair, and his companions
[beta] and [gamma] Aquilae: while towards the right we see the brilliant
Vega.
During this half-hour we shall not confine ourselves to any particular
region of the heavens, but sweep the most conveniently situated
constellations.
[Illustration: PLATE V.]
First, however, we should recommend the observer to try and get a good
view of the great nebula in Andromeda, which is _not_ conveniently
situated for observation, but is so high that after a little trouble the
observer may expect a more distinct view than in the previous quarter.
He will see [beta] Andromedae towards the south-east, about 18 deg. from the
zenith, [mu] and [nu] nearly in a line towards the zenith, and the
nebula about half-way between [beta] and the zenith.
With a similar object it will be well to take another view of the great
cluster in Perseus, about 18 deg. from the zenith towards the
east-north-east (_see_ the pointers [gamma] and [delta] Cassiopeiae in
Map 4, Frontispiece), the cluster being between [delta] Cassiopeiae and
[alpha] Persei.
Not very far off is the wonderful variable Algol, now due east, and
about 58 deg. above the horizon. The variability of this celebrated object
was doubtless discovered in very ancient times, since the name Al-gol,
or "the Demon" seems to point to a knowledge of the peculiarity of this
"slowly winking eye." To Goodricke, however, is due the rediscovery of
Algol's variability. The period of variation is 2d. 20h. 48m.; during
2h. 14m. Algol appears of the second magnitude; the remaining 6-3/4
hours are occupied by the gradual decline of the star to the fourth
magnitude, and its equally gradual return to the second. It will be
found easy to watch the variations of this singular object, though, of
course, many of the minima are attained in the daytime. The following
may help the observer:--
On October 8th, 1867, at about half-past eleven in the evening, I
noticed that Algol had reached its minimum of brilliancy. Hence the next
minimum was attained at about a quarter-past eight on the evening of
October 11th; the next at about five on the evening of October 14th,
and so on. Now, if this process be carried on, it will be found that the
next evening minimum occurred at about 10h. (_circiter_) on the evening
of October 31st, the next at about 11h. 30m. on the evening of November
20th. Thus at whatever hour any minimum occurs, another occurs _six
weeks and a day later_, at about the same hour. This would be exact
enough if the period of variation were _exactly_ 2d. 20m. 48s., but the
period is nearly a minute greater, and as there are fifteen periods in
six weeks and a day, it results that there is a difference of about 13m.
in the time at which the successive recurrences at nearly the same hour
take place. Hence we are able to draw up the two following Tables, which
will suffice to give all the minima conveniently observable during the
next two years. Starting from a minimum at about 11h. 45m. on November
20th, 1867, and noticing that the next 43-day period (with the 13m.
added) gives us an observation at midnight on January 2nd, 1868, and
that successive periods would make the hour later yet, we take the
minimum next after that of January 2nd, viz. that of January 5th, 1868,
8h. 48m., and taking 43-day periods (with 13m. added to each), we get
the series--
h. m.
Jan. 5, 1868, 8 45 P.M.
Feb. 17, ----, 8 58 ----
Mar. 31, ----, 9 11 ----
May 13, ----, 9 24 ----
June 25, ----, 9 37 ----
Aug. 7, ----, 9 50 ----
Sept. 19, ----, 10 3 ----
Nov. 1 ----, 10 16 ----
Dec. 14, ----, 10 29 ----
Jan. 26, 1869, 10 42 ----
Mar. 10, ----, 10 25 ----
Mar. 13, ----, 7 43 ----[7]
Apr. 25, ----, 7 56 ----
June 7, ----, 8 9 ----
July 20, ----, 8 22 ----
Sept. 1, ----, 8 35 ----
Oct. 14, ----, 8 48 ----
Nov. 26, ----, 9 1 ----
Jan. 8, 1870, 9 14 ----
Feb. 20, ----, 9 27 ----
From the minimum at about 10 P.M. on October 31st, 1867, we get in like
manner the series--
h. m.
Dec. 13, 1867, 10 13 P.M.
Jan. 25, 1868, 10 26 ----
Mar. 8, ----, 10 39 ----
Apr. 20, ----, 10 52 ----
June 2, ----, 11 5 ----
June 5, ----, 7 53 ----[8]
July 18, ----, 8 6 ----
Aug. 30, ----, 8 19 ----
Oct. 12, ----, 8 32 ----
Nov. 24, ----, 8 45 ----
Jan. 6, 1869, 8 58 ----
Feb. 18, ----, 9 11 ----
Apr. 2, ----, 9 24 ----
May 15, ----, 9 37 ----
June 27, ----, 9 50 ----
Aug. 9, ----, 10 3 ----
Sept. 21, ----, 10 16 ----
Nov. 3, ----, 10 29 ----
Dec. 16, ----, 10 42 ----
Jan. 28, 1870, 10 55 ----
From one or other of these tables every observable minimum can be
obtained. Thus, suppose the observer wants to look for a minimum during
the last fortnight in August, 1868. The first table gives him no
information, the latter gives him a minimum at 8h. 19m. P.M. on August
30; hence of course there is a minimum at 11h. 31m. P.M. on August 27;
and there are no other conveniently observable minima during the
fortnight in question.
The cause of the remarkable variation in this star's brilliancy has been
assigned by some astronomers to the presence of an opaque secondary,
which transits Algol at regular intervals; others have adopted the view
that Algol is a luminous secondary, revolving around an opaque primary.
Of these views the former seems the most natural and satisfactory. It
points to a secondary whose mass bears a far greater proportion to that
of the primary, than the mass even of Jupiter bears to the sun; the
shortness of the period is also remarkable. It may be noticed that
observation points to a gradual diminution in the period of Algol's
variation, and the diminution seems to be proceeding more and more
rapidly. Hence (assuming the existence of a dark secondary) we must
suppose that either it travels in a resisting medium which is gradually
destroying its motion, or that there are other dependent orbs whose
attractions affect the period of this secondary. In the latter case the
decrease in the period will attain a limit and be followed by an
increase.
However, interesting as the subject may be, it is a digression from
telescopic work, to which we now return.
Within the confines of the second map in Plate 4 is seen the fine star
[gamma] Andromedae. At the hour of our observations it lies high up
towards E.S.E. It is seen as a double star with very moderate telescopic
power, the distance between the components being upwards of 10"; their
magnitudes 3 and 5-1/2, their colours orange and green. Perhaps there is
no more interesting double visible with low powers. The smaller star is
again double in first-class telescopes, the components being yellow and
blue according to some observers, but according to others, both green.
Below [gamma] Andromedae lie the stars [beta] and [gamma] Triangulorum,
[gamma] a fine naked-eye triple (the companions being [delta] and [eta]
Triangulorum), a fine object with a very low power. To the right is
[alpha] Triangulorum, certainly less brilliant than [beta]. Below
[alpha] are the three stars [alpha], [beta], and [gamma] Arietis, the
first an unequal and difficult double, the companion being purple, and
only just visible (under favourable circumstances) with a good 3-inch
telescope; the last an easy double, interesting as being the first ever
discovered (by Hook, in 1664), the colours of components white and grey.
Immediately below [alpha] Arietis is the star [alpha] Ceti, towards the
right of which (a little lower) is Mira, a wonderful variable. This star
has a period of 331-1/3 days; during a fortnight it appears as a star of
the 2nd magnitude,--on each side of this fortnight there is a period of
three months during one of which the star is increasing, while during
the other it is diminishing in brightness: during the remaining five
months of the period the star is invisible to the naked eye. There are
many peculiarities and changes in the variation of this star, into which
space will not permit me to enter.
Immediately above Mira is the star [alpha] Piscium at the knot of the
Fishes' connecting band. This is a fine double, the distance between the
components being about 3-1/2", their magnitudes 5 and 6, their colours
pale green and blue (see Plate 5).
Close to [gamma] Aquarii (see Frontispiece, Map 4), above and to the
left of it, is the interesting double [zeta] Aquarii; the distance
between the components is about 3-1/2", their magnitudes 4 and 4-1/2,
both whitish yellow. The period of this binary seems to be about 750
years.
