ASTRONOMY
FOR YOUNG FOLKS
[Illustration: NORTHERN PORTION OF THE MOON AT LAST QUARTER
Taken with 100-inch Hooker Telescope of the Mt. Wilson Observatory
(See Chapter XXI)]
ASTRONOMY
_for_
YOUNG FOLKS
BY
ISABEL MARTIN LEWIS, A. M.
(_Connected with the Nautical Almanac Office
of the U. S. Naval Observatory_)
NEW YORK
DUFFIELD AND COMPANY
1922
Copyright, 1921, by
THE CENTURY COMPANY
Copyright, 1922, by
DUFFIELD AND COMPANY
Printed in U. S. A.
CONTENTS
CHAPTER PAGE
Preface xiii
I. The Constellations 3
II. January 15
III. February 21
IV. March 28
V. April 35
VI. May 41
VII. June 49
VIII. July 56
IX. August 64
X. September 71
XI. October 78
XII. November 84
XIII. December 90
XIV. Stars of the Southern Hemisphere 96
XV. The Milky Way or Galaxy 107
XVI. The Surface of the Sun 113
XVII. The Solar System 119
XVIII. The Origin of the Earth 127
XIX. Jupiter and His Nine Moons 139
XX. The Rings and Moons of Saturn 148
XXI. Is the Moon a Dead World 156
XXII. Comets 165
XXIII. Meteorites 173
XXIV. The Earth As a Magnet 183
XXV. Some Effects of the Earth's Atmosphere Upon
Sunlight 193
XXVI. Keeping Track of the Moon 207
XXVII. The Motions of the Heavenly Bodies 216
XXVIII. The Evolution of the Stars--From Red Giants
to Red Dwarfs 225
XXIX. Double and Multiple Stars 230
XXX. Astronomical Distances 241
XXXI. Some Astronomical Facts Worth Remembering 250
ILLUSTRATIONS
PAGE
Northern Portion of the Moon at Last Quarter _Frontispiece_
The Great Hercules Cluster--A Universe of Suns _facing page_ 56
A Dark Nebula: The Dark Bay or Dark Horse
Nebula in Orion _facing page_ 110
A. Venus. B. Mars. C. Jupiter. D. Saturn _facing page_ 122
Spiral Nebula in Canes Venatici _facing page_ 216
Spiral Nebula in Andromeda Viewed Edgewise _facing page_ 222
LIST OF TABLES
PAGE
I. The Principal Elements of the Solar System 261
II. The Satellites of the Solar System 262
III. The Twenty Brightest Stars in the Heavens 263
IV. A List of the Principal Constellations 264-265
V. Pronunciations and Meanings of Names of
Stars and Constellations 266-267
PREFACE
Astronomy, it has been said, is the oldest and the noblest of
the sciences. Yet it is one of the few sciences for which most
present-day educators seem to find little, if any, room in their
curriculum of study for the young, in spite of its high cultural
value. It is, we are told, too abstruse a subject for the youthful
student. This is doubtless true of theoretical or mathematical
astronomy and the practical astronomy of the navigator, surveyor and
engineer, but it is not true of general, descriptive astronomy. There
are many different aspects of this many-sided science, and some of
the simplest and grandest truths of astronomy can be grasped by the
intelligent child of twelve or fourteen years of age.
Merely as a branch of nature study the child should have some
knowledge of the sun, moon, stars and planets, their motions and
their physical features, for they are as truly a part of nature as
are the birds, trees and flowers, and the man, woman or child who
goes forth beneath the star-lit heavens at night absolutely blind to
the wonders and beauties of the universe of which he is a part, loses
as much as the one who walks through field or forest with no thought
of the beauties of nature that surround him.
The astronomer is the pioneer and explorer of today in realms
unknown just as the pioneers and explorers of several centuries ago
were to some extent astronomers as they sailed unknown seas and
traversed unexplored regions. As the years pass by the astronomer
extends more and more his explorations of the universe and brings
back among the fruits of discovery measures of giant suns and
estimates of the form and extent of the universe, views of whirling,
seething nebulæ, mysterious dark clouds drifting through space,
tremendous solar upheavals or glimpses of strangely marked surfaces
of nearby planets.
In the following pages the author has endeavored to tell in words
not beyond the comprehension of the average fourteen-year-old child
something of the nature of the heavenly bodies. In Part I an effort
is made to make the child familiar with the stars by indicating when
and where they can be found in the early evening hours. In addition
to identifying the principal constellations and their brightest stars
by means of diagrams an attempt has been made to acquaint the child
with the most interesting recent discoveries that have been made
concerning the principal stars or objects in each group as well as
with some of the stories and legends that have been associated with
these groups of stars for centuries, and that have been handed down
in the folk-lore of all nations.
Chapters 2-13, inclusive, appeared originally with diagrams similar
to those shown here, under the department of Nature and Science
for Young Folk in _St. Nicholas_ from May, 1921, to April, 1922,
inclusive. The Introductory Chapter and Chapters 14 and 15, on the
Milky Way and Stars of the Southern Hemisphere, respectively, are
published here for the first time, as is also the chapter in Part II
on the Evolution of the Stars from Red Giants to Red Dwarfs, which
gives the order of the evolution of the stars as now accepted as a
result of the brilliant astronomical researches of Dr. Henry Norris
Russell of the United States and Prof. A. S. Eddington, of England.
