equator--which occurs each year at the beginning of summer--at the
time when it was passing through the constellation of Cancer. Our
tropic of Cancer, which marks the northern limit of the torrid zone,
received its name from this fact. At the time when the sun reaches
the point farthest north, its height above the horizon changes very
little from day to day, and for a short time it appears to be slowly
crawling sideways through the heavens, as a crab walks, and for this
reason, possibly, the constellation was called Cancer, The Crab.
At the present time the "Precession of the Equinoxes," or westward
shifting of the vernal equinox--the point where the sun crosses
the equator going north in the spring--brings the sun, when it is
farthest north, in Gemini instead of in Cancer. At the present time,
then it would be more accurate to speak of the tropic of Gemini,
though this in turn would be inaccurate after a lapse of centuries,
as the sun passed into another constellation at the beginning of
summer. The tropic of Capricorn, which marks the farthest southern
excursions of the sun in its yearly circuit of the heavens, should
also more appropriately be called the tropic of Sagittarius, as the
sun is now in Sagittarius instead of Capricornus at the time when it
is farthest south, though the point is slowly shifting westward into
Scorpio.
Mythology tells us that Cancer was sent by Juno to distract Hercules
by pinching his toes while he was contending with the many-headed
serpent in the Lernean swamp. Hercules, the legend says, crushed the
crab with a single blow, and Juno by way of reward placed it in the
heavens.
In Cancer, according to the belief of the Chaldeans, was located the
"gate of men," by which souls descended into human bodies, while in
Capricornus was the "gate of the gods," through which the freed souls
of men returned to heaven.
[Illustration: APRIL--HYDRA]
Hydra, the many-headed serpent with which Hercules contended, is
represented by a constellation of great length. It extends from a
point just south of Cancer, where a group of faint stars marks the
heads, to the south and southeast in a long line of faint stars.
It passes in its course just south of Crater and Corvus, the two
small star-groups below Leo (see constellations for May), which
are sometimes called its riders, and it also stretches below the
entire length of the long, straggling constellation of Virgo. At
this time we can trace it only to the point where it disappears
below the horizon in the southeast. It contains but one bright star,
Alphard, or Cor Hydrae as it is also called, standing quite alone and
almost due south at this time. Hydra, as well as Lynx and Cancer,
contains no noteworthy or remarkable object and consists chiefly of
faint stars. Alphard is, in fact, the only bright star that we have
in the constellations for this month. It chances that these three
inconspicuous star-groups, Lynx, Crater, and Hydra, lie nearest to
the meridian at this time, separating the brilliant groups of winter
from those of the summer months.
[Illustration: APRIL--LYNX]
VI
MAY
Ursa Major, the Great Bear, and Ursa Minor, the Lesser Bear, or,
as they are more familiarly called, the Big Dipper and the Little
Dipper, are the best known of all the constellations visible in
northern latitudes. They are called circumpolar constellations, which
means "around the pole." For those who live north of 40° N. Lat. they
never set, but can be seen at all hours of the night and at all times
of the year. In fall and winter evenings Ursa Major lies below the
pole and near the horizon, and so is usually hidden more or less from
view by trees or buildings. It is during the early evening hours of
late spring and summer that this constellation is seen to the best
advantage high in the sky above the pole. If one looks due north at
the time mentioned, it will be impossible to miss either of these
constellations.
To complete the outline of the Great Bear, it is necessary to include
faint stars to the east, which form the head of the Bear, and other
faint stars to the south, which form the feet, but these are all
inconspicuous and of little general interest.
The two stars in the bowl of the Big Dipper through which an arrow is
drawn in the chart, are called the Pointers, because an imaginary
line drawn through these two stars and continued a distance about
equal to the length of the Big Dipper, brings us to the star
Polaris, or the North Star, at the end of the handle of the Little
Dipper, which is very close to the north pole of the heavens, the
direction in which the earth's axis points. The pole lies on the line
connecting the star at the bend in the handle of the Big Dipper with
Polaris, and is only one degree distant from the pole-star.