Turning next towards the south-west we see the second-magnitude star
[epsilon] Pegasi, some 40 deg. above the horizon. This star is a wide but
not easy double, the secondary being only of the ninth magnitude; its
colour is lilac, that of the primary being yellow.
Towards the right of [epsilon] Pegasi and lower down are seen the three
fourth-magnitude stars which mark the constellation Equuleus. Of these
the lowest is [alpha], to the right of which lies [epsilon] Equulei, a
fifth-magnitude star, really triple, but seen as a double star with
ordinary telescopes (Plate 5). The distance between the components is
nearly 11", their colours white and blue, their magnitudes 5-1/2 and
7-1/2. The primary is a very close double, which appears, however, to be
opening out rather rapidly.
Immediately below Equuleus are the stars [alpha]^{1} and [alpha]^2
Capricorni, seen as a naked-eye double to the right of and above [beta].
Both [alpha]^1 and [alpha]^2 are yellow; [alpha]^2 is of the 3rd,
[alpha]^1 of the 4th magnitude; in a good telescope five stars are seen,
the other three being blue, ash-coloured, and lilac. The star [beta]
Capricorni is also a wide double, the components yellow and blue, with
many telescopic companions.
To the right of Equuleus, towards the west-south-west is the
constellation Delphinus. The upper left-hand star of the rhombus of
stars forming the head of the Delphinus is the star [gamma] Delphini, a
rather easy double (see Plate 5), the components being nearly 12" apart,
their magnitudes 4 and 7, their colours golden yellow and flushed grey.
Turn we next to the charming double Albireo, on the beak of Cygnus,
about 36 deg. above the horizon towards the west. The components are 34-1/2"
apart, their magnitudes 3 and 6, their colours orange-yellow, and blue.
It has been supposed (perhaps on insufficient evidence) that this star
is merely an optical double. It must always be remembered that a certain
proportion of stars (amongst those separated by so considerable a
distance) _must_ be optically combined only.
The star [chi] Cygni is a wide double (variable) star. The components
are separated by nearly 26", their magnitudes 5 and 9, their colours
yellow and light blue. [chi] may be found by noticing that there is a
cluster of small stars in the middle of the triangle formed by the stars
[gamma], [delta], and [beta] Cygni (see Map 4, Frontispiece), and that
[chi] is the nearest star _of the cluster_ to [beta]. The star [phi]
Cygni, which is just above and very close to [beta] (Albireo), does not
belong to the cluster. [chi] is about half as far again from [phi] as
[phi] from Albireo. But as [chi] descends to the 11th magnitude at its
minimum the observer must not always expect to find it very easily. It
has been known to be invisible at the epoch when it should have been
most conspicuous. The period of this variable is 406 days.
The star 61 Cygni is an interesting one. So far as observation has yet
extended, it would seem to be the nearest to us of all stars visible in
the northern hemisphere. It is a fine double, the components nearly
equal (5-1/2 and 6), both yellow, and nearly 19" apart. The period of
this binary appears to be about 540 years. To find 61 Cygni note that
[epsilon] and [delta] Cygni form the diameter of a semicircle divided
into two quadrants by [alpha] Cygni (Arided). On this semicircle, on
either side of [alpha], lie the stars [nu] and [alpha] Cygni, [nu]
towards [epsilon]. Now a line from [alpha] to [nu] produced passes very
near to 61 Cygni at a distance from [nu] somewhat greater than half the
distance of [nu] from [alpha].
The star [mu] Cygni lies in a corner of the constellation, rather
farther from [zeta] than [zeta] from [epsilon] Cygni. A line from
[epsilon] to [zeta] produced meets [kappa] Pegasi, a fourth-magnitude
star; and [mu] Cygni, a fifth-magnitude star, lies close above [kappa]
Pegasi. The distance between the components is about 5-1/2", their
magnitudes 5 and 6, their colours white and pale blue.
The star [psi] Cygni may next be looked for, but for this a good map of
Cygnus will be wanted, as [psi] is not pointed to by any well-marked
stars. A line from [alpha], parallel to the line joining [gamma] and
[delta], and about one-third longer than that line, would about mark the
position of [psi] Cygni. The distance between the components of this
double is about 3-1/2", their magnitudes 5-1/2 and 8, their colours
white and lilac.
Lastly, the observer may turn to the stars [gamma]_{1} and [gamma]_{2}
Draconis towards the north-west about 40 deg. above the horizon (they are
included in the second map of Plate 2). They form a wide double, having
equal (fifth-magnitude) components, both grey. (See Plate 5.)
CHAPTER VI.
HALF-HOURS WITH THE PLANETS.
In observing the stars, we can select a part of the heavens which may be
conveniently observed; and in this way in the course of a year we can
observe every part of the heavens visible in our northern hemisphere.
But with the planets the case is not quite so simple. They come into
view at no fixed season of the year: some of them can never be seen _by
night_ on the meridian; and they all shift their place among the stars,
so that we require some method of determining where to look for them on
any particular night, and of recognising them from neighbouring fixed
stars.
The regular observer will of course make use of the 'Nautical Almanac';
but 'Dietrichsen and Hannay's Almanac' will serve every purpose of the
amateur telescopist. I will briefly describe those parts of the almanac
which are useful to the observer.
It will be found that three pages are assigned to each month, each page
giving different information. If we call these pages I. II. III., then
in order that page I. for each month may fall to the left of the open
double page, and also that I. and II. may be open together, the pages
are arranged in the following order: I. II. III.; III. I. II.; I. II.
III.; and so on.
Now page III. for any month does not concern the amateur observer. It
gives information concerning the moon's motions, which is valuable to
the sailor, and interesting to the student of astronomy, but not
applicable to amateur observation.
[Illustration: PLATE VI.]
We have then only pages I. and II. to consider:--
Across the top of both pages the right ascension and declination of the
planets Venus, Jupiter, Mars, Saturn, Mercury, and Uranus are given,
accompanied by those of two conspicuous stars. This information is very
valuable to the telescopist. In the first place, as we shall presently
see, it shows him what planets are well situated for observation, and
secondly it enables him to map down the path of any planet from day to
day among the fixed stars. This is a very useful exercise, by the way,
and also a very instructive one. The student may either make use of the
regular maps and mark down the planet's path in pencil, taking a light
curve through the points given by the data in his almanac, or he may lay
down a set of meridians suited to the part of the heavens traversed by
the planet, and then proceed to mark in the planet's path and the stars,
taking the latter either from his maps or from a convenient list of
stars.[9] My 'Handbook of the Stars' has been constructed to aid the
student in these processes. It must be noticed that old maps are not
suited for the work, because, through precession, the stars are all out
of place as respects R.A. and Dec. Even the Society's maps, constructed
so as to be right for 1830, are beginning to be out of date. But a
matter of 20 or 30 years either way is not important.[10] My Maps,
Handbook and Zodiac-chart have been constructed for the year 1880, so as
to be serviceable for the next fifty years or so.
Next, below the table of the planets, we have a set of vertical
columns. These are, in order, the days of the month, the calendar--in
which are included some astronomical notices, amongst others the
diameter of Saturn on different dates, the hours at which the sun rises
and sets, the sun's right ascension, declination, diameter, and
longitude; then eight columns which do not concern the observer; after
which come the hours at which the moon rises and sets, the moon's age;
and lastly (so far as the observer is concerned) an important column
about Jupiter's system of satellites.
Next, we have, at the foot of the first page, the hours at which the
planets rise, south, and set; and at the foot of the second page we have
the dates of conjunctions, oppositions, and of other phenomena, the
diameters of Venus, Jupiter, Mars, and Mercury, and finally a few words
respecting the visibility of these four planets.
After the thirty-six pages assigned to the months follow four (pp.
42-46) in which much important astronomical information is contained;
but the points which most concern our observer are (i.) a small table
showing the appearance of Saturn's rings, and (ii.) a table giving the
hours at which Jupiter's satellites are occulted or eclipsed, re-appear,
&c.
We will now take the planets in the order of their distance from the
sun: we shall see that the information given by the almanac is very
important to the observer.
