Astronomy for Amateurs Part 2

How, then, is one to distinguish them? How can they be readily found and named? There are so many of them!

Do not fear; it is quite a simple matter. In studying the surface of the Earth we make use of geographical maps on which the continents and seas of which it consists are drawn with the utmost care. Each country of our planet is subdivided into states, each of which has its proper name. We shall pursue the same plan in regard to the Heavens, and it will be all the easier since the Great Book of the Firmament is constantly open to our gaze. Our globe, moreover, actually revolves upon itself so that we read the whole in due sequence. Given a clear atmosphere, and a little stimulus to the will from our love of truth and science, and the geography of the Heavens, or "uranography," will soon be as familiar to us as the geography of our terrestrial atom.

On a beautiful summer's night, when we look toward the starry sky, we are at first aware only of a number of s.h.i.+ning specks. The stars seem to be scattered almost accidentally through s.p.a.ce; they are so numerous and so close to one another that it would appear rash to attempt to name them separately. Yet some of the brighter ones particularly attract and excite our attention. After a little observation we notice a certain regularity in the arrangement of these distant suns, and take pleasure in drawing imaginary figures round the celestial groups.

That is what the ancients did from a practical point of view. In order to guide themselves across the trackless ocean, the earliest Phenician navigators noted certain fixed bearings in the sky, by which they mapped out their routes. In this way they discovered the position of the immovable Pole, and acquired empire over the sea. The Chaldean pastors, too, the nomad people of the East, invoked the Heavens to a.s.sist in their migrations. They grouped the more brilliant of the stars into Constellations with simple outlines, and gave to each of these celestial provinces a name derived from mythology, history, or from the natural kingdoms. It is impossible to determine the exact epoch of this primitive celestial geography. The Centaur Chiron, Jason's tutor, was reputed the first to divide the Heavens upon the sphere of the Argonauts. But this origin is a little mythical! In the Bible we have the Prophet Job, who names Orion, the Pleiades, and the Hyades, 3,300 years ago. The Babylonian Tables, and the hieroglyphs of Egypt, witness to an astronomy that had made considerable advance even in those remote epochs. Our actual constellations, which are doubtless of Babylonian origin, appear to have been arranged in their present form by the learned philosopher Eudoxus of Cnidus, about the year 360 B.C. Aratus sang of them in a didactic poem toward 270. Hipparchus of Rhodes was the first to note the astronomical positions with any precision, one hundred and thirty years before our era. He cla.s.sified the stars in order of magnitude, according to their apparent brightness; and his catalogue, preserved in the Almagest of Ptolemy, contains 1,122 stars distributed into forty-eight Constellations.

The figures of the constellations, taken almost entirely from fable, are visible only to the eyes of the imagination, and where the ancients placed such and such a person or animal, we may see, with a little good-will, anything we choose to fancy. There is nothing real about these figures. And yet it is indispensable to be able to recognize the constellations in order to find our way among the innumerable army of the stars, and we shall commence this study with the description of the most popular and best known of them all, the one that circles every night through our Northern Heavens. Needless to name it; it is familiar to every one. You have already exclaimed--the Great Bear!

This vast and splendid a.s.sociation of suns, which is also known as the Chariot of David, the Plow or Charles's Wain, and the Dipper, is one of the finest constellations in the Heavens, and one of the oldest--seeing that the Chinese hailed it as the divinity of the North, over three thousand years ago.

If any of my readers should happen to forget its position in the sky, the following is a very simple expedient for finding it. Turn to the North--that is, opposite to the point where the sun is to be found at midday. Whatever the season of the year, day of the month, or hour of the night, you will always see, high up in the firmament, seven magnificent stars, arranged in a quadrilateral, followed by a tail, or handle, of three stars. This magnificent constellation never sinks below our horizon. Night and day it watches above us, turning in twenty-four hours round a very famous star that we shall shortly become acquainted with. In the figure of the Great Bear, the four stars of the quadrilateral are found in the body, and the three at the extremity make the tail. As David's Chariot, the four stars represent the wheels, and the three others the horses.

Sometimes our ancestors called them the Seven Oxen, the "oxen of the celestial pastures," from which the word septentrion (_septem triones_, seven oxen of labor) is derived. Some see a Plowshare; others more familiarly call this figure the Dipper. As it rotates round the pole, its outline varies with the different positions.