[Illustration: MAY--URSA MAJOR AND URSA MINOR]
The distance between the Pointers is five degrees of arc, and the
distance from the more northerly of these two stars to Polaris is
nearly thirty degrees. We may find it useful to remember this in
estimating distances between objects in the heavens, which are always
given in angular measure.
A small two and one-half inch telescope will separate Polaris into
two stars eighteen seconds of arc apart. The companion star is a
faint white star of the ninth magnitude.
Twenty years or so ago it was discovered with the aid of the
spectroscope that the brighter of the two stars was also a double
star, but the two stars were so close together that they could not
be seen as separate stars in any telescope. Later it was found that
the brighter star was in reality triple, that is, it consists of
three suns close together. The faint white companion star formed
with these three suns a system of four suns revolving about a common
center of gravity. Still more recently it has been discovered that
the brightest of these four suns varies regularly in brightness in a
period of a little less than four days. It belongs to the important
class of stars known as Cepheid variable stars, whose changes
of light, it is believed, are produced by some periodic form of
disturbance taking place within the stars themselves.
With one exception, Polaris is the nearest to the earth of all these
Cepheid variable stars, which are in most instances at very great
distances from the solar system. The latest measurements of the
distance of Polaris show that its light takes about two centuries
to travel to the earth, or, in other words, that it is distant two
hundred light-years.
Like all Cepheid variables, Polaris is a giant star. It gives forth
about five hundred and twenty-five times as much light as our own
sun. If Polaris and the sun were placed side by side at a distance of
thirty-three light-years, the sun would appear as a star of the fifth
magnitude, just well within the range of visibility of the human
eye, while Polaris would outshine Sirius, the brightest star in the
heavens.
As a practical aid to navigators, Polaris is unsurpassed in
importance by any star of the northern hemisphere of the heavens. At
the equator the pole-star lies in the horizon; at the north pole of
the earth it is in the zenith or directly overhead. Its altitude or
height above the horizon is always equal to the latitude of the place
of observation. As we travel northward from the equator toward the
pole we see Polaris rise higher and higher in the sky. In New York
the elevation of Polaris above the horizon is forty degrees, which is
the latitude of the city.
The Pointers indicate the direction of Polaris and the true north,
while the height of Polaris above the horizon tells us our latitude.
These kindly stars direct us by night when we are uncertain of our
bearings, whether we travel by land or sea or air. They are the
friends and aids of explorers, navigators and aviators, who often
turn to them for guidance.
Bryant writes thus beautifully of Polaris in his _Hymn to the North
Star_:
Constellations come and climb the heavens, and go.
Star of the Pole! and thou dost see them set.
Alone in thy cold skies,
Thou keep'st thy old unmoving station yet,
Nor join'st the dances of that glittering train,
Nor dipp'st thy virgin orb in the blue western main.
On thy unaltering blaze
The half wrecked mariner, his compass lost,
Fixes his steady gaze,
And steers, undoubting, to the friendly coast;
And they who stray in perilous wastes by night,
Are glad when thou dost shine to guide their footsteps right.
The star at the bend in the handle of the Big Dipper, called Mizar,
is of special interest. If one has good eyesight, he will see close
to it a faint star. This is Alcor, which is Arabic for The Test. The
two stars are also called the Horse and the Rider.
Mizar forms with Alcor what is known as a wide double star. It is, in
fact, the widest of all double stars. Many stars in the heavens that
appear single to us are separated by the telescope into double or
triple or multiple stars. They consist of two or more suns revolving
about a common center, known as their center of gravity. Sometimes
the suns are so close together that even the most powerful telescope
will not separate them. Then a most wonderful little instrument,
called the spectroscope, steps in and analyzes the light of the stars
and shows which are double and which are single. A star shown to be
double by the spectroscope, but not by the telescope, is called a
spectroscopic binary star.
Mizar is of historic interest, as being the first double star to
be detected with the aid of the telescope. A very small telescope
will split Mizar up into two stars. The brighter of the two is a
spectroscopic binary star beside, so that it really consists of two
suns instead of one, with the distance between the two so small that
even the telescope cannot separate them. About this system of three
suns which we know as the star Mizar, the faint star Alcor revolves
at a distance equal to sixteen thousand times the distance of the
earth from the sun.
