THE WORLDS IN THE SKIES

The earth upon which we live is only one of many worlds that whirl through space. If we are to understand our own world, we must first learn something about the worlds in the skies. These bodies are arranged in groups, or systems, sweeping through circuits that baffle measurement; and such is the magnitude of the boundless space they occupy that our entire solar system is only a point in comparison. To this vast expanse of worlds, and systems and space we give the general name Universe.

THE SOLAR SYSTEM AND
ITS MEMBERS

First in importance to us in this immense space filled with stars is what astronomers call the Solar System, so-called because the sun is its center. It contains the planets, eight in number, of which our earth is one. They have been named after the ancient deities; the two interior ones, Mercury and Venus, and the exterior ones, Mars, Jupiter, Saturn, Uranus, and Neptune; the first three being smaller than our earth, and the remainder a great deal larger.

Mercury and Venus are known to be interior planets, that is, planets between us and the sun, because they appear to swing on either side of the sun. Mercury very seldom leaves the sun sufficiently to rise so early before the sun, or set so late after him, as to be visible. Venus, however, gets so far away as to be seen long after sunset or before sunrise, and is called the Evening or Morning star, accordingly.

Besides the planets there are other members of the system, namely, comets and falling stars, which will be mentioned again more fully hereafter. All these bodies form a sort of family, having the sun for their head. The illustrations and drawings on separate pages give a view of the entire system.

Comparative Size. The size of the planets, in general, increases with their distance from the sun. The four composing the first group are all comparatively small, the earth being the largest. Those of the second group are all of great size. Jupiter, the largest, is not less than 1,390 times as large as the earth; but as it is much less dense, the amount of matter it contains is only a trifle more than 337 times that of the earth. All the planets together equal but one seven-hundredth part of the mass of the sun.

The Satellites, except our moon, and the two satellites of Mars, belong wholly to the second group of planets. Jupiter has eight; Saturn eight and several revolving rings; Uranus has four, and possibly more; while Neptune, so far as known with certainty, has but one.

MOVEMENTS WITHIN THE
SOLAR SYSTEM

Rotary Motion. The sun, all the primary planets, and their satellites, as far as known, rotate from west to east. Each rotation constitutes a day for the rotating body. The central line of rotary motion is called the axis of rotation, and the extremities of the axis are called the Poles.

Revolution Around the Sun. All the primary planets and asteroids revolve around the sun in the direction of their rotation, that is from west to east; and the planes of the orbits in which they revolve coincide very nearly with the plane of the sun’s equator. One revolution around the sun constitutes the year of a planet.

All the satellites, except those of Uranus and perhaps Neptune, also revolve from west to east.

Most of the comets revolve around the sun in very irregular and elongated orbits, only a few having their entire orbit within the planetary system. Some so move that after having entered our system and made their circuit around the sun, they seem to leave it, never to return.

[Large illustration] (258 kB)

Since the orbits of the planets are in most cases not far removed from the plane of the ecliptic, they are to be seen in a comparatively narrow belt of the heavens called,

The Zodiac. The belt of the sky which occupies 8° on each side of the ecliptic is called the Zodiac, and it is within this belt that the moon and the chief planets confine their movements, as none of their orbits is inclined to that of the earth by more than 8°. The Zodiac, which circles the celestial sphere, is divided into twelve signs each of which occupies 30°, and roughly coincides [15] with a constellation. The following lists give the signs of the Zodiac, with the seasons in which the sun passes through each of them:

Spring: Aries the Ram; Taurus the Bull; Gemini the Twins.

Summer: Cancer the Crab; Leo the Lion; Virgo the Virgin.

Autumn: Libra the Balance; Scorpio the Scorpion; Sagittarius the Archer.

Winter: Capricornus the Goat; Aquarius the Water-bearer; Pisces the Fishes.

Owing to the precession of the equinoxes, the signs of the Zodiac do not now correspond with the constellations of which they bear the names. Thus the sign Aries, in which the sun is seen on March 21st as it passes the vernal equinox, with which the solar year begins, is now in the constellation of Pisces, and in the course of the next 23,000 years it will move steadily backward through the constellations until it returns to the Ram, where it stood when its name was first given to it.

KEPLER’S CELEBRATED LAWS OF
PLANETARY MOVEMENTS

The laws under which the planets move were discovered through the genius of John Kepler, and are known as Kepler’s Laws of Planetary Motion. Kepler derived these laws from observation only, but Newton first explained them by showing that they were the necessary consequences of the laws of motion and the law of universal gravitation.

Kepler’s First Law states: “The earth and the other planets revolve in ellipses with the sun in one focus.”

Kepler’s Second Law states: “The radius vector of each planet moves over equal areas in equal times.”

Kepler’s Third Law states: “The squares of the periodic times of the planets are in proportion to the cubes of their mean distances from the sun.”

DIAGRAMS ILLUSTRATING KEPLER’S FIRST TWO LAWS OF PLANETARY MOTION

The diagram on the top illustrates the ellipse, and explains the first and second laws. The picture-diagram on the bottom illustrates the second law, which is that, as the planet moves round the sun, its radius vector describes equal areas in equal times. That is to say, a planet moves from A to B in the same time as it takes to move from C to D.

These laws cannot be fully understood without some acquaintance with mathematics. They may, however, be briefly explained for the comprehension of the non-mathematical reader. The figure in the [diagram] is an ellipse—what is known in popular language as an oval—which is symmetrical about the line AB, known as its major axis. It has two foci, S and S₁. The fundamental law of the ellipse is that if we take any point P on it, and join this point by a straight line to the two foci, then the sum of these two lines SP and S₁P is always the same—SP + S₁P = C.

The second law is rather less easy to understand. The radius vector is the line joining the sun to the planet at any moment; if we suppose the sun to be at the focus S, and P to be the planet, the radius vector at various positions of the planet will be represented by the lines SP, SP₁, SP₂, and so on. If the positions P, P₁, P₂, and so on, represent those which the planet occupies after equal periods of time—say, once a month—then the sectors of the ellipse bounded by each pair of lines, SP and SP₁, SP₁ and SP₂, will be equal. If a planet were to move in a circle round the sun, it is obvious that this law would imply that it moved with a uniform speed; but since the curvature of the ellipse varies in every part of its course, so must the speed of the planet, in order that its radius vector may describe equal areas in equal times. The planet will, in fact, be moving faster when it is near the sun, as at P, than when it is far off from the sun, as at P₂.