The remaining chapters in Part II have been chosen from a series of
articles that have appeared in _Science and Invention_, formerly
_The Electrical Experimenter_, in the past four years, and have been
considerably revised and in some parts rewritten to adapt them to the
understanding of more youthful readers. These chapters deal with a
variety of astronomical subjects of general popular interest and an
effort has been made to select subjects that would cover as wide an
astronomical field as possible in a limited space.
The author's aim has not been to write a text-book of astronomy or
to treat in detail of any one aspect of this extensive science, but
simply to give the average child some general knowledge of the nature
of the heavenly bodies, both those that form a part of our own solar
system and those that lie in the depths of space beyond.
It has been necessary to write very briefly, and we feel
inadequately, of many topics of special interest such as the sun and
moon. Books have been written on these two subjects alone as well as
upon such subjects as Mars, eclipses, comets, meteors, etc., but the
object has been to acquaint the child with the outstanding features
of a variety of celestial objects rather than to treat of a few in
detail.
If the writer succeeds in arousing the child's interest in the stars
so that he may look forth with intelligence at the heavens and
greet the stars as friends and at the same time grasps some of the
simplest and most fundamental of astronomical truths such as the
distinction between stars and planets, the motions of the heavenly
bodies and their relative distances from us and the place of our own
planet-world in the universe, this book will have served its purpose.
ASTRONOMY
FOR YOUNG FOLKS
"The heavens declare the glory of God, and the firmament showeth
His handiwork."
Psalm XIX.
I
THE CONSTELLATIONS
"Canst thou bind the sweet influences of the Pleiades
Or loose the bands of Orion?
Canst thou bring forth Mazzaroth in his season
Or canst thou guide Arcturus with his sons?"
--BOOK OF JOB.
Who would not like to know the stars and constellations by their
names and in their seasons as we know the birds and the trees and
the flowers, to recognize at their return, year by year, Sirius and
Spica, Arcturus and Antares, Vega and Altair, to know when Ursa Major
swings high overhead and Orion sinks to rest beneath the western
horizon, when Leo comes into view in the east or the Northern Crown
lies overhead?
Often we deprive ourselves of the pleasure of making friends with the
stars and shut our eyes to the glories of the heavens above because
we do not realize how simple a matter it is to become acquainted with
the various groups of stars as they cross our meridian, one by one,
day after day and month after month in the same orderly sequence.
When the robin returns once more to nest in the same orchard in the
spring time, Leo and Virgo may be seen rising above the eastern
horizon in the early evening hours. When the first snow flies in
the late fall and the birds have all gone southward the belt of
Orion appears in the east and Cygnus dips low in the west. When we
once come to know brilliant blue-white Vega, ruddy Arcturus, golden
Capella and sparkling Sirius we watch for them to return each in its
proper season and greet them as old friends.
In the following pages we give for each month the constellations
or star-groups that are nearest to our meridian, that is, that lie
either due north or due south or exactly overhead in the early part
of the month and the early part of the evening.
We do not need to start our study of the constellations in January.
We may start at any month in the year and we will find the
constellations given for that month on or near the meridian at the
time indicated.
In using the charts or diagrams of the constellations, we should hold
them in an inverted position with the top of the page toward the
north or else remember that the left-hand side of the page is toward
the _east_ and the right-hand side of the page toward the _west_,
which is the opposite of the arrangement for charts and maps of the
earth's surface.
We should also bear in mind that the constellations are all
continually shifting westward for the stars and the moon and the
planets as well as the sun rise daily in the east and set in the
west. This is due to the fact that the earth is turning in the
opposite direction on its axis, that is from west to east. In
twenty-four hours the earth turns completely around with respect to
the heavens or through an angle of 360°, so in one hour it turns
through an angle of 360° ÷ 24 or 15°. As a result the stars appear
to shift westward 15° every hour. This is a distance about equal in
length to the handle of the Big Dipper, which I am sure we all know,
even if we do not know another constellation in the heavens.
If, then, we look at the heavens at a later hour than that for which
the constellations are given we will find them farther westward and
if our time of observation is earlier in the evening than the hour
mentioned we will find them farther eastward.
In the course of a year the earth makes one trip around the sun and
faces in turn all parts of the heavens. That is, it turns through
an angle of 360° with respect to the heavens in a year or through
an angle of 360° ÷ 12 or 30° in one month. So as a result of our
revolution around the sun, which is also in a west to east direction,
we see that all the constellations are gradually shifting westward at
the rate of 30° a month. It is for this reason that we see different
constellations in different months, and it is because of the turning
of the earth on its axis that we see different constellations at
different hours of the night.
If we should sit up from sunset to sunrise and watch the stars rise
in the east, pass the meridian and set in the west--as the sun does
by day--we should see in turn the same constellations that are to
pass across the heavens in the next six months. This is because in
twelve hours' time we are carried through the same angle with respect
to the heavens by the earth's rotation on its axis that we are in the
next six months by the motion of the earth around the sun.