Mercury is so close to the sun as to be rarely seen with the naked eye,
since he never sets much more than two hours and a few minutes after the
sun, or rises by more than that interval before the sun. It must not be
supposed that at each successive epoch of most favourable appearance
Mercury sets so long after the sun or rises so long before him. It would
occupy too much of our space to enter into the circumstances which
affect the length of these intervals. The question, in fact, is not a
very simple one. All the necessary information is given in the almanac.
We merely notice that the planet is most favourably seen as an evening
star in spring, and as a morning star in autumn.[11]
The observer with an equatorial has of course no difficulty in finding
Mercury, since he can at once direct his telescope to the proper point
of the heavens. But the observer with an alt-azimuth might fail for
years together in obtaining a sight of this interesting planet, if he
trusted to unaided naked-eye observations in looking for him. Copernicus
never saw Mercury, though he often looked for him; and Mr. Hind tells me
he has seen the planet but once with the naked eye--though this perhaps
is not a very remarkable circumstance, since the systematic worker in an
observatory seldom has occasion to observe objects with the unaided eye.
By the following method the observer can easily pick up the planet.
Across two uprights (Fig. 10) nail a straight rod, so that when looked
at from some fixed point of view the rod may correspond to the sun's
path near the time of observation. The rod should be at right-angles to
the line of sight to its centre. Fasten another rod at right angles to
the first. From the point at which the rods cross measure off and mark
on both rods spaces each subtending a degree as seen from the point of
view. Thus, if the point of view is 9-1/2 feet off, these spaces must
each be 2 inches long, and they must be proportionately less or greater
as the eye is nearer or farther.
[Illustration: _Fig. 10._]
Now suppose the observer wishes to view Mercury on some day, whereon
Mercury is an evening star. Take, for instance, June 9th, 1868. We find
from 'Dietrichsen' that on this day (at noon) Mercury's R.A. is 6h. 53m.
23s.: and the sun's 5h. 11m. 31s. We need not trouble ourselves about
the odd hours after noon, and thus we have Mercury's R.A. greater than
the sun's by 1h. 41m. 52s. Now we will suppose that the observer has so
fixed his uprights and the two rods, that the sun, seen from the fixed
point of view, appears to pass the point of crossing of the rods at
half-past seven, then Mercury will pass the cross-rod at 11m. 52s. past
nine. But where? To learn this we must take out Mercury's declination,
which is 24 deg. 43' 18" N., and the sun's, which is 22 deg. 59' 10" N. The
difference, 1 deg. 44' 8" N. gives us Mercury's place, which it appears is
rather less than 1-3/4 degree north of the sun. Thus, about 1h. 42m.
after the sun has passed the cross-rod, Mercury will pass it between the
first and second divisions above the point of fastening. The sun will
have set about an hour, and Mercury will be easily found when the
telescope is directed towards the place indicated.
It will be noticed that this method does not require the time to be
exactly known. All we have to do is to note the moment at which the sun
passes the point of fastening of the two rods, and to take our 1h. 42m.
from that moment.
This method, it may be noticed in passing, may be applied to give
naked-eye observations of Mercury at proper seasons (given in the
almanac). By a little ingenuity it may be applied as well to morning as
to evening observations, the sun's passage of the cross-rod being taken
on one morning and Mercury's on the next, so many minutes _before_ the
hour of the first observation. In this way several views of Mercury may
be obtained during the year.
Such methods may appear very insignificant to the systematic observer
with the equatorial, but that they are effective I can assert from my
own experience. Similar methods may be applied to determine from the
position of a known object, that of any neighbouring unknown object even
at night. The cross-rod must be shifted (or else two cross-rods used)
when the unknown _precedes_ the known object. If two cross-rods are
used, account must be taken of the gradual diminution in the length of a
degree of right ascension as we leave the equator.
Even simpler methods carefully applied may serve to give a view of
Mercury. To show this, I may describe how I obtained my first view of
this planet. On June 1st, 1863, I noticed, that at five minutes past
seven the sun, as seen from my study window, appeared from behind the
gable-end of Mr. St. Aubyn's house at Stoke, Devon. I estimated the
effect of Mercury's northerly declination (different of course for a
vertical wall, than for the cross-rod in fig. 8, which, in fact, agrees
with a declination-circle), and found that he would pass out opposite a
particular point of the wall a certain time after the sun. I then turned
the telescope towards that point, and focussed for distinct vision of
distant objects, so that the outline of the house was seen out of focus.
As the calculated time of apparition approached, I moved the telescope
up and down so that the field swept the neighbourhood of the estimated
point of apparition. I need hardly say that Mercury did not appear
exactly at the assigned point, nor did I see him make his first
appearance; but I picked him up so soon after emergence that the outline
of the house was in the field of view with him. He appeared as a
half-disc. I followed him with the telescope until the sun had set, and
soon after I was able to see him very distinctly with the naked eye. He
shone with a peculiar brilliance on the still bright sky; but although
perfectly distinct to the view when his place was indicated, he escaped
detection by the undirected eye.[12]
Mercury does not present any features of great interest in ordinary
telescopes; though he usually appears better defined than Venus, at
least as the latter is seen on a dark sky. The phases are pleasingly
seen (as shown in Plate 6) with a telescope of moderate power. For their
proper observation, however, the planet must be looked for with the
telescope in the manner above indicated, as he always shows a nearly
semi-circular disc when he is visible to the naked eye.
We come next to Venus, the most splendid of all the planets to the eye.
In the telescope Venus disappoints the observer, however. Her intense
lustre brings out every defect of the instrument, and especially the
chromatic aberration. A dark glass often improves the view, but not
always. Besides, an interposed glass has an unpleasant effect on the
field of view.
Perhaps the best method of observing Venus is to search for her when she
is still high above the horizon, and when therefore the background of
the sky is bright enough to take off the planet's glare. The method I
have described for the observation of Mercury will prove very useful in
the search for Venus when the sun is above the horizon or but just set.
Of course, when an object is to be looked for high above the horizon,
the two rods which support the cross-rods must not be upright, but
square to the line of view to that part of the sky.
But the observer must not expect to see much during his observation of
Venus. In fact, he can scarcely do more than note her varying phases
(see Plate 6) and the somewhat uneven boundary of the terminator. Our
leading observers have done so little with this fascinating but
disappointing planet, that amateurs must not be surprised at their own
failure.
I suppose the question whether Venus has a satellite, or at any rate
whether the object supposed to have been seen by Cassini and other old
observers were a satellite, must be considered as decided in the
negative. That Cassini should have seen an object which Dawes and Webb
have failed to see must be considered utterly improbable.
Leaving the inferior planets, we come to a series of important and
interesting objects.
First we have the planet Mars, nearly the last in the scale of planetary
magnitude, but far from being the least interesting of the planets. It
is in fact quite certain that we obtain a better view of Mars than of
any object in the heavens, save the Moon alone. He may present a less
distinguished appearance than Jupiter or Saturn, but we see his surface
on a larger scale than that of either of those giant orbs, even if we
assume that we ever obtain a fair view of their real surface.
Nor need the moderately armed observer despair of obtaining interesting
views of Mars. The telescope with which Beer and Maedler made their
celebrated series of views was only a 4-inch one, so that with a 3-inch
or even a 2-inch aperture the attentive observer may expect interesting
views. In fact, more depends on the observer than on the instrument. A
patient and attentive scrutiny will reveal features which at the first
view wholly escape notice.
In Plate 6 I have given a series of views of Mars much more distinct
than an observer may expect to obtain with moderate powers. I add a
chart of Mars, a miniature of one I have prepared from a charming
series of tracings supplied me by Mr. Dawes. The views taken by this
celebrated observer in 1852, 1856, 1860, 1862, and 1864, are far better
than any others I have seen. The views by Beer and Maedler are good, as
are some of Secchi's (though they appear badly drawn), Nasmyth's and
Phillips'; Delarue's two views are also admirable; and Lockyer has given
a better set of views than any of the others. But there is an amount of
detail in Mr. Dawes' views which renders them superior to any yet taken.
I must confess I failed at a first view to see the full value of Mr.