The third law shows that there is a definite numerical relation between the motions of all the planets, and that the time which each of them takes to complete its orbit depends upon its distance from the sun.

On his discovery of his third law Kepler had written: “The book is written to be read either now or by posterity—I care not which; it may well wait a century for a reader, as God has waited six thousand years for an observer.” Twelve years after his death, on Christmas Day, 1642, near Grantham, England, the predestined “reader” was born. The inner meaning of Kepler’s three laws was brought to light by Isaac Newton.

THE GIGANTIC SUN AND HIS FUNCTION
IN THE SOLAR SYSTEM

The great luminary which warms, lights, and rules the solar system is, like the majority of its fellow stars, a gigantic bubble. In other words, it is a globe of glowing gas, which is nowhere solid, though the immense pressure which must exist in its interior probably causes this gas to assume there a density greater than that of any solid which we know.

A PHOTOGRAPH OF THE SUN, SHOWING THE CLOUDS OF FIERY VAPOR WHICH SURROUND IT

Dimensions of the Sun. The sun appears to human vision as a brilliant globe of a little more than half a degree in diameter. It is about the same apparent size as the moon, since the size of the sun is to that of the moon very nearly in the same proportion as their relative distances from the earth. In reality, however, the sun is a gigantic orb, so huge that if the earth were at its center the whole orbit of the moon would lie well within its circumference. The diameter of the sun is about 866,500 miles.

The mass of the sun is about 332,000 times that of the earth, but its specific gravity is only about a quarter that of the earth, 1.41, if that of water be taken as unity. The mean distance of the sun from the earth is about 92,800,000 miles; but, as the earth’s orbit is not circular but elliptic, this distances varies by about 3,000,000 miles, being smallest in January and greatest in July.

The Physical Condition of the sun is very different from that of the earth, though we know it is composed of very similar materials. The white-hot surface that we see, called the photosphere, is believed to be largely a shell of highly heated metallic vapors surrounding the unseen mass beneath. Dark spaces seen in the photosphere are known as sun-spots, and these are often surrounded by brighter patches, termed faculæ. Above the photosphere a shallow envelope of gases, rising here and there into huge prominences, and known as the chromosphere, is seen in red tints when the sun is totally eclipsed. Beyond the chromosphere, there is also seen, at the same time, a faint but far more extensive envelope called the corona.

This diagram illustrates the theory that sun-spots are formed by fragments struck from Saturn’s rings (which are in themselves nothing more than a great meteoric swarm) by the swarm of meteors known as the Leonids, which fragments fall into the solar furnace at a speed of four hundred miles a second.

The sun’s rays supply light and heat not only to the earth, but also to the other planets which revolve round it. Its attraction confines these planets in their orbits and controls their motions.

THE MOON—THE EARTH’S
ONLY SATELLITE

The Moon, the satellite of the earth, is the nearest to us of all the heavenly bodies, being at a mean distance of 240,000 miles. Its diameter is 2,153 miles and, its density being little more than half that of the earth, the force of gravity at its surface is very much less than that at the surface of the earth. A body which weighs a pound here would only weigh about two and one-half ounces if taken to the moon.

THE SYSTEM OF MARS AND ITS MOONS CONTRASTED WITH THAT OF THE EARTH AND MOON

In this diagram the markings on the earth and Mars are to scale, the orbits of the planets are seen in perspective and the measurements are according to Prof. Percival Lowell.

The Moon’s Orbit. Her path is approximately an ellipse with the earth in one focus. Its apparent motion in the sky is from west to east, but she moves much faster than the sun, taking about twenty-seven days eight hours to travel all round the earth. The time between two successive new moons (synodic period or lunation) is twenty-nine and one-half days. The reason of the difference is that the sun moves slowly in his annual course through the stars in the same direction as the moon, which therefore in its revolution round the earth has to overtake him when it returns. The moon rotates on its axis in the same time as it performs a revolution in its orbit; hence the same half is always turned toward us.

When the moon in her orbit lies between the sun and the earth, she is said to be in conjunction with the sun; when the earth is between the moon and the sun, the moon is said to be in opposition to the sun. At either of the two points midway from conjunction and opposition, i. e. 90° from conjunction or opposition, the moon is said to be in quadrature.

The Phases of the Moon. Except at opposition—i. e. when the earth is between the moon and sun—the whole of the moon’s disc does not appear bright to us, and the amount of the bright surface seen by us is found to depend on the relative positions of moon and sun. Half of the moon is always illuminated by the sun; but when it is in conjunction between the earth and sun the whole of the bright surface is on the side away from us; so that the moon is invisible. As it moves farther from the line joining earth and sun, a small portion of the bright side comes into view as a narrow crescent. This increases till half the disc is illuminated, when the lines joining earth and moon and earth and sun are at right angles. From this time the moon loses its crescent shape and becomes convex on both sides, or gibbous (Lat. gibbus, a hump)—the maximum brightness, or full moon, occurring when sun and moon are on opposite sides of the earth. After this the moon becomes gibbous, then crescent, and vanishes before the time of new moon.

It is worthy of note that the moon is higher in the heavens and longer above the horizon in the winter than in summer. This is owing to the plane of its orbit being at night high towards the south in winter and low in summer, as is the ecliptic. The moon’s orbit, like that of other planets, is elliptical, but irregular. When nearest to the earth, she is said to be in perigee; when at the greatest distance, in apogee.