[Illustration: MAY--LEO]
If we follow the imaginary line drawn through the Pointers in a
_southerly_ direction about forty-five degrees, we come to Leo,
The Lion, one of the zodiacal constellations. There should be
no difficulty in finding the constellation Leo, as its peculiar
sickle-shaped group of bright stars makes it distinctive from all
other constellations. At the time we have mentioned, that is, the
early evening hours, it will lie a little to the southwest of the
zenith. Leo is one of the finest constellations and is always
associated with the spring months because it is then high in the sky
in the evening. Regulus is the beautiful white star which marks
the handle of The Sickle, and the heart of Leo; and Denebola is the
second-magnitude star in the tail of Leo.
[Illustration: MAY--CORVUS AND CRATER]
Due south of Denebola, about thirty degrees, we find the small
star-group known as Crater, The Cup, which is composed of rather
faint and inconspicuous stars. Just east of Crater is the group
known as Corvus, The Crow, which forms a very characteristic little
four-sided figure of stars differing very little from one another
in brightness. These two star groups lie far to the south in our
latitudes; but if we lived twenty degrees south of the equator, we
would find them nearly overhead, at this time of the year. Just south
of Corvus and Crater we find Hydra, one of the constellations for
April which extends beneath these constellations and also beneath
Virgo, one of the June constellations.
VII
JUNE
The star-groups that occupy the center of the celestial stage in
mid-latitudes of the northern hemisphere during the early evening
hours of June are Boötes, often called The Hunter, (although the word
means Herdsman or Shouter), which will be found overhead at this
time; Virgo, The Maiden, largest of the zodiacal constellations,
lying nearly due south; Canes Venatici, The Hunting Dogs; Corona
Borealis, The Northern Crown, and Coma Berenices.
The gorgeous orange-hued Arcturus in Boötes and the beautiful
bluish-white Spica in Virgo, like a diamond in its sparkling
radiance, form with Denebola in Leo, which we identified in May, a
huge equal-sided triangle that is always associated with the spring
and early summer months.
To the west of Boötes, below the handle of the Big Dipper, is a
region where there are few conspicuous stars. Here will be found
Canes Venatici (The Hunting Dogs with which Boötes is supposed to be
pursuing the Great Bear around the north pole), and, further south,
Coma Berenices (Bernice's Hair).
The brighter of the two Hunting Dogs, which is also the brightest
star in the entire region covered by these two constellations,
appears as a beautiful blue-and-yellow double star in the telescope.
It was named Cor Caroli (Heart of Charles) by the astronomer Halley
in honor of Charles II of England, at the suggestion of the court
physician, who imagined it shone more brightly than usual the
night before the return of Charles to London. Of more interest to
astronomers is the magnificent spiral nebula in this constellation,
known as the "Whirlpool Nebula," appearing as a faint, luminous patch
in the sky, of which many photographs have been taken with the great
telescopes. This entire region, from Canes Venatici to Virgo, abounds
in faint spiral nebulæ that for some reason not yet understood by
astronomers are crowded together in this part of the heavens where
stars are comparatively few. It is believed that there are between
five hundred thousand and a million of these spiral nebulæ in the
entire heavens, and the problem of their nature and origin and
distance is one that the astronomers are very anxious to solve. Many
wonderful facts are now being learned concerning these faint nebulous
wisps of light which, with a few exceptions, are observable only with
great telescopes. They reveal their spiral structure more clearly to
the photographic plate than to the human eye, and some magnificent
photographs of them have been taken with powerful telescopes.
Coma Berenices, south of Canes Venatici and southwest of Boötes, is
a constellation that consists of a great number of stars closely
crowded together, and just barely visible to the unaided eye. As
a result, it has the appearance of filmy threads of light, which
doubtless suggested its name to the imaginative ancients, who loved
to fill the heavens with fanciful creations associated with their
myths and legends. These stars form a moving cluster of stars
estimated to be at a distance of about 270 light-years from the solar
systems.
[Illustration: JUNE--BOÖTES, CANES VENATICI AND COMA BERENICIS]
This region, so lacking in interesting objects for the naked-eye
observer, is a mine of riches to the fortunate possessors of
telescopes; and the great telescopes of the world are frequently
pointed in this direction, exploring the mysteries of space that
abound here.