Dawes' tracings. Faint marks appeared, which I supposed to be merely
intended to represent shadings scarcely seen. A more careful study
shewed me that every mark is to be taken as the representative of what
Mr. Dawes actually saw. The consistency of the views is perfectly
wonderful, when compared with the vagueness and inconsistency observable
in nearly all other views. And this consistency is not shown by mere
resemblance, which might have been an effect rather of memory
(unconsciously exerted) than observation. The same feature changes so
much in figure, as it appears on different parts of the disc, that it
was sometimes only on a careful projection of different views that I
could determine what certain features near the limb represented. But
when this had been done, and the distortion through the effect of
foreshortening corrected, the feature was found to be as true in shape
as if it had been seen in the centre of the planet's disc.
In examining Mr. Dawes' drawings it was necessary that the position of
Mars' axis should be known. The data for determining this were taken
from Dr. Oudemann's determinations given in a valuable paper on Mars
issued from Mr. Bishop's observatory. But instead of calculating Mars'
presentation by the formulae there given, I found it convenient rather to
make use of geometrical constructions applied to my 'Charts of the
Terrestrial Planets.' Taking Maedler's start-point for Martial
longitudes, that is the longitude-line passing near Dawes' forked bay, I
found that my results agreed pretty fairly with those in Prof. Phillips'
map, so far as the latter went; but there are many details in my charts
not found in Prof. Phillips' nor in Maedler's earlier charts.
I have applied to the different features the names of those observers
who have studied the physical peculiarities presented by Mars. Mr.
Dawes' name naturally occurs more frequently than others. Indeed, if I
had followed the rule of giving to each feature the name of its
discoverer, Mr. Dawes' name would have occurred much more frequently
than it actually does.
On account of the eccentricity of his orbit, Mars is seen much better in
some oppositions than in others. When best seen the southern hemisphere
is brought more into view than the northern because the summer of his
northern hemisphere occurs when he is nearly in aphelion (as is the case
with the Earth by the way).
The relative dimensions and presentation of Mars, as seen in opposition
in perihelion, and in opposition in aphelion, are shown in the two rows
of figures.
In and near quadrature Mars is perceptibly gibbous. He is seen thus
about two months before or after opposition. In the former case, he
rises late and comes to the meridian six hours or so after midnight. In
the latter case, he is well seen in the evening, coming to the meridian
at six. His appearance and relative dimensions as he passes from
opposition to quadrature are shown in the last three figures of the
upper row.
Mars' polar caps may be seen with very moderate powers.
I add four sets of meridians (Plate 6), by filling in which from the
charts the observer may obtain any number of views of the planet as it
appears at different times.
Passing over the asteroids, which are not very interesting objects to
the amateur telescopist, we come to Jupiter, the giant of the solar
system, surpassing our Earth more than 1400 times in volume, and
overweighing all the planets taken together twice over.
Jupiter is one of the easiest of all objects of telescopic observation.
No one can mistake this orb when it shines on a dark sky, and only Venus
can be mistaken for it when seen as a morning or evening star. Sometimes
both are seen together on the twilight sky, and then Venus is generally
the brighter. Seen, however, at her brightest and at her greatest
elongation from the sun, her splendour scarcely exceeds that with which
Jupiter shines when high above the southern horizon at midnight.
Jupiter's satellites may be seen with very low powers; indeed the outer
ones have been seen with the naked eye, and all are visible in a good
opera-glass. Their dimensions relatively to the disc are shown in Plate
7. Their greatest elongations are compared with the disc in the
low-power view.
Jupiter's belts may also be well seen with moderate telescopic power.
The outer parts of his disc are perceptibly less bright than the centre.
More difficult of observation are the transits of the satellites and of
their shadows. Still the attentive observer can see the shadows with an
aperture of two inches, and the satellites themselves with an aperture
of three inches.
The minute at which the satellites enter on the disc, or pass off, is
given in 'Dietrichsen's Almanac.' The 'Nautical Almanac' also gives the
corresponding data for the shadows.
The eclipses of the satellites in Jupiter's shadow, and their
occultations by his disc, are also given in 'Dietrichsen's Almanac.'
In the inverting telescope the satellites move from right to left in the
nearer parts of their orbit, and therefore transit Jupiter's disc in
that direction, and from left to right in the farther parts. Also note
that _before_ opposition, (i.) the shadows travel in front of the
satellites in transiting the disc; (ii.) the satellites are eclipsed in
Jupiter's _shadow_; (iii.) they reappear from behind his _disc_. On the
other hand, _after_ opposition, (i.) the shadows travel _behind_ the
satellites in transiting the disc; (ii.) the satellites are occulted by
the _disc_; (iii.) they reappear from eclipse in Jupiter's _shadow_.
Conjunctions of the satellites are common phenomena, and may be waited
for by the observer who sees the chance. An eclipse of one satellite by
the shadow of another is not a common phenomenon; in fact, I have never
heard of such an eclipse being seen. That a satellite should be quite
extinguished by another's shadow is a phenomenon not absolutely
impossible, but which cannot happen save at long intervals.
The shadows are not _black spots_ as is erroneously stated in nearly all
popular works on astronomy. The shadow of the fourth, for instance, is
nearly all penumbra, the really black part being quite minute by
comparison. The shadow of the third has a considerable penumbra, and
even that of the first is not wholly black. These penumbras may not be
perceptible, but they affect the appearance of the shadows. For
instance, the shadow of the fourth is perceptibly larger but less black
than that of the third, though the third is the larger satellite.
In transit the first satellite moves fastest, the fourth slowest, the
others in their order. The shadow moves just as fast (appreciably) as
the satellite it belongs to. Sometimes the shadow of the satellite may
be seen to overtake (apparently) the disc of another. In such a case the
shadow does not pass over the disc, but the disc conceals the shadow.
This is explained by the fact that the shadow, if visible throughout its
length, would be a line reaching slantwise from the satellite it belongs
to, and the end of the shadow (that is, the point where it meets the
disc) is _not_ the point where the shadow crosses the orbit of any inner
satellite. Thus the latter may be interposed between the end of the
shadow--the only part of the shadow really visible--and the eye; but the
end of the shadow _cannot_ be interposed between the satellite and the
eye. If a satellite _on the disc_ were eclipsed by another satellite,
the black spot thus formed would be in another place from the black spot
on the planet's body. I mention all this because, simple as the question
may seem, I have known careful observers to make mistakes on this
subject. A shadow is seen crossing the disc and overtaking, apparently,
a satellite in transit. It seems therefore, on a first view, that the
shadow will hide the satellite, and observers have even said that they
have _seen_ this happen. But they are deceived. It is obvious that _if
one satellite eclipse another, the shadows of both must occupy the same
point on Jupiter's body_. Thus it is the overtaking of one _shadow_ by
another on the disc, and not the overtaking of a _satellite_ by a
shadow, which determines the occurrence of that as yet unrecorded
phenomenon, the eclipse of one satellite by another.[13]
The satellites when far from Jupiter seem to lie in a straight line
through his centre. But as a matter of fact they do not in general lie
in an exact straight line. If their orbits could be seen as lines of
light, they would appear, in general, as very long ellipses. The orbit
of the fourth would frequently be seen to be _quite clear_ of Jupiter's
disc, and the orbit of the third might in some very exceptional
instances pass _just_ clear of the disc. The satellites move most nearly
in a straight line (apparently) when Jupiter comes to opposition in the
beginning of February or August, and they appear to depart most from
rectilinear motion when opposition occurs in the beginning of May and
November. At these epochs the fourth satellite may be seen to pass above
and below Jupiter's disc at a distance equal to about one-sixth of the
disc's radius.
The shadows do not travel in the same apparent paths as the satellites
themselves across the disc, but (in an inverting telescope) _below_ from
August to January, and _above_ from February to July.