DIAGRAM SHOWING HOW THE MOON’S PHASES ARE CAUSED

In the above diagram, the earth is in the center, and the circle ACFH the orbit of the moon. Since the inclination of the plane of the moon’s orbit to the plane of the ecliptic is only a few degrees, we may neglect it in this case, and suppose the two planes to coincide. Let the sun lie in the direction ES. Since the distance of the sun from the earth is about three hundred and eighty-seven times the distance of the moon from the earth, the lines ES, HS, BS, etc., drawn to the sun from different points of the moon’s orbit, may be considered to be sensibly parallel. Let us first suppose the moon to be in conjunction with the sun at the point A. Here only the dark portion of the moon is turned towards the earth, and the moon is therefore invisible. This is called new moon. As the moon moves on towards B, the enlightened part begins to be visible, and when it reaches C, half the enlightened part is visible, and the moon is at its first quarter. When the moon is at F, in opposition to the sun, all the illuminated part is turned towards the earth, and the moon is full. The moon wanes after leaving F, passes through its last quarter at H, and finally becomes again invisible at A.

Surface of the Moon. The moon is an opaque, cold globe, covered with mountains, extinct volcanoes, and plains. She has neither water nor atmosphere, and always presents the same surface to the earth in consequence of rotating on her axis in the same time as she revolves round the earth. Moonlight is only reflected sunlight, the illuminated hemisphere being always turned towards the sun.

The face of the moon has been studied and mapped on a large scale. Its chief features are three in number: (1) the numerous volcanic craters, such as Tycho and Copernicus, which are mostly named after distinguished men of science; (2) the wide, dark plains which are known as seas, because they were formerly thought to consist of water; (3) the curious systems of bright streaks, which radiate from many of these craters, of which the most remarkable extend in all directions from the great crater Tycho, near the moon’s south pole, and are conspicuous even to the naked eye at the time of full moon.

The Moon and the Tides. The moon has long been known to have an effect upon the tides, and may perhaps influence the winds. It is of enormous importance to navigators for the determination of longitude, and hence its movements have been investigated with the greatest care and precision.

HOW THE MOON FORMS “TIDES” IN THE CRUST OF THE EARTH

By reason of its power of attraction, it is well recognized that the Moon exercises a greater influence on the side of the earth which is nearest to it. In consequence the earth is subject to a stress or pull that tends to lengthen it out toward the moon, and then to recede as the earth turns away on its axis.

The Planet Mars. Nearest to the earth, with the single exception of Venus, resembles the earth more closely than any other of the planets, and is most favorably situated for our observation of all the heavenly bodies, except the moon. It is a globe rather more than half the size of the earth. When Mars comes nearest to the earth its distance from us is about 35,000,000 miles. At these favorable moments its brightness is about equal to Jupiter, and only surpassed by that of Venus. Mars has a very pronounced red color, which is supposed to be due to the prevalence of a rock like our red sandstone on its surface, or possibly to the color of its vegetation.

Its density is much less—about three-quarters that of the earth; so a pound weight placed on its surface would not weigh much more than six ounces, and a ponderous elephant would, if there, be able to jump about with the agility of a fawn.

The heat and light which Mars receives from the sun, therefore, vary enormously, and so cause a difference in the lengths of winter and summer in his north and south hemispheres, the seasons in the north hemisphere being far more temperate than those in the south. Viewed with the telescope, large dark green spots are seen, the rest of the surface being of a ruddy tint, except at the two poles, where two white spots are observed and considered to be due to large masses of snow and ice. It has been supposed that the greenish spots are oceans, and the ruddy parts land. The spectroscope has shown that watery vapor is present in Mars’ atmosphere, and appearances like huge rain-clouds sometimes obscure a part of the planet for a considerable period. Physical processes seem to go on there much the same as on our planet; hence many believe that Mars is inhabited and forms, in fact, a miniature picture of the earth.

Jupiter. By far the largest of the planets is second in brilliancy to Venus, unlike which, however, it is a “superior” planet, having its orbit outside that of the earth. It is about five times as brilliant as Sirius, the brightest of the fixed stars.

The planet is a beautiful object when viewed with a telescope; it is probable that the markings are entirely due to its atmosphere, and that the actual surface of the planet is rarely visible. Jupiter has hardly yet cooled from the condition of incandescence, and it is only slightly solidified. It possesses eight satellites, four of which were discovered by Galileo when he applied the telescope first to the investigation of the heavens. By means of these satellites the first observations of the velocity of light were made. A fifth was discovered in 1892 at the Lick Observatory.

Saturn was recognized as a planet by the ancients, and was the outside member of the solar system as known by them. His diameters at the equator and poles differ considerably, the protuberance at the equator giving him there a diameter of 74,000 miles, while at the poles it is only 68,000. In size Saturn is the largest of the planets except Jupiter, being in fact seven hundred times larger than our earth, but his density is so small that he would be able to float on water far more easily than an iceberg. From this it follows that he cannot consist of solid or liquid matter, and in fact we can only view a mass of clouds intensely heated within, the whole being probably a planet in the early stage of development—younger even than Jupiter.

The most remarkable characteristic of Saturn, which makes him an object of such interest in the sky, is his possession of a luminous ring. The ring is only luminous on account of its reflection of the sun’s light; hence is invisible to us when, for instance, we are endeavoring to look at the ring from below while the sun is shining above. It also sometimes happens that the plane of the rings passes through the sun or through the center of the earth, in which case only the thin edge of the rings can be seen. The ring is divided into two parts, the inner being the wider, while another faint division appears to divide the outer part into two smaller rings. In 1850 another ring was discovered; this is quite different from the outer rings, being dark, and generally known as the dusky ring of Saturn. The outer ones, though far from solid, can receive a shadow of Saturn, and themselves cast one on his disc. The rings are not continuous masses of matter, but consist of countless myriads of tiny satellites, so close together that to the observer they appear as one body. The planet has eight satellites which seldom pass behind or in front of the planet’s disc, and therefore are not objects of great interest.

Uranus is the next planet beyond Saturn. His mass is about fifteen times as much as that of the earth, an amount which makes him more than outweigh Mercury, Venus, the Earth, and Mars combined. All astronomers do not agree in their estimation of these numbers, Uranus being too far away for measurements to be more than approximate. Gravity on his surface is only three-quarters of what it is here. Uranus has four satellites, and possibly faint rings like those which encircle Saturn.

