THE STARRY HEAVENS.
The Constellations--The Great Bear and the Pointers--The Pole
Star--Cassiopeia--Andromeda, Pegasus, and Perseus--The Pleiades:
Auriga, Capella, Aldebaran--Taurus, Orion, Sirius; Castor and
Pollux--The Lion--Booetes, Corona, and Hercules--Virgo and
Spica--Vega and Lyra--The Swan.
The student of astronomy should make himself acquainted with the
principal constellations in the heavens. This is a pleasing acquirement,
and might well form a part of the education of every child in the
kingdom. We shall commence our discussion of the sidereal system with a
brief account of the principal constellations visible in the northern
hemisphere, and we accompany our description with such outline maps of
the stars as will enable the beginner to identify the chief features of
the starry heavens.
In an earlier chapter we directed the attention of the student to the
remarkable constellation of stars which is known to astronomers as Ursa
Major, or the Great Bear. It forms the most conspicuous group in the
northern skies, and in northern latitudes it never sets. At eleven p.m.
in the month of April the Great Bear is directly overhead (for an
observer in the United Kingdom); at the same hour in September it is low
down in the north; at the same hour July it is in the west; by Christmas
it is at the east. From the remotest antiquity this group of stars has
attracted attention. The stars in the Great Bear were comprised in a
great catalogue of stars, made two thousand years ago, which has been
handed down to us. From the positions of the stars given in this
catalogue it is possible to reconstruct the Great Bear as it appeared in
those early days. This has been done, and it appears that the seven
principal stars have not changed in this lapse of time to any large
extent, so that the configuration of the Great Bear remains practically
the same now as it was then. The beginner must first obtain an
acquaintance with this group of seven stars, and then his further
progress in this branch of astronomy will be greatly facilitated. The
Great Bear is, indeed, a splendid constellation, and its only rival is
to be found in Orion, which contains more brilliant stars, though it
does not occupy so large a region in the heavens.
[Illustration: Fig. 80.--The Great Bear and Pole Star.]
[Illustration: Fig. 81.--The Great Bear and Cassiopeia.]
In the first place, we observe how the Great Bear enables the Pole Star,
which is the most important object in the northern heavens, to be
readily found. The Pole Star is very conveniently indicated by the
direction of the two stars, b and a, of the Great Bear, which
are, accordingly, generally known as the "pointers." This use of the
Great Bear is shown on the diagram in Fig. 80, in which the line b
a, produced onwards and slightly curved, will conduct to the Pole
Star. There is no likelihood of making any mistake in this star, as it
is the only bright one in the neighbourhood. Once it has been seen it
will be readily identified on future occasions, and the observer will
not fail to notice how constant is the position which it preserves in
the heavens. The other stars either rise or set, or, like the Great
Bear, they dip down low in the north without actually setting, but the
Pole Star exhibits no considerable changes. In summer or winter, by
night or by day, the Pole Star is ever found in the same place--at
least, so far as ordinary observation is concerned. No doubt, when we
use the accurate instruments of the observatory the notion of the fixity
of the Pole Star is abandoned; we then see that it has a slow motion,
and that it describes a small circle every twenty-four hours around the
true pole of the heavens, which is not coincident with the Pole Star,
though closely adjacent thereto. The distance is at present a little
more than a degree, and it is gradually lessening, until, in the year
A.D. 2095, the distance will be under half a degree.
The Pole Star itself belongs to another inconsiderable group of stars
known as the Little Bear. The two principal members of this group, next
in brightness to the Pole Star, are sometimes called the "Guards." The
Great Bear and the Little Bear, with the Pole Star, form a group in the
northern sky not paralleled by any similarly situated constellation in
the southern heavens. At the South Pole there is no conspicuous star to
indicate its position approximately--a circumstance disadvantageous to
astronomers and navigators in the southern hemisphere.