Let us suppose then that the time we choose for our observation of
the heavens is the last of the month while our charts are given
for the first of the month. We must look then farther westward
for our constellations just as we must look farther westward if
we chose a later hour in the evening for our observations. Let us
suppose that we choose for our time of observation half-past eight
in the early part of December. On or close to the meridian we will
find the constellations outlined in the charts for December. To
the east of the meridian we will find the constellations that are
given for January and February, and to the west of the meridian
the constellations that are given for November and October. So if
we are particularly ambitious or wish to become acquainted with
the constellations more rapidly we may study at the same time the
constellations for the preceding months now west of the meridian and
the constellations for the following months now east of the meridian
as well as the constellations for the month which will be due north
or south or directly overhead as the case may be.
If we were able to see the stars by day as well as by night we
would observe that as the days go by the sun is apparently moving
continuously eastward among certain constellations. This is a result
of the earth's actual motion around the sun in the same direction.
The apparent path of the sun among the stars is called the ecliptic
and the belt of the heavens eight degrees wide on either side of the
ecliptic is called the zodiac. The constellations that lie within
this belt of the zodiac are called zodiacal constellations. The
zodiac was divided by the astronomer Hipparchus, who lived 161-126
B.C., into twelve signs 30° wide, and the signs were named for the
constellations lying at that time within each of these divisions.
These zodiacal constellations are Aries, Taurus, Gemini, Cancer, Leo,
Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius and Pisces.
With the exception of Libra, the Scales, all of these constellations
are named for people or animals and the word zodiac is derived from
the Greek word meaning "of animals."
Each month the sun moves eastward 30° through one of these zodiacal
constellations. In the days of Hipparchus the sun was in Aries at
the beginning of spring, at the point where the ecliptic crosses the
celestial equator--which lies directly above the earth's equator.
This point where the ecliptic crosses the equator was then known
as the First Point in Aries. The autumnal equinox was 180° distant
in Aquarius and the two points were called the equinoxes because
when the sun is at either equinox the day and night are equal in
length all over the world. Now for certain reasons which we will not
explain here the equinoctial points are not fixed in position but
shift gradually westward at the rate of 1° in 70 years. It is as if
the equinoxes were advancing each year to meet the sun on its return
and their westward motion is therefore called "The Precession of the
Equinoxes."
Since the days of Hipparchus this motion has amounted to about 30° so
that the constellations no longer occupy the signs of the zodiac that
bear their names.
The sun is now in Pisces instead of Aries at the beginning of spring
and in Virgo instead of Aquarius at the beginning of fall.
Not only the sun but the moon and planets as well move through the
zodiacal constellations. In fact a limit for the zodiac of 8° on
either side of the ecliptic was chosen because it marks the extent
of the excursions of the moon and planets from the ecliptic. Neither
moon nor planets will be found at a greater distance than 8° on
either side of the ecliptic.
For convenience in determining the positions of the heavenly bodies
the astronomer assumes that they lie upon the surface of a celestial
sphere that has its center at the center of the earth.
The north pole of the celestial sphere lies directly above the north
pole of the earth and the south pole of the celestial sphere directly
above the south pole of the earth. The celestial equator is the
great circle of the celestial sphere that lies midway between its
north and south poles and directly above the earth's equator. The
ecliptic is also a great circle of the celestial sphere and cuts the
celestial equator at an angle of 23-1/2° in the two points 180° apart
known as the equinoctial points, of which we have already spoken.
The zodiacal constellations lie nearly overhead within the tropics
and can be seen to advantage all over the world except in polar
regions.
For every position of the earth's surface except at the equator we
have also our circumpolar constellations which are the ones that
never pass below the horizon for the place of observation.
In 40° N. Latitude the Big Dipper is a circumpolar constellation for
it is above the horizon at all hours of the day and night and all
times of the year. If our latitude is 40° N., all stars within 40° of
the north pole of the heavens are circumpolar and never set, while
stars within 40° of the south pole of the heavens never rise. All
other stars rise and set daily.
If we were at the north pole all stars within 90° of the north pole
of the heavens would be circumpolar and would describe daily circuits
of the pole parallel to the horizon remaining always above it.
If we were at the equator all stars within _zero_ degrees of either
pole would be circumpolar, that is _no_ stars would be circumpolar,
all stars rising and setting daily.
As a general rule, then, we may say that stars within an angular
distance of the nearest pole of the heavens equal to the latitude
never set and stars within an equal distance of the opposite pole
never rise while all stars outside of these limits rise and set daily.
The beginner who attempts to make the acquaintance of the principal
stars and constellations occasionally may find a bright star in a
constellation that is not noted in the diagrams. In this case he has
probably happened upon one of the bright planets.
It is not possible to include the planets in our diagrams for the
reason that they are not fixed in position but apparently wander
among the stars. The name planet is, in fact, derived from a Greek
word meaning "wanderer." The stars shine by their own light but the
planets shine only by reflected light from the sun. Of the seven
planets in the solar system additional to our own planet earth,
there are two, Uranus and Neptune that we need not consider for
Neptune is not visible without the aid of a telescope and Uranus is
fainter than any of the stars included in our diagrams.