Neptune is farthest from the sun, the distance between the two bodies being about 2,750,000,000 miles. At this immense distance it will, according to Kepler’s laws, take a long time to travel once around its orbit, and this time has been found to be one hundred and sixty-five of our years. Although it is ninety-seven times as large as the earth, yet, on account of its enormous distance from us it can only just be seen, even with a powerful telescope. Neptune possesses one satellite, which moves around the planet in rather less than six days.

Mercury is the smallest planet, except the planetoids, in the solar system, and the one nearest the sun. It is never seen for more than two hours before sunrise or after sunset, and is not always visible then; but when it does appear, it is extremely brilliant. Even when it is most distant the sun appears four and a half times as big to it as it does to us, and when the two are at their nearest, this small planet gets ten times as much light and heat as we do. It is, however, so small and difficult to observe, that comparatively little is known of it.

Venus appears to us as the most brilliant of all the planets, sometimes heralding the sun’s approach in the morning and sometimes following him at night. Hence she has been called the “morning” and the “evening” star; and the ancient Greeks, believing her to be two bodies, and not one, called her Hesperus (Vesper) when she appeared at night, but Phosphorus when she preceded the dawn, this last name having been translated in the Latin, Lucifer. We know very little of the actual surface of Venus, for her envelope of clouds remains constantly in front of us to baffle curiosity, and never lifts to give us a glimpse of the planet beneath. These clouds send on to us the light they borrow from the sun, and shine to us with a brilliant silvery lustre interrupted here and there with shadowy markings of short duration. But when Venus shines to us in crescent-form, certain spots near the ends of the horns can be seen more definitely, and the effects of light and shadow round these points suggest that they are lofty peaks, reaching above the clouds.

The Minor Planets or Asteroids. The space between Mars and Jupiter is occupied by a strange and numerous swarm of minor planets or asteroids. The first of these singular bodies was discovered by an Italian astronomer, Piazzi, on the first night of the nineteenth century. Three others were discovered within the course of the next seven years, and the number now known is upward of 600, most of which have been recognized by the record of their motion on photographs of the sky. The four asteroids first discovered, Ceres, Pallas, Juno, and Vesta, are naturally the largest, ranging in diameter from four hundred to one hundred and eighteen miles.

Vesta, though not the largest, is considerably the brightest of the minor planets, and is occasionally visible to the naked eye. None of the other asteroids has a diameter so great as one hundred miles, and probably the majority of them are only ten or twenty miles in diameter.

COMETS, METEORS AND
SKY DUST

In addition to the planets and their satellites, the sun is attended by numerous other bodies, moving with far less regularity, and generally much less conspicuous in the heavens. These are known as comets and meteorites or shooting stars. One of the most interesting of recent astronomical discoveries is that an intimate physical connection exists between these two classes of bodies.

Comets. Comets have been known from the earliest times, because every now and then a very large and conspicuous one hastens up to the sun from the remote regions of space, and perplexes monarchs with the fear of change. They are called comets, from the Latin coma, meaning hair, because when they are bright enough to be seen with the naked eye they look like stars attended by a long stream of hazy light, which was thought to resemble a woman’s hair flowing down her back. This train of light is known as the comet’s tail. Such bright comets are sometimes as brilliant as Venus; their tails have been known to stretch halfway across the visible sky.

These comets are very beautiful and conspicuous objects, which usually appear in the sky without any warning from astronomers, and invariably create a great popular sensation. By far the greater number of comets, however, are only visible through a telescope, and it is rare that a year passes without at least half a dozen of these being reported. Up to the present time nearly a thousand comets of all sizes have been recorded. Not more than one in five of these visitors is visible to the naked eye.

Cometary Orbits. In all cases in which a comet has been observed sufficiently often for its orbit to be calculated, it is found that it moves in one of the curves which are known to the geometer as conic sections. Less than a hundred of the known comets move like the planets in elliptical orbits, and consequently their periodical return to visibility can be predicted. As a rule the eccentricity of these cometary orbits is very much greater than that of any planetary orbit, which means that the comet approaches fairly close to the sun at one end of its orbit, but at the other flies away far beyond the outermost planet, and for a long period disappears from the view of our most powerful telescopes.

The great majority of comets have only been seen once, and their orbits appear to be either parabolic or hyperbolic. Neither of these is a closed curve, and what seems to happen in such cases is that a comet travelling in such an orbit dashes up to the sun from the remote parts of space, swings round it, often at very close quarters, and flies away again forever. Only those comets which have elliptical orbits can be said to belong to the solar system. The others are visitors from space, which in the course of their motion come near the sun and are deflected by it, but then fly away until after a lapse of ages they perhaps come within the sphere of another star’s attraction. Of the comets which move in elliptical orbits, about twenty have been observed at more than one return to the sun. Some of these complete their orbits in quite a short period, like Encke’s comet, which has the shortest period of all, less than three and a half years; the longest periodical comet is known as Halley’s, which returns to the sun after seventy-six years, and last appeared in 1910; it is a bright and conspicuous object.

The Constitution of Comets. The nature of comets was long in doubt, and even today their physical characteristics are not fully understood. They are certainly formed of gravitational matter, because they move in orbits which are subject to the same laws as those of the planets. But they also appear to be acted upon by powerful repulsive forces emanating from the sun, to which is due the remarkable phenomenon of cometary tails. Perhaps there is not much exaggeration in the statement once made by a well-known astronomer that the whole material of a comet stretching halfway across the visible heavens, if properly compressed, could be placed in a hatbox. The old fear that the earth might suddenly be annihilated by a comet striking it is thoroughly dispelled by modern investigation, which leads us to believe that the worst results of such an encounter would be an extremely beautiful display of shooting stars.

Meteors, or Fireballs, are bodies which do not belong to the earth, but come from other parts of space into our atmosphere, and are seen as bright balls of fire crossing the sky, with a train of light behind. Suddenly they are seen to go out, and very often a fall of stones occurs. Sometimes they are observed to break in two, and loud explosions like thunder are heard. They move very fast—ten or twelve miles per second, and are visible when between forty and eighty miles above the earth.