Just to the east of Boötes is the exquisite little circlet of stars
known as Corona Borealis, the Northern Crown. It consists of six
stars arranged in a nearly perfect semicircle, and one will have no
difficulty in recognizing it. Its brightest star, Alpha, known also
by the name of Alphacca, is a star of the second magnitude.
Boötes is one of the largest and finest of the northern
constellations. It can be easily distinguished by its peculiar
kite-shaped grouping of stars or by the conspicuous pentagon
(five-sided figure) of stars which it contains. The most southerly
star in this pentagon is known as Epsilon Boötes and is one of
the finest double stars in the heavens. The two stars of which it
consists are respectively orange and greenish-blue in color.
By far the finest object in Boötes, however, is the magnificent
Arcturus, which is the brightest star in the northern hemisphere
of the heavens. This star will be conspicuous in the evening hours
throughout the summer months, as will also the less brilliant Spica
in Virgo.
Some recent measurements show that Arcturus is one of our nearer
neighbors among the stars. Its distance is now estimated to be about
twenty-one light-years. That is, a ray of light from this star takes
twenty-one years to reach the earth, traveling at the rate of one
hundred and eighty-six thousand miles per second. It would seem as
if we should hardly speak of Arcturus, twenty-one light-years away,
as a near neighbor, yet there are millions of stars that are far
more distant from the earth, and very few that are nearer to us than
Arcturus.
The brightness of Arcturus is estimated to be about forty times that
of the sun. That is, if the two bodies were side by side, Arcturus
would give forth forty times as much light and heat as the sun.
Arcturus is also one of the most rapidly moving stars in the heavens.
In the past sixteen centuries it has traveled so far as to have
changed its position among the other stars by as much as the apparent
width of the moon. Most of the stars, in spite of their motions
through the heavens in various directions, appear today in the same
relative positions in which they were several thousand years ago. It
is for this reason that the constellations of the Egyptians and of
the Greeks and Romans are the same constellations that we see in the
heavens today. Were all the stars as rapidly moving as Arcturus, the
distinctive forms of the constellations would be preserved for only a
very few centuries.
[Illustration: JUNE--VIRGO]
Virgo, which lies south and southwest of Boötes, is a large,
straggling constellation, consisting of a Y-shaped configuration of
rather inconspicuous stars. It lies in the path of our sun, moon and
planets, and so is one of the zodiacal constellations. The cross in
the diagram indicates the present position of the autumnal equinox,
the point where the sun crosses the equator going south, and the
position the sun now occupies at the beginning of fall.
Spica, the brightest star in Virgo, is a bluish-white,
first-magnitude star, standing very much alone in the sky. In fact,
the Arabs referred to this star as "The Solitary One." Its distance
from the earth is not known, but must be very great as it cannot be
found by the usual methods. The spectroscope shows that it consists
of two suns very close together, revolving about a common center in a
period of only four days.
Within the branches of the "Y" in Virgo, and just to the north of it,
is the wonderful nebulous region of this constellation, but it takes
a powerful telescope to show the faint spiral nebulæ that exist here
in great profusion. All of these spirals are receding from the plane
of the Milky Way with enormous velocities. The spiral nebulæ are, in
fact, the most rapidly moving objects in the heavens.
VIII
JULY
Due east of the little circlet of stars known as Corona Borealis, and
almost directly overhead in our latitude (40° N.) about nine o'clock
in the evening in the early part of July, is the large constellation
of Hercules, named for the famous hero of Grecian mythology. There
are no stars of great brilliancy in this group, but it contains a
large number of fairly bright stars arranged in the form outlined in
the chart. The hero is standing with his head, marked by the star
Alpha Herculis, toward the south, and his foot resting on the head of
Draco, The Dragon, a far-northern constellation with which we become
acquainted in August.
