Part 5

It stretches like a vast ragged semicircle over the sky. Indeed, it traces a rough circle, for this line is continued over the southern hemisphere also. The circle is, however, very far from being smooth or even; the path is full of irregularities. It varies in width to an extent of about thirty degrees, and varies also considerably in brightness. Its total area has been estimated to cover rather less than one-fourth of the whole northern hemisphere of the sky, and to cover about one-third of the southern hemisphere. Its track lies through the constellations Cassiopeia and Auriga; it passes between the feet of Gemini and the horns of Taurus, through Orion just above the giant’s club, and through the neck and shoulder of Monoceros. It passes above Sirius into Argo, here entering the southern hemisphere, and through Argo and the Southern Cross into the Centaur. In the Centaur the Milky Way divides into two streams, in a manner which suggests the divided course of a river around an island, a dark rift between the two luminous streams representing the island.

It is a very long island, however, for the double conformation of the Milky Way extends over one-third of its entire course--that is to say, one hundred and twenty degrees of the circle. The divergent branches reunite in the northern hemisphere in the constellation Cygnus. The brighter stream passes through Norma, Ara, Scorpio and Sagittarius; along the bow of Sagittarius into Antinous, here entering the northern hemisphere again; then through Aquila, Sagitta, and Vulpecula it arrives at Cygnus and reunion with the branch which left it in Centaur. From Cygnus the stream, now single, passes through Lacerta and the head of Cepheus to the point whence we started, in Cassiopeia.

As we follow the Milky Way throughout its course, we find it continually sending out streaming appendages of nebulous appearance towards clusters, nebulæ, or groups of stars. In Norma it sends out a complicated series of nebulous streaks and patches, covering the Scorpion’s tail, spreading faintly over the leg of Ophiuchus, and extending beyond, as if to meet a corresponding branch sent off from the region of Cygnus in the northern hemisphere. The latter is a very bright and remarkable streak, running south through Cygnus and Aquila, to become lost in a dim and sparsely starred region. From Cassiopeia a vivid branch proceeds to the chief star of Perseus, and faint streaks appear to continue the “feeler” towards the Hyades and the Pleiades. There are many other “feelers” of the same kind, and they are all of great interest, because they seem to show some sort of influence exercised by the Milky Way upon the whole starry universe.

ANCIENT AND MODERN CONCEPTIONS OF THE NATURE OF THE MILKY WAY. Strange theories as to the nature of the Milky Way have been put forward at various times. Anaxagoras thought it might be due to the shadow of our globe; Aristotle, that it was some kind of mist due to the exhalation of vapors from the earth.

But a grander and truer conception of its nature and situation, removed far from the earth and independent of any terrestrial cause, had early come to several minds. Pythagoras and Democritus both formed the conjecture that its shimmer might be due to innumerable stars, and Galileo’s telescope confirmed their theory.

As we have seen, the Milky Way is by no means a simple stream of stars; with careful observation, even the naked eye can perceive something of its irregular detail, when the atmosphere is unusually clear, and there is no moon. Viewed under these conditions through a good telescope, the effect of the Milky Way, when made to pass progressively before the vision, is one of unexampled grandeur and sublimity.

[Illustration: =THE STARRY GRANDEUR OF THE MILKY WAY=

=COURSE OF THE MILKY WAY THROUGH THE TWO HEMISPHERES OF THE HEAVENS=

These two drawings show the two semi-circles of the Milky Way as they extend from the regions of the Polar Star to the region of the Southern Cross on each side of the apparent sphere of the heavens. It will be noticed that the bright stars congregate near its region, and that there is a characteristic harmony in the way in which the wisps appear to project into space, suggesting some common cause for this appearance throughout the whole galaxy.]

The general effect has been well likened to that of an old, gnarled tree-trunk, marked with knots and curving lines, and riddled with dark holes and passages, linked together by shimmering wisps or arches. This general effect is practically lost as the detail becomes clear in a telescopic view. The detail is extremely various. At one point it may consist of separate stars scattered irregularly upon a background of darkness; at another, of star-clusters, sometimes following one upon another in long, processional line; at another, the stars seem to collect in small, soft clouds, presenting the appearance, as the telescope sweeps over them, of drifting foam.