Mercury will never appear except in the morning or evening twilight,
when none but the very brightest stars are visible, since it never
departs far from the sun. It will only be seen under certain
favorable conditions, and usually it will escape our notice
altogether unless we know exactly where to look for it although
there are but two or three stars in the heavens that surpass it in
brightness.
Venus, we will probably never mistake for any star in the heavens for
it far surpasses all stars in brightness. It will always be seen in
the west after sunset or in the east before sunrise and it is never
seen more than three hours before or after the sun.
This leaves us but three planets, Jupiter, Saturn and Mars that we
may mistake for bright stars. There is little chance that Jupiter
will be thus mistaken for it also is far brighter than all of the
stars except Sirius which differs greatly from Jupiter in color.
Sirius is a brilliant white and Jupiter is a golden yellow. The
planets do not twinkle as the stars do and this is particularly true
of Jupiter which is remarkable for the quiet steadiness of its yellow
light. This alone would serve to identify it.
Saturn is probably mistaken for a star oftener than any of the other
planets. It moves so slowly among the stars that we would have to
watch it for a number of successive evenings before we could discover
that it is moving with respect to the stars. Saturn is yellowish in
color and we can probably best distinguish it by the steadiness of
its light. If we find in one of the zodiacal groups of stars--for
the planets appear among no other constellations--a bright yellowish
star where no bright star is indicated on the diagram we may be
reasonably certain that we have found the planet Saturn.
Mars is the only planet that is reddish in color. Once in fifteen
or seventeen years, when it is particularly near to the earth, it
surpasses even Jupiter in brightness, but ordinarily it appears
no more brilliant than one of the brighter stars. There are only
two stars with which we are likely to confuse Mars,--Aldebaran and
Antares--which are very similar to it in color, and, at times, in
brightness. Moreover, both of these stars are zodiacal stars and Mars
frequently passes through the constellations to which they belong.
There should be no trouble about identifying Aldebaran and Antares,
however, from their distinctive positions in the diagrams so that any
other reddish star appearing in any of the zodiacal groups we may
feel certain is the planet Mars.
In the following diagrams of the constellations the brightest
and most conspicuous stars, called first-magnitude stars, are
represented by white stars. These are the stars we should all be
able to recognize and call by name and in every instance the name of
a first-magnitude star is given on the diagram. All other stars are
represented by circles, and the size of the circle is an indication
of the brightness of the star.
Stars visible without the aid of a telescope are referred to usually
as "naked-eye stars." They are classed as first, second, third,
four, fifth or sixth magnitude stars, according to their relative
brightness. A star of the first magnitude is about two and one-half
times brighter than a star of the second magnitude, which in turn is
two and one-half times brighter than a star of the third magnitude
and so on. A first-magnitude star is, then, one hundred times
brighter than a sixth magnitude star which is the faintest star that
can be seen without the aid of the telescope.
This ratio between successive magnitudes continues among the
telescopic stars. A star of the sixth magnitude is one hundred times
brighter than a star of the eleventh magnitude which in turn is one
hundred times brighter than a star of the sixteenth magnitude.
The faintest stars that can be seen visually in the greatest
telescopes are of the seventeenth or eighteenth magnitude, though
stars two or three magnitudes fainter can be photographed.
The faintest stars shown in the diagrams are fifth-magnitude stars
and stars of this magnitude as well as stars of the fourth magnitude
are only given when needed to fill out the distinctive outlines of
the constellations which have been formed by connecting the principal
stars in each group by dotted lines.
All stars of first, second and third magnitude are given in the
diagrams without exceptions as such stars are visible to everyone on
clear nights.
The constellations given in the following pages include practically
all of the constellations that can be seen in 40° N. Latitude. A
diagram is given for each constellation.
In this latitude it is impossible to see the constellations of the
southern hemisphere that lie within 40° of the south pole of the
heavens. A brief chapter with diagram treats of these constellations
that are invisible in mid-latitudes of the northern hemisphere.
II
JANUARY
One of the most easily recognized constellations in the heavens is
Taurus, The Bull, a zodiacal group which lies just south of the
zenith in our latitudes in the early evening hours about the first of
January.
Taurus is distinguished by the V-shaped group of The Hyades,
which contains the bright, red, first-magnitude star Aldebaran,
representing the fiery eye of the bull. It also contains the famous
cluster of faint stars known as The Pleiades, lying a short distance
northwest of The Hyades.
No group of stars is more universally known than The Pleiades.
All tribes and nations of the world, from the remotest days of
recorded history up to the present time, have sung the praises of
The Pleiades. They were "The Many Little Ones" of the Babylonians,
"The Seven Sisters" of the Greeks, "The Seven Brothers" of the
American Indians, "The Hen and Chickens" of many nations of Europe,
"The Little Eyes" of the South Sea Islanders. They were honored in
the religious ceremonies of the Aztecs, and the savage tribes of
Australia danced in their honor. Many early tribes of men began their
year with November, the Pleiad month; and on November 17th, when
The Pleiades crossed the meridian at midnight, it was said that no
petition was ever presented in vain to the kings of ancient Persia.