It is not easy to guess why this constellation should have been called the Bear. Yet the name has had a certain influence. From the Greek word _arctos_ (bear) has come arctic, and for its ant.i.thesis, antarctic. From the Latin word _trio_ (ox of labor) has come septentrion, the seven oxen. Etymology is not always logical. Is not the word "venerate"

derived from Venus?

In order to distinguish one star from another, the convention of denoting them by the letters of the Greek Alphabet has been adopted, for it would be impossible to give a name to each, so considerable is their number.[3]

[alpha] and [beta] denote the front wheels of the Chariot generally known as the "pointers;" [gamma] and [delta] the hind wheels; [epsilon], [zeta], [eta] the three horses. All these stars are of the second order of magnitude (the specific meaning of this expression will be explained in the next chapter), except the last ([delta]) of the quadrilateral, which is of the third order.

[Ill.u.s.tration: FIG. 3.--The Great Bear (or Dipper), and the Pole-Star.]

Figure 3 gives the outline of this primitive constellation. In revolving in twenty-four hours round the Pole, which is situated at the prolongation of a line drawn from [beta] to [alpha], it occupies every conceivable position,--as if this page were turned in all directions.

But the relative arrangement of the seven stars remains unaltered. In contemplating these seven stars it must never be forgotten that each is a dazzling sun, a center of force and life. One of them is especially remarkable: [zeta], known as Mizar to the Arabs. Those who have good sight will distinguish near it a minute star, Alcor, or the Cavalier, also called Saidak by the Arabs--that is, the Test, because it can be used as a test of vision. But further, if you have a small telescope at your disposal, direct it upon the fine star Mizar: you will be astonished at discovering two of the finest diamonds you could wish to see, with which no brilliant is comparable. There are several double stars; these we shall become acquainted with later on.

Meantime, we must not forget our celestial geography. The Great Bear will help us to find all the adjacent constellations.

[Ill.u.s.tration: FIG. 4.--To find the Pole-Star.]

If a straight line is drawn (Fig. 4) from [beta] through [alpha], which forms the extremity of the square, and is prolonged by a quant.i.ty equal to the distance of [alpha] from the tip of the handle, we come on a star of second magnitude, which marks the extremity of a figure perfectly comparable with the Great Bear, but smaller, less brilliant, and pointing in the contrary direction. This is the Little Bear, composed, like its big brother, of seven stars; the one situated at the end of the line by which we have found it is the Pole-Star.

Immovable in the region of the North Pole, the Pole-Star has captivated all eyes by its position in the firmament. It is the providence of mariners who have gone astray on the ocean, for it points them to the North, while it is the pivot of the immense rotation accomplished round it by all the stars in twenty-four hours. Hence it is a very important factor, and we must hasten to find it, and render it due homage. It should be added that its special immobility, in the prolongation of the Earth's axis, is merely an effect caused by the diurnal movements of our planet. Our readers are of course aware that it is the earth that turns and not the sky. But evidence of this will be given later on. In looking at the Pole-Star, the South is behind one, the East to the right, and the West to the left.

Between the Great and the Little Bear, we can distinguish a winding procession of smaller stars. These const.i.tute the Dragon.

We will continue our journey by way of Ca.s.siopeia, a fine constellation placed on the opposite side of the Pole-Star in relation to the Great Bear, and shaped somewhat like the open limbs of the letter W. It is also called the Chair. And, in fact, when the figure is represented with the line [alpha] [beta] below, the line [chi] [gamma] forms the seat, and [gamma] [delta] [epsilon] its back.

If a straight line is drawn from [delta] of the Great Bear, and prolonged beyond the Pole-Star in a quant.i.ty equal to the distance which separates these two stars, it is easy to find this constellation (Fig.

5). This group, like the preceding, never sets, and is always visible, opposite to the Great Bear. It revolves in twenty-four hours round the Pole-Star, and is to be seen, now above, now below, now to the right, now to the left.

[Ill.u.s.tration: FIG. 5.--To find Ca.s.siopeia.]

[Ill.u.s.tration: FIG. 6.--To Find Pegasus and Andromeda.]