It will now be easy to add a third constellation to the two already
acquired. On the opposite side of the Pole Star to the Great Bear, and
at about the same distance, lies a very pleasing group of five bright
stars, forming a W. These are the more conspicuous members of the
constellation Cassiopeia, which contains altogether about sixty stars
visible to the naked eye. When the Great Bear is low down in the north,
then Cassiopeia is high overhead. When the Great Bear is high overhead,
then Cassiopeia is to be looked for low down in the north. The
configuration of the leading stars is so striking that once the eye has
recognised them future identification will be very easy--the more so
when it is borne in mind that the Pole Star lies midway between
Cassiopeia and the Great Bear (Fig. 81). These important constellations
will serve as guides to the rest. We shall accordingly show how the
learner may distinguish the various other groups visible from the
British Islands or similar northern latitudes.
The next constellation to be recognised is the imposing group which
contains the Great Square of Pegasus. This is not, like Ursa Major, or
like Cassiopeia, said to be "circumpolar." The Great Square of Pegasus
sets and rises daily. It cannot be seen conveniently during the spring
and the summer, but in autumn and in winter the four stars which mark
the corners of the square can be easily recognised. There are certain
small stars within the region so limited; perhaps about thirty can be
counted by an unaided eye of ordinary power in these latitudes. In the
south of Europe, with its pure and bright skies, the number of visible
stars appears to be greatly increased. An acute observer at Athens has
counted 102 in the same region.
[Illustration: Fig. 82.--The Great Square of Pegasus.]
The Great Square of Pegasus can be reached by a line from the Pole Star
over the end of Cassiopeia. If it be produced about as far again it will
conduct the eye to the centre of the Great Square of Pegasus (Fig. 82).
The line through b and a in Pegasus continued 45 deg. to the
south points out the important star Fomalhaut in the mouth of the
Southern Fish. To the right of this line, nearly half-way down, is the
rather vague constellation of Aquarius, where a small equilateral
triangle with a star in the centre may be noticed.
The square of Pegasus is not a felicitous illustration of the way in
which the boundaries of the constellations should be defined. There can
be no more naturally associated group than the four stars of this
square, and they ought surely to be included in the same constellation.
Three of the stars--marked a, b, g--do belong to Pegasus; but that at
the fourth corner--also marked a--is placed in a different figure, known
as Andromeda, whereof it is, indeed, the brightest member. The remaining
bright stars of Andromeda are marked b and g, and they are readily
identified by producing one side of the Square of Pegasus in a curved
direction. We have thus a remarkable array of seven stars, which it is
both easy to identify and easy to remember, notwithstanding that they
are contributed to by three different constellations. They are
respectively a, b, and g of Pegasus; a, b, and g of Andromeda; and a of
Perseus. The three form a sort of handle, as it were, extending from one
side of the square, and are a group both striking in appearance, and
useful in the further identification of celestial objects. b Andromedae,
with two smaller stars, form the girdle of the unfortunate heroine.
a Persei lies between two other stars (g and d) of the
same constellation. If we draw a curve through these three and prolong
it in a bold sweep, we are conducted to one of the gems of the northern
heavens--the beautiful star Capella, in Auriga (Fig. 83). Close to
Capella are three small stars forming an isosceles triangle--these are
the Hoedi or Kids. Capella and Vega are, with the exception of
Arcturus, the two most brilliant stars in the northern heavens; and
though Vega is probably the more lustrous of the two, yet the opposite
opinion has been entertained. Different eyes will frequently form
various estimates of the relative brilliancy of stars which approach
each other in brightness. The difficulty of making a satisfactory
comparison between Vega and Capella is greatly increased by the wide
distance in the heavens at which they are separated, as well as by a
slight difference in colour, for Vega is distinctly whiter than Capella.
This contrast between the colour of stars is often a source of
uncertainty in the attempt to compare their relative brilliancy; so
that when actual measurements have to be effected by instrumental means,
it is necessary to compare the two stars alternately with some object of
intermediate hue.
[Illustration: Fig. 83.--Perseus and its Neighbouring Stars.]
On the opposite side of the pole to Capella, but not quite so far away,
will be found four small stars in a quadrilateral. They form the head of
the Dragon, the rest of whose form coils right round the pole.