We come now to the most charming telescopic object in the heavens--the
planet Saturn. Inferior only to Jupiter in mass and volume, this planet
surpasses him in the magnificence of his system. Seen in a telescope of
adequate power, Saturn is an object of surpassing loveliness. He must be
an unimaginative man who can see Saturn for the first time in such a
telescope, without a feeling of awe and amazement. If there is any
object in the heavens--I except not even the Sun--calculated to impress
one with a sense of the wisdom and omnipotence of the Creator it is
this. "His fashioning hand" is indeed visible throughout space, but in
Saturn's system it is most impressively manifest.
Saturn, to be satisfactorily seen, requires a much more powerful
telescope than Jupiter. A good 2-inch telescope will do much, however,
in exhibiting his rings and belts. I have never seen him satisfactorily
myself with such an aperture, but Mr. Grover has not only seen the
above-named features, but even a penumbra to the shadow on the rings
with a 2-inch telescope.
Saturn revolving round the sun in a long period--nearly thirty
years--presents slowly varying changes of appearance (see Plate 7). At
one time the edge of his ring is turned nearly towards the earth; seven
or eight years later his rings are as much open as they can ever be;
then they gradually close up during a corresponding interval; open out
again, exhibiting a different face; and finally close up as first seen.
The last epoch of greatest opening occurred in 1856, the next occurs in
1870: the last epoch of disappearance occurred in 1862-63, the next
occurs in 1879. The successive views obtained are as in Plate 7 in order
from right to left, then back to the right-hand figure (but sloped the
other way); inverting the page we have this figure thus sloped, and the
following changes are now indicated by the other figures in order back
to the first (but sloped the other way and still inverted), thus
returning to the right-hand figure as seen without inversion.
The division in the ring can be seen in a good 2-inch aperture in
favourable weather. The dark ring requires a good 4-inch and good
weather.
Saturn's satellites do not, like Jupiter's, form a system of nearly
equal bodies. Titan, the sixth, is probably larger than any of
Jupiter's satellites. The eighth also (Japetus) is a large body,
probably at least equal to Jupiter's third satellite. But Rhea, Dione,
and Tethys are much less conspicuous, and the other three cannot be seen
without more powerful telescopes than those we are here dealing with.
So far as my own experience goes, I consider that the five larger
satellites may be seen distinctly in good weather with a good 3-1/2-inch
aperture. I have never seen them with such an aperture, but I judge from
the distinctness with which these satellites may be seen with a 4-inch
aperture. Titan is generally to be looked for at a considerable distance
from Saturn--_always_ when the ring is widely open. Japetus is to be
looked for yet farther from the disc. In fact, when Saturn comes to
opposition in perihelion (in winter only this can happen) Japetus may be
as far from Saturn as one-third of the apparent diameter of the moon. I
believe that under these circumstances, or even under less favourable
circumstances, Japetus could be seen with a good opera-glass. So also
might Titan.
Transits, eclipses, and occulations of Saturn's satellites can only be
seen when the ring is turned nearly edgewise towards the earth. For the
orbits of the seven inner satellites lying nearly in the plane of the
rings would (if visible throughout their extent) then only appear as
straight lines, or as long ellipses cutting the planet's disc.
The belts on Saturn are not very conspicuous. A good 3-1/2-inch is
required (so far as my experience extends) to show them satisfactorily.
The rings when turned edgewise either towards the earth or sun, are not
visible in ordinary telescopes, neither can they be seen when the earth
and sun are on opposite sides of the rings. In powerful telescopes the
rings seem never entirely to disappear.
The shadow of the planet on the rings may be well seen with a good
2-inch telescope, which will also show Ball's division in the rings. The
shadow of the rings on the planet is a somewhat more difficult feature.
The shadow of the planet on the rings is best seen when the rings are
well open and the planet is in or near quadrature. It is to be looked
for to the left of the ball (in an inverting telescope) at quadrature
preceding opposition, and to the right at quadrature following
opposition. Saturn is more likely to be studied at the latter than at
the former quadrature, as in quadrature preceding opposition he is a
morning star. The shadow of the rings on the planet is best seen when
the rings are but moderately open, and Saturn is in or near quadrature.
When the shadow lies outside the rings it is best seen, as the dark ring
takes off from the sharpness of the contrast when the shadow lies within
the ring. It would take more space than I can spare here to show how it
is to be determined (independently) whether the shadow lies within or
without the ring. But the 'Nautical Almanac' gives the means of
determining this point. When, in the table for assigning the appearance
of the rings, _l_ is less than _l'_ the shadow lies outside the ring,
when _l_ is greater than _l'_ the shadow lies within the ring.
Uranus is just visible to the naked eye when he is in opposition, and
his place accurately known. But he presents no phenomena of interest. I
have seen him under powers which made his disc nearly equal to that of
the moon, yet could see nothing but a faint bluish disc.
Neptune also is easily found if his place be accurately noted on a map,
and a good finder used. We have only to turn the telescope to a few
stars seen in the finder nearly in the place marked in our map, and
presently we shall recognise the one we want by the peculiarity of its
light. What is the lowest power which will exhibit Neptune as a disc I
do not know, but I am certain no observer can mistake him for a fixed
star with a 2-inch aperture and a few minutes' patient scrutiny in
favourable weather.
[Illustration: PLATE VII.]
CHAPTER VII.
HALF-HOURS WITH THE SUN AND MOON.
The moon perhaps is the easiest of all objects of telescopic
observation. A very moderate telescope will show her most striking
features, while each increase of power is repaid by a view of new
details. Yet in one sense the moon is a disappointing object even to the
possessor of a first-class instrument. For the most careful and
persistent scrutiny, carried on for a long series of years, too often
fails to reward the observer by any new discoveries of interest. Our
observer must therefore rather be prepared to enjoy the observation of
recognised features than expect to add by his labours to our knowledge
of the earth's nearest neighbour.
Although the moon is a pleasing and surprising telescopic object when
full, the most interesting views of her features are obtained at other
seasons. If we follow the moon as she waxes or wanes, we see the true
nature of that rough and bleak mountain scenery, which when the moon is
full is partially softened through the want of sharp contrasts of light
and shadow. If we watch, even for half an hour only, the changing form
of the ragged line separating light from darkness on the moon's disc, we
cannot fail to be interested. "The outlying and isolated peak of some
great mountain-chain becomes gradually larger, and is finally merged in
the general luminous surface; great circular spaces, enclosed with rough
and rocky walls many miles in diameter, become apparent; some with flat
and perfectly smooth floors, variegated with streaks; others in which
the flat floor is dotted with numerous pits or covered with broken
fragments of rock. Occasionally a regularly-formed and unusually
symmetrical circular formation makes its appearance; the exterior
surface of the wall bristling with terraces rising gradually from the
plain, the interior one much more steep, and instead of a flat floor,
the inner space is concave or cup-shaped, with a solitary peak rising in
the centre. Solitary peaks rise from the level plains and cast their
long narrow shadows athwart the smooth surface. Vast plains of a dusky
tint become visible, not perfectly level, but covered with ripples,
pits, and projections. Circular wells, which have no surrounding wall
dip below the plain, and are met with even in the interior of the
circular mountains and on the tops of their walls. From some of the
mountains great streams of a brilliant white radiate in all directions
and can be traced for hundreds of miles. We see, again, great fissures,
almost perfectly straight and of great length, although very narrow,
which appear like the cracks in moist clayey soil when dried by the
sun."[14]
But interesting as these views may be, it was not for such discoveries
as these that astronomers examined the surface of the moon. The
examination of mere peculiarities of physical condition is, after all,
but barren labour, if it lead to no discovery of physical variation. The
principal charm of astronomy, as indeed of all observational science,
lies in the study of change--of progress, development, and decay, and
specially of systematic variations taking place in regularly-recurring
cycles. And it is in this relation that the moon has been so
disappointing an object of astronomical observation. For two centuries
and a half her face has been scanned with the closest possible scrutiny;
her features have been portrayed in elaborate maps; many an astronomer
has given a large portion of his life to the work of examining craters,
plains, mountains, and valleys, for the signs of change; but until
lately no certain evidence--or rather, no evidence save of the most
doubtful character--has been afforded that the moon is other than "a
dead and useless waste of extinct volcanoes." Whether the examination of
the remarkable spot called Linne--where lately signs were supposed to
have been seen of a process of volcanic eruption--will prove an
exception to this rule, remains to be seen. The evidence seems to me
strongly to favour the supposition of a change of some sort having taken
place in this neighbourhood.