Other meteors dart across the sky and disappear, all in a very short time. These are known as shooting stars, and are sometimes big and bright, like planets. It is estimated that about six or eight meteors which drop stones come into our atmosphere every year; but some 20,000,000 of small bodies pass through the air every day—these would all appear as shooting stars if they occurred at night.

At some periods of the year there are so many shooting stars that they appear like a shower of fire. On November 14th this happens, the shower being greatest every thirty-three years. A stream of meteors is travelling round the sun, and every thirty-three years the earth just comes through them. Meteoric showers also occur about August 9th to 11th, and smaller ones in April.

The luminosity of meteors is due to the intense heat caused by the resistance of the air to their passage, and in support of this theory it is found that meteoric stones are always covered, either wholly or in part, with a crust of cement that has recently been melted.

THE FIXED STARS
IN THE HEAVENS

We shall now study the so-called fixed stars, those stars, namely, which preserve the same relative position and configuration from night to night, only varying, and that with perfect regularity, in the times at which they reach the meridian. For this reason they have been known from the dawn of astronomy as fixed stars, in contrast with the planets or wandering stars.

The observer who watches the nightly changes in the sky with close attention will soon perceive that all these fixed stars appear to move in circles or parts of circles. Some of them describe larger circles than others, and the further south a star is when it passes the meridian, the larger circle will it describe.

It cannot be too often repeated that this motion of the stars is only apparent, being due to the real rotation of the earth, along with the observer on its surface, in the contrary direction. It is estimated that there are about three thousand stars visible to the naked eye in our latitude, though not all these are visible at the same time, many of them being below the horizon, while others are elevated in the sky at different times and seasons.

THE MAGNITUDES AND GROUPING
OF THE STARS

In beginning our study of the stars, let us put ourselves in the position of the earliest observers. Let us first, like them, watch the stars, and see how they appear from night to night.

We see, at the first glance, that the stars vary much in brightness. The brightest ones—like Sirius, Capella, Arcturus, and Vega—are called stars of the first magnitude. Those less brilliant, like the six brightest of “the Dipper,” are said to be of the second magnitude. All the stars which can be seen with the unaided eye are thus divided into six classes or magnitudes, according to their brightness.

Constellations. We also see that the stars are not uniformly distributed over the sky. They seem to be arranged in groups, some of which take the form of familiar objects. Every one knows the seven bright stars which are called “the Dipper.” Another group resembles a sickle, another a cross, and so on. All the stars in the heavens have been divided into groups called constellations. Many of these were recognized and named at a very early period.

We should become familiar with these constellations in order to study the stars with any profit.

It is necessary, in the first place, to have some way of designating the stars in each constellation. Many of the brighter stars have proper names as Sirius, Arcturus, and Vega; but the great majority of them are marked by the letters of the Greek alphabet. The brightest star in each constellation is called α (alpha); the next brightest, β (beta); the next, γ (gamma); and so on. The characters and names of the Greek alphabet are as follows:

α,	Alpha.
β,	Beta.
γ,	Gamma.
δ,	Delta.
ε,	Epsilon.
ζ,	Zeta.
η,	Eta.
θ,	Theta.
ι,	Iota.
κ,	Kappa.
λ,	Lambda.
μ,	Mu.
ν,	Nu.
ξ,	Xi.
ο,	Omicron.
π,	Pi.
ρ,	Rho.
σ,	Sigma.
τ,	Tau.
υ,	Upsilon.
φ,	Phi.
χ,	Chi.
ψ,	Psi.
ω,	Omega.

These letters are followed by the Latin name of the constellation. Thus Aldebaran is called α Tauri; Rigel, β Orionis; Sirius, α Canis Majoris.

If there are more stars in a constellation than can be named from the Greek alphabet, the Roman alphabet is used in the same way; and when both alphabets are exhausted, numbers are used.

Circumpolar Constellations. One of the most important constellations, and one easily recognized, is the Great Bear, or Ursa Major. It is represented in [Plate 1] on the Star Chart. It may be known by the seven stars forming “the Dipper.” The Bear’s feet are marked by three pairs of stars. These and the star in the nose can be readily found by means of the lines drawn on the chart. It may be remarked here, that in all cases the stars thus connected by lines are the leading stars of the constellation. The stars α and β are called the Pointers. If a line be drawn from β to α, and prolonged about five times the distance between them, it will pass near an isolated star of the second magnitude known as the Pole Star, or Polaris. This is the brightest star in the Little Bear, or Ursa Minor ([Plate 2]). It is in the end of the handle of a second “dipper,” smaller than the one in the Great Bear.

On the opposite side of the Pole Star from the Great Bear, and at about the same distance, is another conspicuous constellation, called Cassiopeia. Its five brightest stars form an irregular W, opening towards the Pole Star ([Plate 2]).

About half-way between the two Dippers three stars of the third magnitude will be seen, the only stars at all prominent in that neighborhood. These belong to Draco, or the Dragon. The chart will show that the other stars in the body of the monster form an irregular curve around the Little Bear, while the head is marked by four stars arranged in a trapezium. Two of these stars, β and γ, are quite bright. A little less than half-way from Cassiopeia to the head of the Dragon is a constellation known as Cepheus, five stars of which form an irregular K.

These five constellations never set in our latitude, and are called circumpolar constellations.

Constellations Visible in September. At this time the Great Bear will be low down in the northwest, and the Dragon’s head nearly in the zenith. If we draw a line from ζ to η of the Great Bear and prolong it, we shall find that it will pass near a reddish star of the first magnitude. This star is called Arcturus, or α Boötis, since it is the brightest star in the constellation Boötes. Of its other conspicuous stars, four form a cross. These and the remaining stars of the constellation can be readily traced with the aid of [Plate 3].

Near the Dragon’s head ([Plate 4]) may be seen a very bright star of the first magnitude, shining with a pure white light. This star is Vega, or α Lyræ.