If we continue the curve formed by the three stars g, a, and
d in Perseus, and if we bend round this curve gracefully into one
of an opposite flexion, in the manner shown in Fig. 83, we are first
conducted to two other principal stars in Perseus, marked e and
z. The region of Perseus is one of the richest in the heavens. We
have here a most splendid portion of the Milky Way, and the field of
the telescope is crowded with stars beyond number. Even a small
telescope or an opera-glass directed to this teeming constellation
cannot fail to delight the observer, and convey to him a profound
impression of the extent of the sidereal heavens. We shall give in a
subsequent paragraph a brief enumeration of some of the remarkable
telescopic objects in Perseus. Pursuing in the same figure the line
e and z, we are conducted to the remarkable little group
known as the Pleiades.
[Illustration: Fig. 84.--The Pleiades.]
The Pleiades form a group so universally known and so easily identified
that it hardly seems necessary to give any further specific instructions
for their discovery. It may, however, be observed that in these
latitudes they cannot be seen before midnight during the summer. Let us
suppose that the search is made at about 11 p.m. at night: on the 1st of
January the Pleiades will be found high up in the sky in the south-west;
on the 1st of March, at the same hour, they will be seen to be setting
in the west. On the 1st of May they are not visible; on the 1st of July
they are not visible; on the 1st of September they will be seen low down
in the east. On the 1st of November they will be high in the heavens in
the south-east. On the ensuing 1st of January the Pleiades will be in
the same position as they were on the same date in the previous year,
and so on from year to year. It need, perhaps, hardly be explained here
that these changes are not really due to movements of the
constellations; they are due, of course, to the apparent annual motion
of the sun among the stars.
[Illustration: Fig. 85.--Orion, Sirius, and the Neighbouring Stars.]
The Pleiades are shown in the figure (Fig. 84), where a group of ten
stars is represented, this being about the number visible with the
unaided eye to those who are gifted with very acute vision. The lowest
telescopic power will increase the number of stars to thirty or forty
(Galileo saw more than forty with his first telescope), while with
telescopes of greater power the number is largely increased; indeed, no
fewer than 625 have been counted with the aid of a powerful telescope.
The group is, however, rather too widely scattered to make an effective
telescopic object, except with a large field and low power. Viewed
through an opera-glass it forms a very pleasing spectacle.
[Illustration: Fig. 86.--Castor and Pollux.]
If we draw a ray from the Pole Star to Capella, and produce it
sufficiently far, as shown in Fig. 85, we come to the great
constellation of our winter sky, the splendid group of Orion. The
brilliancy of the stars in Orion, the conspicuous belt, and the
telescopic objects which it contains, alike render this group
remarkable, and place it perhaps at the head of the constellations. The
leading star in Orion is known either as a Orionis, or as
Betelgeuze, by which name it is here designated. It lies above the three
stars, d, e, z, which form the belt. Betelgeuze is a
star of the first magnitude, and so also is Rigel, on the opposite side
of the belt. Orion thus enjoys the distinction of containing two stars
of the first magnitude in its group, while the five other stars shown in
Fig. 85 are of the second magnitude.
The neighbourhood of Orion contains some important stars. If we carry on
the line of the belt upwards to the right, we are conducted to another
star of the first magnitude, Aldebaran, which strongly resembles
Betelgeuze in its ruddy colour. Aldebaran is the brightest star in the
constellation of Taurus. It is this constellation which contains the
Pleiades already referred to, and another more scattered group known as
the Hyades, which can be discovered near Aldebaran.
[Illustration: Fig. 87.--The Great Bear and the Lion.]
The line of the belt of Orion continued downwards to the left conducts
the eye to the gem of the sky, the splendid Sirius, which is the most
brilliant star in the heavens. It has, indeed, been necessary to create
a special order of magnitude for the reception of Sirius alone; all the
other first magnitude stars, such as Vega and Capella, Betelgeuze and
Aldebaran, coming a long way behind. Sirius, with a few other stars of
much less lustre, form the constellation of Canis Major.
It is useful for the learner to note the large configuration, of an
irregular lozenge shape, of which the four corners are the first
magnitude stars, Aldebaran, Betelgeuze, Sirius, and Rigel (Fig. 85).
The belt of Orion is placed symmetrically in the centre of the group,
and the whole figure is so striking that once perceived it is not likely
to be forgotten.