THE STRANGE, DARK RIFTS IN THE SKYSCAPE WHERE NO STARS APPEAR. At yet another point the track may be involved in nebulosity in which many stars appear to be imbedded. Perhaps the most characteristic features are several which have already been remarked as conspicuous in star-clusters or nebulæ, such as lines of stars, dark lanes or rifts, and dark holes. The lines of stars, which are evidently connected by some actual physical relation, are either straight, curved, radiated, or in parallels. In Sagittarius is a very striking collection of about thirty stars resembling in form a forked twig with a curved hook at the unforked end. The dark rifts in the Milky Way show the same features as those in star-clusters. Sometimes they are parallel; sometimes they radiate like branches from a common center; sometimes they are lines with bright stars; sometimes they are quite black, as if utterly void; sometimes slightly luminous, as if powdered with small stars.

It can be by no accident or chance that in the vast edifice of the heavens objects of certain classes should crowd into the belt of the Milky Way, and other classes avoid it; it points to the whole forming a single growth, an essential unity. For there is but one belt in the heavens, like the Milky Way, a belt in which small stars, new stars, and planetary nebulæ find their favorite home; and that belt encircles the entire heavens; and similarly that belt is the only region from which the white nebulæ appear to be repelled. The Milky Way forms the foundation, the strong and buttressed wall of the celestial building; the white nebulæ close in the roof of its dome.

NEBULAE AND THE THEORY OF THE UNIVERSE

It has already been observed that a number of stars are arranged in clusters of groups, while others, like our own sun, are at vast distances from their nearest neighbors. Some of these clusters, of which the Pleiades afford the best example to the naked eye, can be resolved by a keen eye into separate stars; some, like Præsepe in Cancer, which only show to the naked eye as a hazy spot of light, break up in a good field-glass into clusters of stars; but the majority of stellar clusters require a powerful telescope for their resolution.

It was long ago noticed that, the more powerful a telescope was, the greater was the number of these hazy spots of light which it would resolve into clusters of stars. Consequently the opinion was formed that all the hazy little clouds or nebulæ which are so prevalent throughout a large part of the sky were simply clusters of stars, so far away that their light merged into a single impression on the eye. A great number of these nebulæ were only resolved by large telescopes; many were found to be irresolvable by any telescope. It was simply concluded from this that they were still more distant than the clusters which had yielded to the resolving powers of the telescope; and it was further supposed that each of these clusters of stars might be a separate universe or galaxy, comparable in extent and importance with our own universe, bounded by the vast girdle of the Milky Way.

THE NEBULAR HYPOTHESIS. This grand conception of innumerable universes scattered throughout space was speedily destroyed by the spectroscope, which distinguishes with entire certainty between the light sent to us from a solid star and that emitted by a gas. When it was turned upon the nebulæ which had been supposed in reality to be star-clusters so distant that no telescope could resolve them, it showed unmistakably that these nebulæ were not star-groups, but simply masses of incandescent gas.

Besides, nebulæ vary greatly in form and appearance; some are clearly clusters of stars, others are perfectly hazy. A round or oval form is sometimes exhibited, with a gradual condensation towards the center, and a number of stars standing in the center of a nebulous haze can be observed. Such observations on nebulæ caused Kant and Laplace to suggest a theory--now known as the nebular theory--as to the formation of worlds. They considered that the solar system, for example, originally existed as uncondensed nebulous matter. This gradually condensed towards the center, forming the nucleus of the sun, and later the outer parts separated into distinct parts, each part condensing into a planet. The different forms of nebulæ observed in the heavens are then supposed to be systems in different stages of development.

THE VARIED COLOR OF THE STARS

Many of the stars shine with colored light, as red, blue, green, or yellow.

These colors are exhibited in striking contrast in many of the double stars. Combinations of blue and yellow, or green and yellow, are not uncommon; while in fewer cases we find one star white and the other purple, or one white and the other red. In several instances each star has a rosy light.