If in the next place, starting from the stars [alpha] and [delta] in the Great Bear, we draw two lines which join at Polaris and are prolonged beyond Ca.s.siopeia, we arrive at the Square of Pegasus (Fig. 6), a vast constellation that terminates on one side in a prolongation formed of three stars.

These three last stars belong to Andromeda, and themselves abut on Perseus. The last star in the Square of Pegasus is also the first in Andromeda.

[gamma] of Andromeda is a magnificent double orb, to which we shall return in the next chapter, _i.e._, the telescope resolves it into two marvelous suns, one of which is topaz-yellow, and the other emerald-green. Three stars, indeed, are visible with more powerful instruments.

[Ill.u.s.tration: FIG. 7.--Perseus, the Pleiades, Capella.]

Above [beta] and near a small star, is visible a faint, whitish, luminous trail: this is the oblong nebula of Andromeda, the first mentioned in the history of astronomy, and one of the most beautiful in the Heavens, perceptible to the unaided eye on very clear nights.

The stars [alpha], [beta] and [gamma] of Perseus form a concave bow which will serve in a new orientation. If it is prolonged in the direction of [delta], we find a very brilliant star of the first magnitude. This is Capella, the Goat, in the constellation of the Charioteer (Fig. 7).

If coming back to [delta] in Perseus, a line is drawn toward the South, we reach the Pleiades, a gorgeous cl.u.s.ter of stars, scintillating like the finest dust of diamonds, on the shoulder of the Bull, to which we shall come shortly, in studying the Constellations of the Zodiac.

Not far off is a very curious star, [beta] of Perseus, or Algol, which forms a little triangle with two others smaller than itself. This star is peculiar in that, instead of s.h.i.+ning with a fixed light, it varies in intensity, and is sometimes pale, sometimes brilliant. It belongs to the category of variable stars which we shall study later on. All the observations made on it for more than two hundred years go to prove that a dark star revolves round this sun, almost in the plane of our line of sight, producing as it pa.s.ses in front of it a partial eclipse that reduces it from the second to the fourth magnitude, every other two days, twenty hours, and forty-nine minutes.

And now, let us return to the Great Bear, which aided us so beneficently to start for these distant sh.o.r.es, and whence we shall set out afresh in search of other constellations.

If we produce the curved line of the tail, or handle, we encounter a magnificent golden-yellow star, a splendid sun of dazzling brilliancy: let us make our bow to Arcturus, [alpha] of the Herdsman, which is at the extremity of this pentagonal constellation. The princ.i.p.al stars of this asterism are of the third magnitude, with the exception of [alpha], which is of the first. Alongside of the Herdsman is a circle consisting of five stars of the third and fourth magnitude, save the third, [alpha], or the Pearl, which is of the second magnitude. This is the Corona Borealis. It is very easily recognized (Fig. 8).

[Ill.u.s.tration: FIG. 8.--To find Arcturus, the Herdsman, and the Northern Crown.]

A line drawn from the Pole-Star to Arcturus forms the base of an equilateral triangle, the apex of which, situated opposite the Great Bear, is occupied by Vega, or [alpha] of the Lyre, a splendid diamond of ideal purity scintillating through the ether. This magnificent star, of first magnitude, is, with Arcturus, the most luminous in our Heavens. It burns with a white light, in the proximity of the Milky Way, not far from a constellation that is very easily recognized by the arrangement of its princ.i.p.al stars in the form of a cross. It is named Cygnus, the Bird, or the Swan (Fig. 9), and is easy to find by the Square of Pegasus, and the Milky Way. This figure, the brilliancy of whose const.i.tuents (of the third and fourth magnitudes) contrasts strongly with the pallor of the Milky Way, includes at its extremity at the foot of the Cross, a superb double star, [beta] or Albirio: [alpha] of Cygnus is also called Deneb. The first star of which the distance was calculated is in this constellation. This little orb of fifth magnitude, which hangs 69,000,000,000,000 kilometers (42,000,000,000,000 miles) above our Earth, is the nearest of all the stars to the skies of Europe.

[Ill.u.s.tration: FIG. 9.--The Swan, Vega, the Eagle.]