The sort of scrutiny required for the discovery of changes, or for the
determination of their extent, is far too close and laborious to be
attractive to the general observer. Yet the kind of observation which
avails best for the purpose is perhaps also the most interesting which he
can apply to the lunar details. The peculiarities presented by a spot upon
the moon are to be observed from hour to hour (or from day to day,
according to the size of the spot) as the sun's light gradually sweeps
across it, until the spot is fully lighted; then as the moon wanes and the
sun's light gradually passes from the spot, the series of observations is
to be renewed. A comparison of them is likely--especially if the observer
is a good artist and has executed several faithful delineations of the
region under observation, to throw much light upon the real contour of the
moon's surface at this point.
In the two lunar views in Plate 7 some of the peculiarities I have
described are illustrated. But the patient observer will easily be able
to construct for himself a set of interesting views of different
regions.
It may be noticed that for observation of the waning moon there is no
occasion to wait for those hours in which only the waning moon is
visible _during the night_. Of course for the observation of a
particular region under a particular illumination, the observer has no
choice as to hour. But for generally interesting observations of the
waning moon he can wait till morning and observe by daylight. The moon
is, of course, very easily found by the unaided eye (in the day time)
when not very near to the sun; and the methods described in Chapter V.
will enable the observer to find the moon when she is so near to the sun
as to present the narrowest possible sickle of light.
One of the most interesting features of the moon, when she is observed
with a good telescope, is the variety of colour presented by different
parts of her surface. We see regions of the purest white--regions which
one would be apt to speak of as _snow-covered_, if one could conceive
the possibility that snow should have fallen where (now, at least) there
is neither air nor water. Then there are the so-called seas, large grey
or neutral-tinted regions, differing from the former not merely in
colour and in tone, but in the photographic quality of the light they
reflect towards the earth. Some of the seas exhibit a greenish tint, as
the Sea of Serenity and the Sea of Humours. Where there is a central
mountain within a circular depression, the surrounding plain is
generally of a bluish steel-grey colour. There is a region called the
Marsh of Sleep, which exhibits a pale red tint, a colour seen also near
the Hyrcinian mountains, within a circumvallation called Lichtenburg.
The brightest portion of the whole lunar disc is Aristarchus, the peaks
of which shine often like stars, when the mountain is within the
unillumined portion of the moon. The darkest regions are Grimaldi and
Endymion and the great plain called Plato by modern astronomers--but, by
Hevelius, the Greater Black Lake.
The Sun.--Observation of the sun is perhaps on the whole the most
interesting work to which the possessor of a moderately good telescope
can apply his instrument. Those wonderful varieties in the appearance of
the solar surface which have so long perplexed astronomers, not only
supply in themselves interesting subjects of observation and
examination, but gain an enhanced meaning from the consideration that
they speak meaningly to us of the structure of an orb which is the
source of light and heat enjoyed by a series of dependent worlds whereof
our earth is--in size at least--a comparatively insignificant member.
Swayed by the attraction of this giant globe, Jupiter and Saturn, Uranus
and Neptune, as well as the four minor planets, and the host of
asteroids, sweep continuously in their appointed orbits, in ever new but
ever safe and orderly relations amongst each other. If the sun's light
and heat were lost, all life and work among the denizens of these orbs
would at once cease; if his attractive energy were destroyed, these orbs
would cease to form a _system_.
The sun may be observed conveniently in many ways, some more suited to
the general observer who has not time or opportunity for systematic
observation; others more instructive, though involving more of
preparation and arrangement.
The simplest method of observing the sun is to use the telescope in the
ordinary manner, protecting the eye by means of dark-green or
neutral-tinted glasses. Some of the most interesting views I have ever
obtained of the sun, have resulted from the use of the ordinary
terrestrial or erecting eye-piece, capped with a dark glass. The
magnifying power of such an eye-piece is, in general, much lower than
that available with astronomical eye-pieces. But vision is very pleasant
and distinct when the sun is thus observed, and a patient scrutiny
reveals almost every feature which the highest astronomical power
applicable could exhibit. Then, owing to the greater number of
intervening lenses, there is not the same necessity for great darkness
or thickness in the coloured glass, so that the colours of the solar
features are seen much more satisfactorily than when astronomical
eye-pieces are employed.
In using astronomical eye-pieces it is convenient to have a rotating
wheel attached, by which darkening glasses of different power may be
brought into use as the varying illumination may require.
Those who wish to observe carefully and closely a minute portion of the
solar disc, should employ Dawes' eye-piece: in this a metallic screen
placed in the focus keeps away all light but such as passes through a
minute hole in the diaphragm.
Another convenient method of diminishing the light is to use a glass
prism, light being partially reflected from one of the exterior
surfaces, while the refracted portion is thrown out at another.
Very beautiful and interesting views may be obtained by using such a
pyramidal box as is depicted in fig. 11.
[Illustration: _Fig. 11._]
This box should be made of black cloth or calico fastened over a light
framework of wire or cane. The base of the pyramid should be covered on
the inside with a sheet of white glazed paper, or with some other
uniform white surface. Captain Noble, I believe, makes use of a surface
of plaster of Paris, smoothed while wet with plate-glass. The door _b
c_ enables the observer to "change power" without removing the box,
while larger doors, _d e_ and _g f_, enable him to examine the image; a
dark cloth, such as photographers use, being employed, if necessary, to
keep out extraneous light. The image may also be examined from without,
if the bottom of the pyramid be formed of a sheet of cut-glass or oiled
tissue-paper.
When making use of the method just described, it is very necessary that
the telescope-tube should be well balanced. A method by which this may
be conveniently accomplished has been already described in Chapter I.
But, undoubtedly, for the possessor of a moderately good telescope there
is no way of viewing the sun's features comparable to that now to be
described, which has been systematically and successfully applied for a
long series of years by the Rev. F. Howlett. To use his own words: "Any
one possessing a good achromatic of not more than three inches'
aperture, who has a little dexterity with his pencil, and a little time
at his disposal (all the better if it be at a somewhat early hour of the
morning)" may by this method "deliberately and satisfactorily view,
measure, and (if skill suffice) delineate most of those interesting and
grand solar phenomena of which he may have read, or which he may have
seen depicted, in various works on physical astronomy."[15]
The method in question depends on the same property which is involved in
the use of the pyramidal box just described, supplemented (where exact
and systematic observation is required) by the fact that objects lying
on or between the lenses of the eye-piece are to be seen faithfully
projected on the white surface on which the sun's image is received. In
place, however, of a box carried upon the telescope-tube, a darkened
room (or true _camera obscura_) contains the receiving sheet.
A chamber is to be selected, having a window looking towards the
south--a little easterly, if possible, so as to admit of morning
observation. All windows are to be completely darkened save one, through
which the telescope is directed towards the sun. An arrangement is to be
adopted for preventing all light from entering by this window except
such light as passes down the tube of the telescope. This can readily be
managed with a little ingenuity. Mr. Howlett describes an excellent
method. The following, perhaps, will sufficiently serve the purposes of
the general observer:--A plain frame (portable) is to be constructed to
fit into the window: to the four sides of this frame triangular pieces
of cloth (impervious to light) are to be attached, their shape being
such that when their adjacent edges are sewn together and the flaps
stretched out, they form a rectangular pyramid of which the frame is the
base. Through the vertex of this pyramid (near which, of course, the
cloth flaps are not sewn together) the telescope tube is to be passed,
and an elastic cord so placed round the ends of the flaps as to prevent
any light from penetrating between them and the telescope. It will now
be possible, without disturbing the screen (fixed in the window), to
move the telescope so as to follow the sun during the time of
observation. And the same arrangement will serve for all seasons, if so
managed that the elastic cord is not far from the middle of the
telescope-tube; for in this case the range of motion is small compared
to the range of the tube's extremity.
