Astronomy for Amateurs - Camille Flammarion (ebook reader with android os TXT) 📗
- Author: Camille Flammarion
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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 antithesis, 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]
α and β denote the front wheels of the Chariot generally known as the "pointers;" γ and δ the hind wheels; ε, ζ, η 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 (δ) of the quadrilateral, which is of the third order.
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 β to α, 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: ζ, 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.
If a straight line is drawn (Fig. 4) from β through α, which forms the extremity of the square, and is prolonged by a quantity equal to the distance of α 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 constitute the Dragon.
We will continue our journey by way of Cassiopeia, 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 α β below, the line χ γ forms the seat, and γ δ ε its back.
If a straight line is drawn from δ of the Great Bear, and prolonged beyond the Pole-Star in a quantity 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.
If in the next place, starting from the stars α and δ in the Great Bear, we draw two lines which join at Polaris and are prolonged beyond Cassiopeia, 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.
γ 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.
Above β 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 α, β and γ of Perseus form a concave bow which will serve in a new orientation. If it is prolonged in the direction of δ, 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 δ in Perseus, a line is drawn toward the South, we reach the Pleiades, a gorgeous cluster 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, β of Perseus, or Algol, which forms a little triangle with two others smaller than itself. This star is peculiar in that, instead of shining 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 passes 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 shores, 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, α of the Herdsman, which is at the extremity of this pentagonal constellation. The principal stars of this asterism are of the third magnitude, with the exception of α, 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, α, or the Pearl, which is of the second magnitude. This is the Corona Borealis. It is very easily recognized (Fig. 8).
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 α 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 principal 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 constituents (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, β or Albirio: α 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.
Not far off is the fine Eagle, which spreads its wings in the Milky Way, and in which the star Altaïr, α, of first magnitude, is situated between its two satellites, β and γ.
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 principal 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 circumpolar. 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 passes 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,"
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