[Illustration: JANUARY--TAURUS]
Poets of all ages have felt the charm of The Pleiades. Tennyson gives
the following beautiful description of The Pleiades in _Locksley
Hall_:
"Many a night I saw the Pleiades, rising through the mellow shade,
Glitter like a swarm of fireflies tangled in a silver braid."
A well-known astronomer, not so many years ago, also felt the
mysterious charm of The Pleiades and seriously expressed the belief
that Alcyone, the brightest star of The Pleiades, was a central sun
about which all other suns were moving. But we know that there is no
foundation whatever for such a belief.
A fairly good eye, when the night is clear and dark, will make out
six stars in this group arranged in the form of a small dipper. A
seventh star lies close to the star at the end of the handle and is
more difficult to find. It is called Pleione, and is referred to in
many legends as the lost Pleiad. Persons gifted with exceptionally
fine eyesight have made out as many as eleven stars in the group;
and with the aid of an ordinary opera-glass, anyone can see fully
one hundred stars in this cluster. Astronomers have found that The
Pleiades cluster contains at least two hundred and fifty stars, all
drifting slowly in the same general direction through space, and that
the entire group is enveloped in a fiery, nebulous mist which is
most dense around the brightest stars. It is not known whether the
stars are being formed from the nebula or whether the nebula is being
puffed off from the stars. The brightest star, Alcyone, is at least
two hundred times more brilliant than our own sun, and all of the
brighter stars in the group surpass the sun many times in brightness.
It is believed that this cluster is so large that light takes many
years to cross from one end of it to the other, and that it is so far
from the earth that its light takes over three centuries to reach
us, traveling at the rate of 186,000 miles a second.
The Hyades is a group of stars scarcely less famous than The
Pleiades, and the stars in the group also form a moving cluster of
enormous extent at a distance of 140 light-years from the earth.
Among the ancients, The Hyades were called the rain-stars, and the
word Hyades is supposed to come from the Greek word for rain. Among
the many superstitions of the past was the belief that the rising or
setting of a group of stars with the sun had some special influence
over human affairs. Since The Hyades set just after the sun in the
showery springtime and just before sunrise in the stormy days of late
fall, they were always associated with rain. In Tennyson's _Ulysses_
we read:
"Through scudding drifts the rainy Hyades
Vex'd the dim sea."
The Hyades outline the forehead of Taurus, while two bright stars
some distance to the northeast of the V form the tips of the
horns. Only the head and forequarters of the bull are shown in the
star-atlases that give the mythological groups, for, according to one
legend, he is swimming through the sea and the rest of his body is
submerged. According to another legend, Taurus is charging down upon
Orion, The Warrior, represented by the magnificent constellation just
to the southeast of Taurus, of which we shall have more to say next
month.
Aldebaran is the Arabic word for "The Hindmost," and the star is so
called because it follows The Pleiades across the sky. It is one of
the most beautiful of all the many brilliant stars visible at this
time and we might profit by following the advice of Mrs. Sigourney in
_The Stars_:
"Go forth at night
And talk with Aldebaran, where he flames
In the cold forehead of the wintry sky."
Next to Aldebaran in the V is the interesting double star Theta,
which we can see as two distinct stars without a telescope.
Directly south of Taurus is Eridanus, sometimes called Fluvius
Eridanus, or The River Eridanus. Starting a little to the southeast
of Taurus, close to the brilliant blue-white star Rigel in Orion, it
runs to the westward for a considerable distance in a long curving
line of rather faint stars, bends sharply southward for a short
distance, then curves backward toward the east once more, and, after
running for some distance, makes another sharp curve to the southwest
and disappears below the southern horizon. Its course is continued
far into the southern hemisphere. Its brightest star, Achernar, is
a star of the first magnitude, but it lies below the horizon in our
latitudes.
[Illustration: JANUARY--ERIDANUS]
Eridanus contains no star of particular interest to us. Most of the
numerous stars that mark its course are of the fourth and fifth
magnitude. It contains but two stars of the third magnitude, one
at the beginning of its course and one close to the southwestern
horizon. The beautiful constellation of Perseus lies just to the
north of Taurus and should rightfully be considered among the
constellations lying nearest to the meridian in January, but we give
this constellation among the star groups for December because of
its close association with the nearby constellations Andromeda and
Pegasus in legend and story.
III
FEBRUARY
Across the meridian, due south, between eight and nine o'clock in
the evening in the early part of February, lies Orion, The Warrior,
generally considered to be the finest constellation in the heavens.
Orion is directly overhead at the equator, and so is seen to
advantage from all parts of the world except the extreme northern and
southern polar regions.