If we draw a line from Arcturus to Vega ([Plate 3]), it will pass through two constellations, the Crown, or Corona Borealis and Hercules. The former is about one-third of the way from Arcturus to Vega, and consists of a semicircle of six stars, the brightest of which is called Alphecca or Gemma Coronæ,—“the gem of the crown.”

Hercules is about half-way between the Crown and Vega. This constellation is marked by a trapezoid of stars of the third magnitude. A star in one foot is near the Dragon’s head; there is also a star in each shoulder, and one in the face.

Just across the Milky Way from Vega ([Plate 5]) is a star of the first magnitude, called Altair, or α Aquilæ. This star marks the constellation Aquila, or the Eagle, and may be recognized by a small star on each side of it. These are the only important stars in this constellation.

In the Milky Way, between Altair and Cassiopeia ([Plate 4]), there is a large constellation called Cygnus, or the Swan. Six of its stars form a large cross, by which it will be readily known. α Cygni is often called Deneb. It forms a large isosceles triangle with Altair and Vega.

Low down in the south, on the edge of the Milky Way ([Plate 6]), is a constellation called Sagittarius, or the Archer. It may be known by [25] five stars forming an inverted dipper, often called “the Milk-dipper.” The head is marked by a small triangle. The other stars, as seen by the map, may be grouped so as to represent a bow and an arrow.

I. STAR CHART OF THE PRINCIPAL CONSTELLATIONS

Large illustrations (all less than 100 kB):
[Plate 1], [Plate 2], [Plate 3], [Plate 4],
[Plate 5], [Plate 6], [Plate 7], [Plate 8]

Low in the southwest is a bright red star called Antares, or α Scorpionis.

The space between Sagittarius and Hercules and Scorpio is occupied by the Serpent (Serpens) and the Serpent-bearer, or Ophiuchus ([Plates 6 and 7]). The head of the Serpent is near the Crown, and marked by a small triangle. The head of Ophiuchus is close to the head of Hercules, and may be known by a star of the second magnitude. Each shoulder is marked by a pair of stars. His feet are near the Scorpion.

Nearly on a line with Arcturus and γ Ursæ Majoris ([Plate 1]), and rather nearer the latter, is an isolated star of the third magnitude, called Cor Caroli, or Charles’ Heart. This is the only prominent star in the constellation of Canes Venatici, or the Hunting Dogs.

Cassiopeia is almost due east of the Pole Star. A line drawn from the latter through β Cassiopeiæ [26] and prolonged, passes through two stars of the second and third magnitude. These, with two others farther to the south, form a large square, called the Square of Pegasus. Three of these, as seen by the chart ([Plate 5]), belong to the constellation Pegasus, or the Winged Horse. α Pegasi is called Markab, and β is called Algenib. The bright stars in the neck and nose can be found by the chart.

II. STAR CHART OF THE PRINCIPAL CONSTELLATIONS

Large illustrations (all less than 100 kB):
[Plate 9], [Plate 10], [Plate 11], [Plate 12],
[Plate 13], [Plate 14], [Plate 15], [Plate 16]

The fourth star in the Square of Pegasus belongs ([Plate 8]) to the constellation Andromeda. Nearly in a line with α Pegasi and this star are two other bright stars belonging to Andromeda. The stars in her belt may be found by the chart.

Following the direction of the line of stars in Andromeda just mentioned, and bending a little towards the east, we come to Algol, or β Persei, a remarkable variable star. This star may be readily recognized from the fact, together with β and γ Andromeda and the four stars in the Square of Pegasus, it forms a figure similar in outline to the Dipper in Ursa Major, but much larger. If the handle of this great Dipper is made straight instead of being bent, the star in the end of it is α [27] Persei, of the second magnitude. This star has one of the third magnitude on each side of it. The other stars in Perseus may be found by the chart.

Just below θ in the head of Pegasus ([Plate 9]) are three stars of the third and fourth magnitudes, forming a small arc. These mark the urn of Aquarius, the Water-bearer. His body consists of a trapezium of four stars of the third and fourth magnitudes. Small clusters of stars show the course of the water flowing from his urn.

This stream enters the mouth of the Southern Fish, or Piscis Australis. The only bright star in this constellation is Fomalhaut, which is of the first magnitude, and at this time will be low down in the southeast.

To the south of Aquarius is Capricornus, or the Goat. He is marked by three pairs of stars arranged in a triangle. One pair is in his head, another in his tail, and the third in his knees.

Near Altair ([Plate 5]), and a little higher up, is a small diamond of stars forming the Dolphin, or Delphinus.

A little to the west of the Dolphin, in the Milky Way, are four stars of the fourth magnitude, which form the constellation Sagitta, or the Arrow.

Constellations Visible in October. If we look at the heavens at eight o’clock on the 15th of October, we shall see that all the constellations described above have shifted somewhat towards the west. Arcturus and Antares have set. In the east, below Andromeda ([Plate 10]), we see a pair of bright stars, which are the only conspicuous ones in the constellation Aries, or the Ram.

About half-way between Aries and γ Andromedæ are three stars which form a small triangle. This constellation is called Triangulum, or the Triangle.

Between Aries and Pegasus is the constellation Pisces, or the Fishes. The southernmost Fish may be recognized by a pentagon of small stars lying below the back of Pegasus. There are no conspicuous stars in the other Fish, which is directly below Andromeda.

Constellations Visible in November. At eight o’clock in the evening on the 15th of November, we see at a glance that the constellations with which we have become acquainted have moved yet farther to the westward. Boötes, the Crown, Ophiuchus, and the Archer have set; Pegasus, Cassiopeia, and Andromeda are overhead; while new constellations appear in the east.

We notice at once ([Plate 11]) a very bright star in the northeast, directly below Perseus. This is Capella, or α Aurigæ. There are five other conspicuous stars in Auriga, or the Charioteer; and with Capella they form an irregular pentagon.

Somewhat to the eastward ([Plate 12]), and a little lower down, is a very bright red star. This is Aldebaran, or α Tauri. It is familiarly known as the Bull’s eye. It will be noticed by the map that it is at one end of a V which forms the face of the Bull. This group is known as the Hyades. Somewhat above the Hyades is a smaller group, called the Pleiades,—more commonly known as the Seven Stars, though few persons can distinguish more than six. The bright star on the northern horn, or β Tauri, is also in the foot of Auriga, and counts as γ of that constellation.