The following are a few of the most interesting colored double stars:

Color of Color of Smaller Name of Star Larger One One

γ Andromedæ Orange Sea-Green. α Piscium Pale Green Blue. β Cygni Yellow Sapphire Blue. η Cassiopeiæ Yellow Purple. σ Cassiopeiæ Greenish Bright Blue. ζ Coronæ White Light Purple. ι Cancri Orange Blue. α Herculis Orange Emerald Green.

Single stars of a fiery red or deep orange color are common enough. Of the first color may be mentioned Aldebaran, Antares and Betelgeuse. Arcturus is a good example of an orange star. Isolated stars of a deep blue or green color are very rarely found; among the conspicuous stars, β Libræ appears to be the only instance.

It is now a well-established fact that the stars change their color. Sirius was described as a fiery red star by the ancients, is now decided green color.

=NAMES OF IMPORTANT STARS=

INCLUDING THOSE OF FIRST MAGNITUDE

Individual Constellation in Name Meaning Which Found

Achernar The End of The River α Eridani. Alcor The Near One 80 Ursæ Majoris. Alcyone Daughter of Atlas and Pleione η Tauri. Aldebaran The Follower α Tauri. Algenib The Side γ Pegasi. Algol The Demon Star β Persei. Alioth The Tail (of the Sheep) ε Ursæ Majoris. Altair The Soaring Eagle α Aquilæ. Antares The Rival of Mars α Scorpii. Arcturus The Watcher of the Bear α Boötis. Bellatrix The Woman Warrior γ Orionis. Betelgeux The Shoulder of the Giant α Orionis. Canopus The Pilot of Menelaus α Argûs. Capella The Goat α Aurigæ. Caph The Hand β Cassiopeiæ. Castor Son of Zeus and Leda α Geminorum. Cor Caroli Charles’ Heart α Canum Ven. Deneb The Tail α Cygni. Denebola The Lion’s Tail β Leonis. Dubhe The Bear α Ursæ Majoris. Fomalhaut The Fish’s Mouth α Piscis Australis. Markab The Saddle α Pegasi. Mira Ceti The Wonderful Star of Cetus ο Ceti. Mizar The Girdle ζ Ursæ Majoris. Polaris The Pole Star α Ursæ Minoris. Pollux Son of Zeus and Leda β Geminorum. Procyon Before the Dog α Canis Minoris. Regulus The Little King α Leonis. Rigel The Foot β Orionis. Sirius Chief α Canis Majoris. Spica The Ear of Corn α Virginis. Vega The Swooping Eagle α Lyræ.

WHAT CAUSES THE ECLIPSES

When the earth is between the moon and the sun in a line, the moon lies in the shadow of the earth, and so suffers temporary obscuration; a _lunar eclipse_ then takes place. When the moon passes between the earth and the sun, the latter is at certain places on the earth obscured by the dark body of the moon, and a _solar eclipse_ takes place.

LUNAR ECLIPSES. The shadow cast by the earth is conical, and may be shown to extend about one million miles from its surface. At a distance of a quarter of a million miles away the width of this shadow is about six thousand miles; and if the moon passes into it at that approximate distance from the earth, its disc of two thousand miles diameter may be partially or totally obscured. The moon and sun may be on opposite sides of the earth, and yet the former not in shadow. This is due to the fact that the moon’s orbit round the earth is not exactly in the same plane as that of the earth’s orbit round the sun. If it were so, we should have total eclipses at every full moon; but since the two planes are inclined to each other at an angle of 5° 9′, eclipses will occur when the moon is at or near its _nodes_ or positions of coincidence with the plane of the ecliptic. Partial eclipses are produced when only a portion of the moon passes into shadow; annular eclipses such as are sometimes observed in the case of the sun cannot occur with the moon.

[Illustration: =GIANT SHADOWS CAST BY THE EARTH AND MOON=

=HOW THE MOON THROWS ITS SHADOW ON THE EARTH, SHUTTING OFF THE LIGHT OF THE SUN=]

[Illustration: =HOW THE MOON COMES BETWEEN THE EARTH AND SUN, CAUSING THE SHADOW SHOWN ABOVE=]

[Illustration: =HOW THE EARTH THROWS ITS SHADOW ACROSS THE MOON=

On its way through space the moon passes sometimes between the sun and the earth, shutting off the sunlight from the earth, as shown in the top picture. The drawing in the middle shows us that the moon does not hide the sunlight from the whole of the earth, but only from a part of it. But in the part from which the sun is hid the moon’s shadow makes day so dark that we can see the stars. We call this an eclipse of the sun. Sometimes, too, the earth passes between the moon and the sun so as to cut off all sunlight from the moon, as shown in the bottom picture. We call this an eclipse of the moon.]