A group of three faint stars outlines the head of Orion. His right
shoulder is marked by the deep-red, first-magnitude star Betelgeuze
(meaning armpit), and his left shoulder by the bright white star
Bellatrix, The Amazon. Orion stands facing Taurus, The Bull, and
brandishes in his right hand a club, outlined by a number of faint
stars extending from Betelgeuze toward the northeast. The top of the
club lies near the tips of the horns of Taurus. In his left hand he
holds up a lion's skin, which we can trace in another curving line of
faint stars to the west and northwest of Bellatrix. The brilliant,
blue-white, first-magnitude star Rigel lies in the left foot, and the
second-magnitude star Saiph, a little to the east of Rigel, is in
the right knee. Three evenly spaced stars lying in a straight line
that is exactly three degrees in length form the Belt of Orion,
and from the Belt hangs the Sword of Orion, outlined by three faint
stars. The central star in the Sword appears somewhat blurred and is
the multiple star Theta, in the midst of the great Orion nebula, the
finest object of its kind in the heavens. Entangled in the meshes of
this glowing nebula are a number of brilliant suns, appearing to us
as faint stars because of their great distance. The star Theta, in
the heart of the nebula, is seen with a powerful telescope to consist
of six stars; that is, it is a sextuple star. Even with a small
telescope, four of these stars can readily be seen, arranged in the
form of a small trapezium. The lowest star in the Sword is a triple
star, and the entire constellation abounds in double, triple, and
multiple stars.
From the central portion of the nebula extend many branches and
streamers of nebulous light, and it is known that the entire
constellation of Orion is enwrapped in the folds of this nebulosity,
which forms a glowing, whirling mass of fiery gases in rapid
rotation. This constellation is remarkable for the fact that all of
its brighter stars, with the exception of the deep-red Betelgeuze,
form one enormous, connected group of stars. They are all more or
less associated with the great nebula and its branches, and are all
extremely hot, white or bluish-white stars, known as helium stars,
because the gas helium is so conspicuous in their atmospheres. The
Orion stars are the hottest and brightest of all the stars.
Blazing Rigel, Bellatrix, and Saiph, marking three corners of the
great quadrilateral, of which Betelgeuze marks the fourth corner, are
all brilliant helium stars. So are the three stars in the Belt and
the fainter stars in the Sword and the great nebula.
[Illustration: FEBRUARY--ORION]
It has been estimated that the great Orion group of stars is over
six hundred light-years from the earth, or about forty million times
more distant than the sun. For more than six centuries the rays of
light that now enter our eyes from these stars have been traveling
through space with the speed of lightning. So we see Orion not as it
exists today, but as it was six centuries ago. The extent of the
Orion group of stars is also inconceivably great. Even the central
part of the great nebula, which appears to our unaided eyes only as a
somewhat fuzzy star, would extend from here to the nearest star and
beyond, while our entire solar system would be the merest speck in
its midst.
Betelgeuze, the red star that marks the right shoulder of Orion,
is, as we have said, not a member of the Orion group. It has been
estimated that it is about two hundred light-years from the earth,
or only about one-third as far away as the other stars of the
constellation.
Betelgeuze very recently has attracted universal attention, and will
probably be considered an object of historic interest in the future,
because it is the first star to have its diameter measured with the
new Michelson interferometer, which is now being used so successfully
to measure the diameters of the largest stars. The truly sensational
discovery has been made that Betelgeuze is a supergiant of the
universe, with a diameter of about 275,000,000 miles. Our own sun,
which is known as a "dwarf" star, has a diameter of 864,000 miles.
That is, Betelgeuze would make about thirty million suns the size of
our own. If placed at the center of the solar system, it would fill
all of the space within the orbit of Mars; and the planets Mercury,
Venus, and the Earth would lie far beneath its surface. Measurements
of the diameters of other giant stars which are now being made with
the interferometer give results quite as startling as have been
obtained in the case of Betelgeuze; and it has been found that
several of these stars may even exceed Betelgeuze in size. Such a
star is Antares, the fiery-red star in the heart of Scorpio, which
is such a conspicuous object in the summer evening skies. All these
huge stars are deep red in color, and some of them vary irregularly
in brightness. Betelgeuze is one of the stars that changes in
brightness in a peculiar manner from time to time. When shining with
its greatest brilliancy it is a brighter object than the nearby star
Aldebaran, in Taurus; but a few months or a year later it may lose
so much of its light as to be decidedly inferior to Aldebaran. We
may note for ourselves this remarkable change in the brightness of
Betelgeuze by comparing the two stars from time to time.
Directly south of Orion lies the small constellation of Lepus, The
Hare, which is made up of third-magnitude and fourth-magnitude stars.
The four brighter stars are arranged in the form of a small, but
distinct, quadrilateral, or four-sided figure, which may be easily
seen in our latitudes. The small constellation of Columba, The Dove,
which lies just south of Lepus, is so close to the horizon that it
can not be seen to advantage in the mid-latitudes of the northern
hemisphere. Neither Lepus nor Columba contain any object of unusual
interest.
[Illustration: FEBRUARY--AURIGA]
Due north of Orion, and lying in the zenith at this time, is Auriga,
The Charioteer, represented, strange to say, with Capella, a goat, in
his arms. The beautiful first-magnitude star Capella, golden-yellow
in color, serves us in identifying the constellation. Close at hand
are The Kids, represented by a group of three faint stars. Capella
is one of the most brilliant stars of the northern hemisphere. It
is almost exactly equal in brightness to Arcturus and Vega, stars
conspicuous in the summer months, and it is a shade brighter than
magnificent blue-white Rigel in Orion. Capella is about fifty
light-years distant from the earth and is fully two hundred times
more brilliant than our own sun. At the distance of Capella, the sun
would appear to be considerably fainter than any one of the three
stars in the nearby group of The Kids.