A large screen of good drawing-paper should next be prepared. This
should be stretched on a light frame of wood, and placed on an easel,
the legs of which should be furnished with holes and pegs that the
screen may be set at any required height, and be brought square to the
tube's axis. A large T-square of light wood will be useful to enable the
observer to judge whether the screen is properly situated in the last
respect.
We wish now to direct the tube towards the sun, and this "without
dazzling the eyes as by the ordinary method." This may be done in two
ways. We may either, before commencing work--that is, before fastening
our elastic cord so as to exclude all light--direct the tube so that its
shadow shall be a perfect circle (when of course it is truly directed),
then fasten the cord and afterwards we can easily keep the sun in the
field by slightly shifting the tube as occasion requires. Or (if the
elastic cord has already been fastened) we may remove the eye-tube and
shift the telescope-tube about--the direction in which the sun lies
being roughly known--until we see the spot of light received down the
telescope's axis grow brighter and brighter and finally become a _spot
of sun-light_. If a card be held near the focus of the telescope there
will be seen in fact an image of the sun. The telescope being now
properly directed, the eye-tube may be slipped in again, and the sun may
be kept in the field as before.
There will now be seen upon the screen a picture of the sun very
brilliant and pleasing, but perhaps a little out of focus. The focusing
should therefore next be attended to, the increase of clearness in the
image being the test of approach to the true focus. And again, it will
be well to try the effect of slight changes of distance between the
screen and the telescope's eye-piece. Mr. Howlett considers one yard as
a convenient distance for producing an excellent effect with almost any
eye-piece that the state of the atmosphere will admit of. Of course, the
image becomes more sharply defined if we bring the screen nearer to the
telescope, while all the details are enlarged when we move the screen
away. The enlargement has no limits save those depending on the amount
of light in the image. But, of course, the observer must not expect
enlargement to bring with it a view of new details, after a certain
magnitude of image has been attained. Still there is something
instructive, I think, in occasionally getting a very magnified view of
some remarkable spot. I have often looked with enhanced feelings of awe
and wonder on the gigantic image of a solar spot thrown by means of the
diagonal eye-piece upon the ceiling of the observing-room. Blurred and
indistinct through over-magnifying, yet with a new meaning to me,
_there_ the vast abysm lies pictured; vague imaginings of the vast and
incomprehensible agencies at work in the great centre of our system
crowd unbidden into my mind; and I seem to _feel_--not merely think
about--the stupendous grandeur of that life-emitting orb.
To return, however, to observation:--By slightly shifting the tube,
different parts of the solar disc can be brought successively upon the
screen and scrutinized as readily as if they were drawn upon a chart.
"With a power of--say about 60 or 80 linear--the most minute solar spot,
properly so called, that is capable of formation" (Mr. Howlett believes
"they are never less than three seconds in length or breadth) will be
more readily detected than by any other method," see Plate 7; "as also
will any faculae, mottling, or in short, any other phenomena that may
then be existing on the disc." "Drifting clouds frequently sweep by, to
vary the scene, and occasionally an aerial hail- or snow-storm." Mr.
Howlett has more than once seen a distant flight of rooks pass slowly
across the disc with wonderful distinctness, when the sun has been at a
low altitude, and likewise, much more frequently, the rapid dash of
starlings, which, very much closer at hand, frequent his church-tower."
An eclipse of the sun, or a transit of an inferior planet, is also much
better seen in this way than by any other method of observing the solar
disc. In Plate 7 are presented several solar spots as they have appeared
to Mr. Howlett, with an instrument of moderate power. The grotesque
forms of some of these are remarkable; and the variations the spots
undergo from day to day are particularly interesting to the thoughtful
observer.
A method of measuring the spots may now be described. It is not likely
indeed that the ordinary observer will care to enter upon any systematic
series of measurements. But even in his case, the means of forming a
general comparison between the spots he sees at different times cannot
fail to be valuable. Also the knowledge--which a simple method of
measurement supplies--of the actual dimensions of a spot in miles
(roughly) is calculated to enhance our estimate of the importance of
these features of the solar disc. I give Mr. Howlett's method in his own
words:--
"Cause your optician to rule for you on a circular piece of glass a
number of fine graduations, the 200th part of an inch apart, each fifth
and tenth line being of a different length in order to assist the eye in
their enumeration. Insert this between the anterior and posterior lenses
of a Huygenian eye-piece of moderate power, say 80 linear. Direct your
telescope upon the sun, and having so arranged it that the whole disc of
the sun may be projected on the screen, count carefully the number of
graduations that are seen to exactly occupy the solar diameter.... It
matters not in which direction you measure your diameter, provided only
the sun has risen some 18 deg. or 20 deg. above the horizon, and so escaped the
distortion occasioned by refraction.[16]
"Next let us suppose that our observer has been observing the sun on any
day of the year, say, if you choose, at the time of its mean apparent
diameter, namely about the first of April or first of October, and has
ascertained that" (as is the case with Mr. Howlett's instrument)
"sixty-four graduations occupy the diameter of the projected image. Now
the semi-diameter of the sun, at the epochs above mentioned, according
to the tables given for every day of the year in the 'Nautical Almanac'
(the same as in Dietrichsen and Hannay's very useful compilation) is
16' 2", and consequently his mean total diameter is 32' 4" or 1924". If
now we divide 1924" by 64" this will, of course, award as nearly as
possible 30" as the value in celestial arc of each graduation, either as
seen on the screen, or as applied directly to the sun or any heavenly
body large enough to be measured by it."
Since the sun's diameter is about 850,000 miles, each graduation (in the
case above specified) corresponds to one-64th part of 850,000
miles--that is, to a length of 13,256 miles on the sun's surface. Any
other case can be treated in precisely the same manner.
It will be found easy so to place the screen that the distance between
successive graduations (as seen projected upon the screen) may
correspond to any desired unit of linear measurement--say an inch. Then
if the observer use transparent tracing-paper ruled with faint lines
forming squares half-an-inch in size, he can comfortably copy directly
from the screen any solar phenomena he may be struck with. A variety of
methods of drawing will suggest themselves. Mr. Howlett, in the paper I
have quoted from above, describes a very satisfactory method, which
those who are anxious to devote themselves seriously to solar
observation will do well to study.
It is necessary that the observer should be able to determine
approximately where the sun's equator is situated at the time of any
observation, in order that he may assign to any spot or set of spots its
true position in relation to solar longitude and latitude. Mr. Howlett
shows how this may be done by three observations of the sun made at any
fixed hour on successive days. Perhaps the following method will serve
the purpose of the general observer sufficiently well:--
The hour at which the sun crosses the meridian must be taken for the
special observation now to be described. This hour can always be learnt
from 'Dietrichsen's Almanac'; but noon, civil time, is near enough for
practical purposes. Now it is necessary first to know the position of
the ecliptic with reference to the celestial equator. Of course, at noon
a horizontal line across the sun's disc is parallel to the equator, but
the position of that diameter of the sun which coincides with the
ecliptic is not constant: at the summer and winter solstices this
diameter coincides with the other, or is horizontal at noon; at the
spring equinox the sun (which travels on the ecliptic) is passing
towards the north of the equator, crossing that curve at an angle of
23-1/2 deg., so that the ecliptic coincides with that diameter of the sun
which cuts the horizontal one at an angle of 23-1/2 deg. and has its _left_
end above the horizontal diameter; and at the autumn equinox the sun is
descending and the same description applies, only that the diameter
(inclined 23-1/2 deg. to the horizon) which has its _right_ end uppermost,
now represents the ecliptic. For intermediate dates, use the following
little table:--
--------------------------------------------------------------------------
Date. |Dec. 22|Jan. 5|Jan. 20|Feb. 4|Feb. 19|Mar. 5 |Mar. 21
(Circiter.) | |June 6|May 21 |May 5 |Apr. 20|Apr. 5 |
-------------------+-------+------+-------+------+-------+-------+--------
Inclination of |Left |Left |Left |Left |Left |Left |Left
Ecliptical Diameter| | | | | | |
of Sun to the |0 deg. 0' |6 deg.24' |12 deg.14' |17 deg.3' |20 deg.36' |22 deg.44' |23 deg.27'
Horizon.[17] |Right |Right |Right |Right |Right |Right |Right
-------------------+-------+------+-------+------+-------+-------+--------
Date. | |Dec. 7|Nov. 22|Nov. 7|Oct. 23|Oct. 8 |
(Circiter.) |Jan. 21|July 7|July 23|Aug. 6|Aug. 23|Sept. 7|Sept. 23
--------------------------------------------------------------------------
Now if our observer describe a circle, and draw a diameter inclined
according to above table, this diameter would represent the sun's
equator if the axis of the sun were square to the ecliptic-plane. But
this axis is slightly inclined, the effect of which is, that on or about
June 10 the sun is situated as shown in fig. 14 with respect to the
ecliptic _ab_; on or about September 11 he is situated as shown in fig.