SOLAR ECLIPSES. The shadow cast by the moon is also conical, and extends over a slightly varying distance of about a quarter of a million miles from the moon’s surface. This being the approximate distance of the moon from the earth, it is seen that when the moon is between the earth and the sun the shadow may reach the earth. The extreme limit of the shadow may range from twenty-three thousand miles short of the earth, in which case an entire eclipse of the sun is impossible, to fifteen thousand miles beyond the earth. In the latter case a circular shadow will be projected on the surface of the globe, travelling onwards slowly in the direction of the motion of the moon. Within this shadow or _umbra_ the body of the sun cannot be observed, and a total eclipse prevails. A circular region exists round this shadow, in which only part of the sun is visible; this region is therefore partly in shadow, and is called the _penumbra_. Outside the penumbra the whole sun may be viewed; the moon’s shadow is not nearly large enough to render a solar eclipse co-existent over all parts of the earth’s face towards the sun.

THE MYTHOLOGY OF THE CONSTELLATIONS

To the Greeks the starry heavens were an illustrated mythological poem. Every constellation was a picture, connected with some old fable of gods or heroes.

The two Bears have one story. Callisto was a nymph beloved by Jupiter, who changed her into a she-bear to save her from the jealous wrath of Juno. But Juno learned the truth, and induced Diana to kill the bear in the chase. Jupiter then placed her among the stars as Ursa Major, and her son Arcas afterwards became Ursa Minor. Juno, indignant at the honor thus shown the objects of her hatred, persuaded Tethys and Oceanus to forbid the Bears to descend, like the other stars, into the sea.

According to Ovid, Juno changed Callisto into a bear; and when Arcas, in hunting, was about to kill his mother, Jupiter placed both among the stars.

Ursa Minor was also called Phœnice, because the Phœnicians made it their guide in navigation, while the Greeks preferred the Great Bear for that purpose. It was also known as Cynosura (dog’s tail) from its resemblance to the upturned curl of a dog’s tail. The Great Bear was sometimes called Helice (winding), either from its shape or its curved path.

Boötes (the Herdsman) was also called Arctophylax and Arcturus, both of which names mean the guard or keeper of the bear. According to some of the stories, Boötes was Arcas; according to others, he was Icarus, the unfortunate son of Dædalus. The name Arcturus was afterwards given to the chief star of the constellation.

Cepheus, Cassiopeia, Andromeda, Perseus, and Pegasus are a group of star-pictures illustrating a single story.

Cepheus and Cassiopeia were the king and queen of Ethiopia, and had a very beautiful daughter, Andromeda. Her mother boasted that the maiden was fairer than the Nereids, who in their anger persuaded Neptune to send a sea-monster to ravage the shores of Ethiopia. To appease the offended deities Andromeda, by the command of an oracle, was exposed to this monster. The hero Perseus rescued her and married her.

Pegasus, the winged horse, sprang from the blood of the frightful Gorgon, Medusa, whom Perseus had slain not long before he rescued Andromeda from the sea-monster. According to the most ancient account, Pegasus became the horse of Jupiter, for whom he carried the thunder and lightning; but he afterward came to be considered the horse of Aurora, and finally of the Muses. Modern poets rarely speak of him except as connected with the Muses.

The Dragon, according to some of the poets, was the one that guarded the golden apples of the Hesperides; according to others, the monster sacred to Mars which Cadmus killed in Bœotia.

The Lyre is said to be the one which Apollo gave to Orpheus. After the death of Orpheus, Jupiter placed it among the stars at the intercession of Apollo and the Muses.

The Crown was the bridal gift of Bacchus to Ariadne, transferred to the heavens after her death.

Aquila is probably the eagle into which Merops was changed. It was placed among the stars by Juno. Some, however, make it the Eagle of Jupiter.