Capella is attended by a companion star so close to its brilliant
ruler that it can not be seen as a separate star save with the aid of
the most powerful telescopes. Its distance from Capella has been very
accurately measured, however, by means of the interferometer, which
is giving us the measurements of the diameters of the giant stars. It
is known that this companion sun is closer to Capella than our planet
earth is to the sun.
At no time of the year shall we find near the meridian so many
brilliant and beautiful stars as appear in the month of February at
this time in the evening. In addition to Capella, which is one of
the three most brilliant stars in the northern hemisphere of the
heavens, we have, in Orion alone, two stars of the first magnitude,
Betelgeuze and Rigel, and five stars of the second magnitude,
Bellatrix and Saiph and the three stars in the Belt. In addition, we
have not far distant in the western sky, fiery Aldebaran in Taurus,
and close on the heel of Orion in the east, Sirius, the brightest
star in the heavens, in the constellation of Canis Major, The Greater
Dog, as well as the first-magnitude star Procyon in Canis Minor, The
Lesser Dog. Of these two groups we shall have more to say under the
constellations for March.
IV
MARCH
To the southeast of Orion and almost due south at eight o'clock in
the evening on the first of March lies the constellation of Canis
Major, The Greater Dog, containing Sirius, the Dog-star, which far
surpasses all other stars in the heavens in brilliancy.
Sirius lies almost in line with the three stars that form the Belt
of Orion. We shall not have the slightest difficulty in recognizing
it, owing to its surpassing brilliancy as well as to the fact that it
follows so closely upon the heels of Orion.
Sirius is the Greek for "scorching" or "sparkling," and the ancients
attributed the scorching heat of summer to the fact that Sirius
then rose with the sun. The torrid days of midsummer they called
the "dog-days" for this reason, and we have retained the expression
to the present time. Since Sirius was always associated with the
discomforts of the torrid season, it did not have an enviable
reputation among the Greeks. We find in Pope's translation of the
_Iliad_ this reference to Sirius:
"Terrific glory! for his burning breath
Taints the red air with fever, plagues, and death."
In Egypt, however, many temples were dedicated to the worship of
Sirius, for the reason that some five thousand years ago it rose
with the sun at the time of the summer solstice, which marks the
beginning of summer, and heralded the approaching inundation of the
Nile, which was an occasion for great rejoicing among the Egyptians.
It was, therefore, called the Nile Star and regarded by them with the
greatest reverence.
Sirius is an intensely white hydrogen star; but owing to its great
brilliancy and to the fact that it does not attain a great height
above the horizon in our latitudes, its rays are greatly refracted
or broken up by the atmosphere, which is most dense near the horizon,
and as a result, it twinkles or scintillates more noticeably than
other stars and flashes the spectrum colors--chiefly red and
green--like a true "diamond in the sky"--a magnificent object in the
telescope.
Sirius is one of our nearest neighbors among the stars. Only two
stars are known to be nearer to the solar system. Yet its light takes
about eight and a half years to flash with lightning speed across the
great intervening chasm. It is attended also by a very faint star
that is so lost in the rays of its brilliant companion that it can
only be found with the aid of a powerful telescope. The two stars
are separated by a distance of 1,800,000,000 miles; that is they are
about as far apart as Neptune and the sun. They swing slowly and
majestically about a common center, called their center of gravity,
in a period of about forty-nine years. So faint is the companion of
Sirius that it is estimated that twenty thousand such stars would be
needed to give forth as much light as Sirius. The two stars together,
Sirius and its companion, give forth twenty-six times as much light
as our own sun. They weigh only about three times as much, however.
The companion of Sirius, in spite of its extreme faintness, weighs
fully half as much as the brilliant star.
[Illustration: MARCH--CANIS MAJOR]
There are a number of bright stars in the constellation of Canis
Major. A fairly bright star a little to the west of Sirius marks the
uplifted paw of the dog, and to the southeast, in the tail and hind
quarters, are several conspicuous stars of the second magnitude.
A little to the east and much farther to the north, we find Canis
Minor, The Lesser Dog, containing the beautiful first-magnitude
star Procyon, "Precursor of the Dog"--that is, of Sirius. Since
Procyon is so much farther north than Sirius and very little to the
east, we see its brilliant rays in the eastern sky some time before
Sirius appears above the southeastern horizon, hence its name. Long
after Sirius has disappeared from view beneath the western horizon
in the late spring, Procyon may still be seen low in the western
sky. Procyon, also is one of our nearer neighbors among the stars,
being only about ten light-years distant from the solar system. Like
Sirius, it is a double star with a much fainter companion, that by
its attraction sways the motion of Procyon to such an extent that
we should know of its existence, even if it were not visible, by
the disturbances it produces in the motion of Procyon. The period
of revolution of Procyon and its companion about a common center is
about forty years, and the two stars combined weigh about a third
more than our own sun and give forth six times as much light. Canis
Minor contains only one other bright star, Beta, a short distance to
the northwest of Procyon. Originally, the name Procyon was given to
the entire constellation, but it was later used only with reference
to the one star. Procyon, Sirius, and Betelgeuze in Orion form a huge
equal-sided triangle that lies across the meridian at this time and
is a most conspicuous configuration in the evening sky.