All the space between Taurus and the Southern Fish, and below Aries and Pisces ([Plate 13]), is occupied by Cetus, the Whale. The head is marked by a triangle of rather conspicuous stars below Aries; the tail, by a bright star of the second magnitude, which is now just about as far above the horizon as Fomalhaut. On the body there are five stars, forming a sort of sickle. About halfway between this sickle and the triangle, in the head, is σ Ceti, which is also called Mira, or the wonderful star.

Constellations Visible in December. At eight o’clock in the evening in the middle of December, we shall find that Hercules, Aquila, and Capricornus have sunk below the horizon; while Vega and the Swan are on the point of setting. The Great Bear is climbing up in the northeast. In the east we behold by far the most brilliant group of constellations we have yet seen. Capella and Aldebaran are now high up; and below the former ([Plate 12]) is the splendid constellation of Orion. His belt, made up of three stars in a straight line, will be recognized at once. Above this, on one shoulder, is a star of the first magnitude, called Betelgeuse, or α Orionis. About as far from the belt, on the other side, is another star of the first magnitude, called Rigel. There are two other fainter stars which form a large trapezium with Betelgeuse and Rigel. The three small stars below the belt are upon the sword.

Below Orion ([Plate 14]) is a small trapezium of stars which are in the constellation of Lepus, or the Hare. The head is marked by a small triangle, as seen on the map.

To the north of Orion, and a little lower down ([Plate 12]), are two bright stars near together, one of the first and the other of the second magnitude. The latter is called Castor, and the former Pollux. These stars are in the constellation of Gemini, or the Twins. A line of three smaller stars just in the edge of the Milky Way marks the feet, and another line of three the knees. Pollux forms a large triangle with Capella and Betelgeuse.

Constellations Visible in January. At eight in the evening on the 15th of January, Vega, Altair, the Dolphin, Aquarius, and Fomalhaut have disappeared in the west; Deneb and the Square of Pegasus are near the horizon; while Capella and Aldebaran are nearly overhead. Two stars of exceeding brilliancy have come up in the west. The one farthest to the south ([Plate 14]) is the brightest star in the whole heavens. It is called Sirius, or the Dogstar; and is in the constellation of Canis Major, or the Great Dog, which can be readily traced by the lines on the map.

The other bright star is between Sirius and Pollux ([Plate 12]), and is called Procyon. It is in Canis Minor, or the Little Dog. The only other prominent star in this constellation is one of the third magnitude near Procyon.

Procyon, Sirius, and Betelgeuse form a large equilateral triangle.

Orion and the group of constellations about it constitute by far the most brilliant portion of the heavens, as seen in our latitude. There are, in all, only about twenty stars of the first magnitude, and seven of these are in this immediate vicinity.

Constellations Visible in February. If we look at the heavens at the same time in the evening about the middle of February, we shall miss Cygnus and Pegasus from the west. Auriga and Orion are nearly overhead.

Southeast of the Great Bear ([Plate 15]) is a red star of the first magnitude, called Regulus, in the [28] constellation of Leo, or the Lion. There are five stars near Regulus, which together with it form a group often called the Sickle. The star in the tail is Denebola, which makes a right-angled triangle with two others near it.

MAP SHOWING THE LOCATIONS OF NORTHERN CONSTELLATIONS

[Large illustration] (363 kB)

Between Leo and Gemini is the constellation Cancer, or the Crab. It contains no bright stars, but a remarkable cluster of small stars called Præsepe, or the Beehive.

Below Regulus ([Plate 14]) is a bright red star of the second magnitude, called Cor Hydræ, or the Hydra’s Heart. The head of Hydra is marked by five small stars. The coils of the monster can be traced by the map. A portion of the constellation is on [Plate 16].

Constellations Visible in March. At the middle of March, the heavens will have shifted round somewhat towards the west; but all the conspicuous constellations of the preceding month are still visible, while no new ones at all brilliant have come into view.

If we draw a line from the end of the Great Bear’s tail to Denebola, it will pass through two constellations,—Canes Venatici, described above; and Coma Berenices, or Berenice’s Hair, a large cluster of faint stars. ([Plate 15]).

MAP SHOWING THE LOCATIONS OF THE SOUTHERN CONSTELLATIONS AND ALSO MANY REMARKABLE NEBULAR FORMS

1. Double nebula in Gemini. 2. Double nebula of great brilliancy in Coma Berenicis. 3. Small double nebula. 4. Curiously shaped nebula in Ophiuchus. 5. Two nebulous spots in Canes Venatici. 6. Remarkable veil-like nebula in Lyra. 7. Elliptical nebula in Perseus. 8. Nebulous spot in Sagittarius, split into three pieces; a double star in center. 9. Large curiously-shaped nebula in Rober Caroli, filled with minute stars. 10. Great nebula in Andromeda, visible to the eye. 11. Nebula in Cetus. 12. Elongated nebula in Cygnus. 13. Brilliant round spots in Sagittarius. 14. Round spots in Andromeda. 15-16. Spots in Orion and Ursa Major. 17. Most remarkable of all nebula, in Orion. 18. Great oval nebula in Vulpes, containing two darker nebulae. 19. Nebulous figure in Canis Venaticus. 20. Nebular clouds in the Southern hemisphere.

[Large illustration] (497 kB)

Constellations Visible in April. At the middle of April, Aries and Andromeda have set; Taurus, Orion and Canis Major are sinking towards the west; the Great Bear and the Lion are overhead; Arcturus has risen in the northeast ([Plate 16]); and some way to the south of this is seen a star of the first magnitude, which forms a large triangle with Arcturus and Denebola. It is called Spica [30] Virginis, and is the chief star in the constellation Virgo, or the Virgin. The stars on the breast and wings can be found with the aid of the map.

South of Virgo is a trapezium of four stars, which are in the constellation of Corvus, or the Crow.