Cygnus or Cycnus, according to Ovid, was a relative of Phaëthon. While lamenting the unhappy fate of his kinsman on the banks of the Eridanus, he was changed by Apollo into a swan, and placed among the stars.

Sagittarius was said by the Greeks to be the Centaur Cheiron, the instructor of Peleus, Achilles and Diomed. It is pretty certain, however, that all the zodiacal constellations are of Egyptian origin, and represent twelve Egyptian deities who presided over the months of the year. Thus Aries was Jupiter Ammon; Taurus, the bull Apis; Gemini, the inseparable gods Horus and Harpocrates; and so on. The Greeks adopted the figures, and invented stories of their own to explain them.

Scorpio, in the Egyptian zodiac, represented the monster Typhon. Originally this constellation extended also over the space now filled by Libra.

Ophiuchus represents Æsculpius, the god of medicine. Serpents were sacred to him, probably because they were a symbol of prudence and renovation, and were believed to have the power of discovering herbs of wondrous powers.

Aquarius, in Greek fable, was Ganymede, the Phrygian boy who became the cup-bearer of the gods in place of Hebe.

Taurus, as has been stated above, was the Egyptian Apis. The Greeks made it the bull which carried off Europa. The Pleiades are usually called the daughters of Atlas, whence their name Atlantides. Milton speaks of them as “the seven Atlantic Sisters.”

According to one legend the seventh was Sterope, who became invisible because she had loved a mortal; according to another, her name was Electra, and she left her place that she might not witness the downfall of Troy, which was founded by her son, Dardanus.

The Hyades, according to one of several stories, were sisters of the Pleiades. The name probably means “the Rainy,” since their rising announced wet weather.

Cetus is said by most writers to be the sea-monster from which Perseus rescued Andromeda.

Orion was a famous giant and hunter, who loved the daughter of Oinopion, King of Chios. As her father was slow to consent to her marriage, Orion attempted to carry off the maiden; whereupon Oinopion, with the help of Bacchus, put out his eyes. But the hero, in obedience to an oracle, exposed his eye-balls to the rays of the rising sun, and thus regained his sight. The accounts of his subsequent life, and of his death, are various and conflicting. According to some, Aurora loved him and carried him off; but, as the gods were angry at this, Diana killed him with an arrow. Others say that Diana loved him, and that Apollo, indignant at his sister’s affection for the hero, once pointed out a distant object on the surface of the sea, and challenged her to hit it. It was the head of Orion swimming, and the unerring shot of the goddess pierced it with a fatal wound. Another fable asserts that Orion boasted that he would conquer every animal; but the earth sent forth a scorpion which destroyed him.

Canis Major and Minor are the dogs of Orion, and are pursuing the Hare.

The Twins, Castor and Pollux, the sons of Jupiter and Leda, are the theme of many a fable. They were especially worshipped as the protectors of those who sailed the seas, for Neptune had rewarded their brotherly love by giving them power over winds and waves, that they might assist the shipwrecked.

Leo, according to the Greek story, was the famous Nemean lion slain by Hercules. Jupiter placed it in the heavens in honor of the exploit.

The Hydra also commemorates one of the twelve labors of Hercules--the destruction of the hundred-headed monster of the Lernæan lake.

Virgo represents Astræa, the goddess of innocence and purity, or, as some say, of justice. She was the last of the gods to withdraw from earth at the close of “the golden age.”

Libra, or the Balance, is the emblem of justice, and is usually associated with the fable of Astræa.

Argo Navis is the famous ship in which Jason and his companions sailed to find the Golden Fleece.

This slight sketch of the leading fables connected with the constellations will serve to show how completely the Greeks “nationalized the heavens.”

SCIENTIFIC TERMS USED IN ASTRONOMY

=Astronomy= (_as-tron´om-i_). The science which treats of the heavenly bodies, explaining the motions, times and causes of the motions, distances, magnitudes, gravities, light, etc., of the sun, moon, and stars, the nature and causes of the eclipses of the sun and moon, the conjunction and apposition of the planets, and any other of their mutual aspects, with the times when they did or will happen.

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