[Illustration: MARCH--GEMINI AND CANIS MINOR]
Directly south of the zenith we find Gemini, The Twins, one of the
zodiacal constellations. It is in Gemini that the sun is to be found
at the beginning of summer. The two bright stars Castor and Pollux
mark the heads of the twins, and the two stars in the opposite
corners of the four-sided figure shown in the chart mark their feet.
Castor and Pollux, according to the legend, were the twin brothers of
Helen of Troy who went on the Argonautic expedition. When a storm
overtook the vessel on its return voyage, Orpheus invoked the aid of
Apollo, who caused two stars to shine above the heads of the twins,
and the storm immediately ceased. It was for this reason that Castor
and Pollux became the special deities of seamen, and it was customary
to place their effigies upon the prows of vessels. The "By Jimini!"
of today is but a corruption of the "By Gemini!" heard so frequently
among the sailors of the ancient world.
The astronomical name for Castor, the fainter star, is Alpha
Geminorum, meaning Alpha of Gemini. As it was customary to call the
brightest star in a constellation by the first letter in the Greek
alphabet, it is believed that Castor has decreased considerably in
brightness since the days of the ancients, for it is now decidedly
inferior to Pollux in brightness, which is called Beta Geminorum.
Of the two stars, Castor is the more interesting because it is a
double star that is readily separated into two stars with the aid of
a small telescope. The two principal stars are known to be, in turn,
extremely close double stars revolving almost in contact in periods
of a few days. Where we see but one star with the unaided eye, there
is, then a system of four suns, the two close pairs revolving slowly
about a common center of gravity in a period of several centuries and
at a great distance apart.
The star Pollux, which we can easily distinguish by its superior
brightness, is the more southerly of the twin stars and lies due
north of Procyon and about as far from Procyon as Procyon is from
Sirius.
The appearance of Gemini on the meridian in the early evening and
of the huge triangle, with its corners marked by the brilliants,
Procyon, Sirius, and Betelgeuze, due south, with "Great Orion sloping
slowly to the west," is as truly a sign of approaching spring as
the gradual lengthening of the days, the appearance of crocuses and
daffodils, and the first robin. It is only a few weeks later--as
pictured by Tennyson in _Maud_--
"When the face of the night is fair on the dewy downs,
And the shining daffodil dies, and the Charioteer
And starry Gemini hang like glorious crowns
Over Orion's grave low down in the west."
V
APRIL
In the early evening hours of April the western sky is still adorned
with the brilliant jewels with which we became familiar on the
clear frosty evenings of winter. Orion is now sinking fast to his
rest beneath the western horizon. Beautiful, golden Capella in
Auriga glows in the northwest. Sirius sparkles and scintillates, a
magnificent diamond of the sky, just above the southwestern horizon,
while Procyon in Canis Minor, The Lesser Dog, and Castor and Pollux,
The Twins, in the constellation of Gemini, are still high in the
western part of the heavens.
In the northeast and east may be seen the constellations that will
be close to the meridian at this time next month. Ursa Major, The
Greater Bear, with its familiar Big Dipper, is now in a favorable
position for observation. The Sickle in Leo is high in the eastern
sky, and Spica, the brilliant white diamond of the evening skies of
spring, is low in the southeast in Virgo.
Near the meridian this month we find between Auriga and Ursa Major,
and east of Gemini, the inconspicuous constellation of Lynx, which
contains not a single bright star and is a modern constellation
devised simply to fill the otherwise vacant space in circumpolar
regions between Ursa Major and Auriga.
[Illustration: APRIL--CANCER]
Just south of the zenith at this time, and lying between Gemini and
Leo, is Cancer, The Crab, the most inconspicuous of all the zodiacal
constellations. There are no bright stars in this group, and there
is also nothing distinctive about the grouping of its faint stars,
though we can readily find it, from its position between the two
neighboring constellations of Gemini and Leo by reference to the
chart.
In the position indicated there we will see on clear evenings a
faint, nebulous cloud of light, which is known as Praesepe, The
Beehive, or as The Manger, the two faint stars flanking it on either
side being called Aselli, The Asses. This faint cloud can be easily
resolved by an opera-glass into a coarse cluster of stars that lie
just beyond the range of the unaided human vision.
To the ancients, Praesepe served as an indicator of weather
conditions, and Aratus, an ancient astronomer, wrote of this cluster:
"A murky manger, with both stars
Shining unaltered, is a sign of rain.
If while the northern ass is dimmed
By vaporous shroud, he of the south gleam radiant,
Expect a south wind; the vaporous shroud and radiance
Exchanging stars, harbinger Boreas."
This was not entirely a matter of superstition, as we might possibly
imagine, for the dimness of the cluster is simply an indication
that vapor is gathering and condensing in the atmosphere, just as
a ring around the moon is an indication of the same gathering and
condensation of vapor that precedes a storm.