[Illustration: JULY--HERCULES]
Alpha Herculis, the best known star in this constellation, is of
unusual interest. Not only is it a most beautiful double star, the
brighter of the two stars of which it is composed being orange, and
the fainter greenish-blue, but it is also a star that changes in
brightness irregularly. Both the orange and the blue star share in
this change of brightness. There are a number of stars in the heavens
that vary in brightness, some in very regular periods, and others,
like Alpha Herculis, irregularly. These latter stars are nearly
always deep orange or reddish in color. One may note this variation
in the brightness of Alpha Herculis by comparing it from time to time
with some nearby star that does not vary in brightness.
[Illustration: THE GREAT HERCULES CLUSTER--A UNIVERSE OF SUNS
Taken with 60-inch Reflector of the Mt. Wilson Observatory]
The constellation of Hercules is a very rich field for the possessor
of even a small telescope. Here are to be found beautifully colored
double stars in profusion, and, in addition, two remarkable clusters
of stars. The brighter of the two is known as the Great Hercules
Cluster. Its position is shown on the chart, and, under favorable
conditions--that is, on a clear, dark night, when there is no
moonlight--it may be seen without the aid of a telescope as a small,
faint patch of light. One would never suspect from such a view what
a wonderful object this cluster becomes when seen with the aid of a
powerful telescope. Photographs taken with the great telescopes show
this faint wisp of light as a magnificent assemblage of thousands of
stars, each a sun many times more brilliant than our own sun. The
crowded appearance of the stars in the cluster is due partly to the
fact that it is very distant from the earth, though neighboring stars
in the cluster are indeed much nearer to one another than are the
stars in the vicinity of our solar system. It has been found that
this cluster is so far away that its light takes over thirty-six
thousand years to reach the earth. At the distance of this cluster,
a sun equal in brightness to our own sun would be so faint that the
most powerful telescope in the world would not show it. So we know
that the stars that are visible in the Hercules cluster are far more
brilliant than our sun. A fair-sized telescope will show about four
thousand stars in this cluster, but the greatest telescopes show over
one hundred thousand in it, and there are without doubt many more too
faint to be seen at all. The Hercules cluster is called a globular
star-cluster, because the stars in it are arranged nearly in the form
of a sphere. There are in the heavens about ninety such clusters
whose distances have been found, and they are among the most distant
of all objects. Most of them are very faint, and a few are over two
hundred thousand light-years distant from the earth. The Hercules
cluster is one of the nearest and is the most noted of all of these
globular clusters. It is considered to be one of the finest objects
in the heavens. The other cluster in Hercules is also very fine, but
not to be compared with this one.
[Illustration: JULY--OPHIUCHUS AND SERPENS]
Just to the south of Hercules are two constellations, Ophiuchus, The
Serpent-Bearer, and Serpens, The Serpent, which are so intermingled
that it is difficult to distinguish them. There are in these two
constellations, as in Hercules, no stars of unusual brilliancy,
but a large number of fairly bright stars. The brightest star in
Ophiuchus is known as Alpha Ophiuchi and it marks the head of the
Serpent-Bearer. The two stars, Alpha Ophiuchi and Alpha Herculis,
are close together, being separated by a distance about equal to
that between the Pointers of the Big Dipper. Alpha Ophiuchi is the
brighter of the two, and it is farther east.
Ophiuchus, according to one legend, was once a physician on earth,
and was so successful as a healer that he could raise the dead.
Pluto, the god of the lower world, became alarmed for fear his
kingdom would become depopulated, and persuaded Jupiter to remove
Ophiuchus to a heavenly abode, where he would be less troublesome.
The serpent is supposed to be a symbol of his healing powers. The
head of Serpens is marked by a group of faint stars just south of
Corona Borealis and southwest of Hercules. From here a line of fairly
bright stars marks the course of Serpens southward to the hand of
Ophiuchus. Two stars close together and nearly equal in brightness
mark the hand with which the hero grasps the body of the serpent. The
other hand is marked by an equally bright single star some distance
to the eastward where the two constellations again meet. Ophiuchus
is thus represented as holding the serpent with both hands. It is
not an easy matter to make out the outlines of these straggling
groups, but there are in them several pairs of stars nearly equal in
brightness and about as evenly spaced as the two stars in the one
hand of Ophiuchus, and these, as well as the diagram, will be of aid
in tracing the two groups.