Constellations Visible in May. At the middle of May, Taurus, Orion, and Canis Major have set; Vega has just come up in the northeast; and between Vega and Arcturus we again see Hercules and Corona. Below Spica are two stars of the second magnitude, belonging to the constellation Libra, or the Balance. Another star of the fourth magnitude forms a triangle with these, and marks one pan of the balance. ([Plate 7]).

Constellations Visible in June. In June we shall find that Canis Minor, Perseus, Auriga, and Gemini have either set, or are on the point of setting; Arcturus is overhead; Cygnus and Aquila are just rising. Ophiuchus is well up; and low in the southeast we see again the red star Antares, in the constellation Scorpio, or the Scorpion ([Plate 6]). There is a star of the third magnitude on each side of Antares, and several stars of the third and fourth magnitudes in the head and claws. The configuration of these stars is much like a boy’s kite with a long tail. Scorpio is a very brilliant constellation, and is seen to better advantage in July and August.

Constellations Visible in July and August. We have now described all the important constellations visible in our latitude. Those which are seen in July and August are mainly those described under the last two or three months, and under September.

Southern Circumpolar Constellations. There are a number of constellations near the South Pole of the heavens which never rise in our latitude, just as there are certain ones near the North Pole which never set. These are called the southern circumpolar constellations.

CONSTELLATIONS VISIBLE EACH MONTH

The following table gives the constellations visible at eight o’clock in the evening about the middle of each month. The stars opposite the names of the constellations indicate those visible in the month designated at the top.

NAME OF CONSTELLATION	Sept.	Oct.	Nov.	Dec.	Jan.	Feb.	Mar.	April	May	June	July	Aug.
Ursa Major (er´sa mā´jor). The Greater Bear.	★	★	★	★	★	★	★	★	★	★	★	★
Ursa Minor (er´sa mī´nor). The Lesser Bear.	★	★	★	★	★	★	★	★	★	★	★	★
Draco (drak´ō). Dragon.	★	★	★	★	★	★	★	★	★	★	★	★
Cassiopeia (kas-si-o-pē´a). Lady’s Chair.	★	★	★	★	★	★	★	★	★	★	★	★
Cepheus (sē´fe-us).	★	★	★	★	★	★	★	★	★	★	★	★
Bootes (bo-ō´tēz). The Oxdriver or Plowman.	★							★	★	★	★	★
Corona Borealis (kō-rō´na bō-rē-ā´lis). The Northern Crown.	★	★							★	★	★	★
Ophiuchus (of-i-u´kus). The Serpent Bearer.	★									★	★	★
Sagittarius (saj-i-tā´ri-us). The Archer.	★										★	★
Hercules (her´ku-lēz).	★	★							★	★	★	★
Lyra (lī´ra). The Lyre.	★	★	★							★	★	★
Aquila (ak´wil-a).	★	★	★								★	★
Delphinus (del´fin-us). Dolphin.	★	★	★								★	★
Capricornus (kap-ri-kor´nus). The Goat.	★	★	★									★
Cygnus (sig´nus). The Swan.	★	★	★	★						★	★	★
Sagitta (saj´it-ta). The Arrow.	★	★	★								★	★
Aquarius (a-kwā´ri-us). The Water-bearer.	★	★	★	★
Piscis Australis (pis´sis aw-strā´lis). The Southern Fish.	★	★	★	★
Pegasus (peg´a-sus). The Winged Horse.	★	★	★	★	★
Andromeda (an-drom´e-da).	★	★	★	★	★	★	★
Perseus (per´sus).	★	★	★	★	★	★	★	★
Aries (a´ri-ēz). Ram.		★	★	★	★	★	★
Pisces (pis´sēz). Fishes.		★	★	★	★
Cetus (sē´tus). The Whale.			★	★	★	★
Triangulum (trī-ang´u-lum). The Triangle.			★	★	★	★	★
Auriga (aw-ri´ga). The Waggoner or The Charioteer.			★	★	★	★	★	★	★
Taurus (tau´rus). The Bull.			★	★	★	★	★	★
Lepus (lep´us). The Hare.				★	★	★	★
Orion (ō-ri´on). Giant and Hunter.				★	★	★	★	★
Gemini (jem´i-ni). The Twins.				★	★	★	★	★	★
Canis Major (kā´nis mā´jor). The Great Dog.					★	★	★	★
Canis Minor (kā´nis mī´nor). The Little Dog.					★	★	★	★	★
Cancer (kan´ser). The Crab.						★	★	★	★	★
Hydra (hī´dra). The Snake.						★	★	★	★	★
Leo (lē´ō). The Lion.						★	★	★	★	★	★
Coma Berenices (kō´ma ber-e-nī´sēz). Hair of Berenice.							★	★	★	★	★	★
Canes Venatici (ka´nēz vē-nā´ti-si). The Hunter’s Dogs.							★	★	★	★	★	★
Virgo (ver´gō). The Virgin.								★	★	★	★	★
Corvus (kor´vus). The crow.								★	★	★	★
Libra (li´bra). Balance.								★	★	★	★
Scorpio (skor´pi-ō). The Scorpion.										★	★	★

THE WONDERFUL
MILKY WAY

Everyone knows the Milky Way. It is one of the most striking sights of a clear night, for only on clear, moonless nights can we see its cloudy track of light across the heavens. More than any other celestial object it affects us with a sense of mystery and of unknown destiny as, indeed, it has affected men at all times and in all countries. To the American Indian it was the “path of souls.” In ancient mythology it had various meanings: thus, it was the highway of the gods to Olympus; or it sprang from the ears of corn dropped by Isis as she fled from her pursuer; or it marked the original course of the sun, which he later abandoned. In mediæval times it became associated by pilgrims with their own journeys.

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.

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.

[Large illustration] (235 kB)

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:

Name of Star	Color of Larger One	Color of Smaller 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 Name	Meaning	Constellation in 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.

GIANT SHADOWS CAST BY THE EARTH AND MOON

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

HOW THE MOON COMES BETWEEN THE EARTH AND SUN, CAUSING THE SHADOW SHOWN ABOVE

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.