Not far off is the fine Eagle, which spreads its wings in the Milky Way, and in which the star Altar, [alpha], of first magnitude, is situated between its two satellites, [beta] and [gamma].

The Constellation of Hercules, toward which the motions of the Sun are impelling us, with all the planets of its system, is near the Lyre. Its princ.i.p.al stars can be recognized inside the triangle formed by the Pole-Star, Arcturus, and Vega.

All the Constellations described above belong to the Northern Hemisphere. Those nearest the pole are called circ.u.mpolar. They revolve round the pole in twenty-four hours.

Having now learned the Northern Heavens, we must come back to the Sun, which we have left behind us. The Earth revolves round him in a year, and in consequence he seems to revolve round us, sweeping through a vast circle of the celestial sphere. In each year, at the same period, he pa.s.ses the same points of the Heavens, in front of the same constellations, which are rendered invisible by his light. We know that the stars are at a fixed position from the Earth, whatever their distance, and that if we do not see them at noon as at midnight, it is simply because they are extinguished by the dazzling light of the orb of day. With the aid of a telescope it is always possible to see the more brilliant of them.

The Zodiac is the zone of stars traversed by the Sun in the course of a year. This word is derived from the Greek _Zodiakos_, which signifies "animal," and this etymology arose because most of the figures traced on this belt of stars represent animals. The belt is divided into twelve parts that are called the twelve Signs of the Zodiac, also named by the ancients the "Houses of the Sun," since the Sun visits one of them in each month. These are the signs, with the primitive characters that distinguish them: the Ram [Aries], the Bull [Taurus], the Twins [Gemini], the Crab [Cancer], the Lion [Leo], the Virgin [Virgo], the Balance [Libra], the Scorpion [Scorpio], the Archer [Sagittarius], the Goat [Capricorn], the Water-Carrier [Aquarius], the Fishes [Pisces]. The sign [Aries] represents the horns of the Ram, [Taurus] the head of the Bull, and so on.

If you will now follow me into the Houses of the Sun you will readily recognize them again, provided you have a clear picture of the princ.i.p.al stars of the Northern Heavens. First, you see the Ram, the initial sign of the Zodiac; because at the epoch at which the actual Zodiac was fixed, the Sun entered this sign at the vernal equinox, and the equator crossed the ecliptic at this point. This constellation, in which the horns of the Ram (third magnitude) are the brightest, is situated between Andromeda and the Pleiades. Two thousand years ago, the Ram was regarded as the symbol of spring; but owing to the secular movement of the precession of the equinoxes, the Sun is no longer there on March 21: he is in the Fishes.

To the left, or east of the Ram, we find the Bull, the head of which forms a triangle in which burns Aldebaran, of first magnitude, a magnificent red star that marks the right eye; and the Hyades, scintillating pale and trembling, on its forehead. The timid Pleiades, as we have seen, veil themselves on the shoulder of the Bull--a captivating cl.u.s.ter, of which six stars can be counted with the unaided eye, while several hundred are discovered with the telescope.

Next the Twins. They are easily recognized by the two fine stars, [alpha] and [beta], of first magnitude, which mark their heads, and immortalize Castor and Pollux, the sons of Jupiter, celebrated for their indissoluble friends.h.i.+p.

Cancer, the Crab, is the least important sign of the Zodiac. It is distinguished only by five stars of fourth and fifth magnitudes, situated below the line of Castor and Pollux, and by a pale cl.u.s.ter called Praesepe, the Beehive.

The Lion next approaches, superb in his majesty. At his heart is a gorgeous star of first magnitude, [alpha] or Regulus. This figure forms a grand trapezium of four stars on the celestial sphere.

The Virgin exhibits a splendid star of first magnitude; this is Spica, which with Regulus and Arcturus, form a triangle by which this constellation can be recognized.

The Balance follows the Virgin. Its scales, marked by two stars of second magnitude, are situated a little to the East of Spica.

We next come to the eighth constellation of the Zodiac, which is one of the most beautiful of this belt of stars. Antares, a red star of first magnitude, occupies the heart of the venomous and accursed Scorpion. It is situated on the prolongation of a line joining Regulus to Spica, and forms with Vega of the Lyre, and Arcturus of the Herdsman, a great isosceles triangle, of which this latter star is the apex.