13; on or about December 11 as shown in fig. 12; and on or about March
10 as shown in fig. 15. The inclination of his equator to the ecliptic
being so small, the student can find little difficulty in determining
with sufficient approximation the relation of the sun's polar axis to
the ecliptic on intermediate days, since the equator is never more
_inclined_ than in figs. 12 and 14, never more _opened out_ than in
figs. 13 and 15. Having then drawn a line to represent the sun's
ecliptical diameter inclined to the horizontal diameter as above
described, and having (with this line to correspond to _ab_ in figs.
12-15) drawn in the sun's equator suitably inclined and opened out, he
has the sun's actual presentation (at noon) as seen with an erecting
eye-piece. Holding his picture upside down, he has the sun's
presentation as seen with an astronomical eye-piece--and, finally,
looking at his picture from behind (without inverting it), he has the
presentation seen when the sun is projected on the screen. Hence, if he
make a copy of this last view of his diagram upon the centre of his
screen, and using a low power, bring the whole of the sun's image to
coincide with the circle thus drawn (to a suitable scale) on the screen,
he will at once see what is the true position of the different
sun-spots. After a little practice the construction of a suitably sized
and marked circle on the screen will not occupy more than a minute or
two.
[Illustration: _Fig. 12._]
[Illustration: _Fig. 13._]
[Illustration: _Fig. 14._]
[Illustration: _Fig. 15._]
It must be noticed that the sun's apparent diameter is not always the
same. He is nearer to us in winter than in summer, and, of course, his
apparent diameter is greater at the former than at the latter season.
The variation of the apparent diameter corresponds (inversely) to the
variation of distance. As the sun's greatest distance from the earth is
93,000,000 miles (pretty nearly) and his least 90,000,000, his greatest,
mean, and least apparent diameters are as 93, 91-1/2, and 90
respectively; that is, as 62, 61, and 60 respectively.
Mr. Howlett considers that with a good 3-inch telescope, applied in the
manner we have described, all the solar features may be seen, except the
separate granules disclosed by first-class instruments in the hands of
such observers as Dawes, Huggins, or Secchi. Faculae may, of course, be
well seen. They are to be looked for near spots which lie close to the
sun's limb.
When the sun's general surface is carefully scrutinised, it is found to
present a mottled appearance. This is a somewhat delicate feature. It
results, undoubtedly, from the combined effect of the granules
separately seen in powerful instruments. Sir John Herschel has stated
that he cannot recognise the marbled appearance of the sun with an
achromatic. Mr. Webb, however, has seen this appearance with such a
telescope, of moderate power, used with direct vision; and certainly I
can corroborate Mr. Howlett in the statement that this appearance may be
most distinctly seen when the image of the sun is received within a
well-darkened room.
My space will not permit me to enter here upon the discussion of any of
those interesting speculations which have been broached concerning solar
phenomena. We may hope that the great eclipse of August, 1868, which
promises to be the most favourable (for effective observation) that has
ever taken place, will afford astronomers the opportunity of resolving
some important questions. It seems as if we were on the verge of great
discoveries,--and certainly, if persevering and well-directed labour
would seem in any case to render such discoveries due as man's just
reward, we may well say that he deserves shortly to reap a harvest of
exact knowledge respecting solar phenomena.
THE END.
FOOTNOTES:
[Footnote 1: Such a telescope is most powerful with the shortest sight.
It may be remarked that the use of a telescope often reveals a
difference in the sight of the two eyes. In my own case, for instance, I
have found that the left eye is very short-sighted, the sight of the
right eye being of about the average range. Accordingly with my left eye
a 5-1/2-foot object-glass, alone, forms an effective telescope, with
which I can see Jupiter's moons quite distinctly, and under favourable
circumstances even Saturn's rings. I find that the full moon is too
bright to be observed in this way without pain, except at low
altitudes.]
[Footnote 2: Betelgeuse--commonly interpreted the Giant's
Shoulder--_ibt-al-jauza_. The words, however, really signify, "the
armpit of the central one," Orion being so named because he is divided
centrally by the equator.]
[Footnote 3: I have never been able to see more than four with a
3-3/4-inch aperture. I give a view of the trapezium as seen with an
8-inch equatorial.]
[Footnote 4: Sir W. Herschel several times saw [epsilon] Lyrae as a
double. Bessel also relates that when he was a lad of thirteen he could
see this star double. I think persons having average eye-sight could see
it double if they selected a suitable hour for observation. My own
eye-sight is not good enough for this, but I can distinctly see this
star wedged whenever the line joining the components is inclined about
45 deg. to the horizon, and also when Lyra is near the zenith.]
[Footnote 5: They were so described by Admiral Smyth in 1839. Mr. Main,
in 1862, describes them as straw-coloured and reddish, while Mr. Webb,
in 1865, saw them pale-yellow and _lilac_!]
[Footnote 6: Or the observer may sweep from [omicron] towards [nu],
looking for R about two-fifths of the way from [omicron] to [nu].]
[Footnote 7: Here a single period only is taken, to get back to a
convenient hour of the evening.]
[Footnote 8: Here a single period only is taken, to get back to a
convenient hour of the evening.]
[Footnote 9: I have constructed a zodiac-chart, which will enable the
student to mark in the path of a planet, at any season of the year, from
the recorded places in the almanacs.]
[Footnote 10: It is convenient to remember that through precession a
star near the ecliptic shifts as respects the R.A. and Dec. lines,
through an arc of one degree--or nearly twice the moon's diameter--in
about 72 years, all other stars through a less arc.]
[Footnote 11: Mercury is best seen when in quadrature to the sun, but
_not_ (as I have seen stated) at those quadratures in which he attains
his maximum elongation from the sun. This will appear singular, because
the maximum elongation is about 27 deg., the minimum only about 18 deg.. But it
happens that in our northern latitudes Mercury is always _south_ of the
sun when he attains his maximum elongation, and this fact exercises a
more important effect than the mere amount of elongation.]
[Footnote 12: It does not seem to me that the difficulty of detecting
Mercury is due to the difficulty "of identifying it amongst the
surrounding stars, during the short time that it can be seen" (Hind's
'Introduction to Astronomy'). There are few stars which are comparable
with Mercury in brilliancy, when seen under the same light.]
[Footnote 13: I may notice another error sometimes made. It is said that
the shadow of a satellite _appears_ elliptical when near the edge of the
disc. The shadow is _in reality_ elliptical when thus situated, but
_appears_ circular. A moment's consideration will show that this should
be so. The part of the disc concealed by a _satellite_ near the limb is
also elliptical, but of course appears round.]
[Footnote 14: From a paper by Mr. Breen, in the 'Popular Science
Review,' October, 1864.]
[Footnote 15: 'Intellectual Observer' for July, 1867, to which magazine
the reader is referred for full details of Mr. Howlett's method of
observation, and for illustrations of the appliances he made use of, and
of some of his results.]
[Footnote 16: As the sun does not attain such an altitude as 18 deg. during
two months in the year, it is well to notice that the true length of the
sun's apparent solar diameter is determinable even immediately after
sun-rise, if the line of graduation is made to coincide with the
_horizontal_ diameter of the picture on the screen--for refraction does
not affect the length of this diameter.]
[Footnote 17: The words "Left" and "Right" indicate which end of the
sun's ecliptical diameter is uppermost at the dates in upper or lower
row respectively.]
LONDON:
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CROSS.
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