About half way from the Square of Pegasus to Aldebaran is the chief star
in the Ram--a bright orb of the second magnitude; with two others it
forms a curve, at the other end of which will be found g of the
same constellation, which was the first double star ever noticed.
We can again invoke the aid of the Great Bear to point out the stars in
the constellation of Gemini (Fig. 86). If the diagonal joining the stars
d and b of the body of the Bear be produced in the direction
opposite to the tail, it will lead to Castor and Pollux, two remarkable
stars of the second magnitude. This same line carried a little further
on passes near the star Procyon, of the first magnitude, which is the
only conspicuous object in the constellation of the Little Dog.
[Illustration: Fig. 88.--Booetes and the Crown.]
[Illustration: Fig. 89.--Virgo and the neighbouring Constellations.]
The pointers in the Great Bear marked a b will also serve to
indicate the constellation of the Lion. If we produce the line joining
them in the direction opposite from that used in finding the Pole, we
are brought into the body of the Lion. This group will be recognised by
the star of the first magnitude called Regulus. It is one of a series of
stars forming an object somewhat resembling a sickle: three of the group
are of the second magnitude. The Sickle has a special claim on our
notice because it contains the radiant point from which the periodic
shooting star shower known as the Leonids diverges. Regulus lies
alongside the sun's highway through the stars, at a point which he
passes on the 21st of August every year.
Between Gemini and Leo the inconspicuous constellation of the Crab may
be found; the most striking object it contains is the misty patch called
Praesepe or the Bee-Hive, which the smallest opera-glass will resolve
into its component stars.
[Illustration: Fig. 90.--The Constellation of Lyra.]
The tail of the Great Bear, when prolonged with a continuation of the
curve which it possesses, leads to a brilliant star of the first
magnitude known as Arcturus, the principal star in the constellation of
Booetes (Fig. 88). A few other stars, marked b, g, d,
and e in the same constellation, are also shown in the figure.
Among the stars visible in these latitudes Arcturus is to be placed next
to Sirius in point of brightness. Two stars in the southern hemisphere,
invisible in these latitudes, termed a Centauri and Canopus, are
nearly as bright as Vega and Capella, but not quite as bright as
Arcturus.
In the immediate neighbourhood of Booetes is a striking semicircular
group known as the Crown or Corona Borealis. It will be readily found
from its position as indicated in the figure, or it may be identified by
following the curved line indicated by b, d, e, and z in the Great Bear.
[Illustration: Fig. 91.--Vega, the Swan, and the Eagle.]
The constellation of Virgo is principally characterised by the first
magnitude star called Spica, or a Virginis. This may be found from
the Great Bear; for if the line joining the two stars a and
g in that constellation be prolonged with a slight curve, it will
conduct the eye to Spica. We may here notice another of those large
configurations which are of great assistance in the study of the stars.
There is a fine equilateral triangle, whereof Arcturus and Spica form
two of the corners, while the third is indicated by Denebola, the bright
star near the tail of the Lion (Fig. 89).
In the summer evenings when the Crown is overhead, a line from the Pole
Star through its fainter edge, continued nearly to the southern horizon,
encounters the brilliant red star Cor Scorpionis, or the Scorpion's
Heart (Antares), which was the first star mentioned as having been seen
with the telescope in the daytime.
The first magnitude star, Vega, in the constellation of the Lyre, can
be readily found at the corner of a bold triangle, of which the Pole
Star and Arcturus form the base (Fig. 90). The brilliant whiteness of
Vega will arrest the attention, while the small group of neighbouring
stars which form the Lyre produces one of the best defined
constellations.
Near Vega is another important constellation, known as the Swan or
Cygnus. The brightest star will be identified as the vertex of a
right-angled triangle, of which the line from Vega to the Pole Star is
the base, as shown in Fig. 91. There are in Cygnus five principal stars,
which form a constellation of rather remarkable form.
The last constellation which we shall here describe is that of Aquila or
the Eagle, which contains a star of the first magnitude, known as
Altair; this group can be readily found by a line from Vega over b
Cygni, which passes near the line of three stars, forming the
characteristic part of the Eagle.
We have taken the opportunity to indicate in these sketches of the
constellations the positions of some other remarkable telescopic
objects, the description of which we must postpone to the following
chapters.