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In our celestial journey we have reached Neptune, the sentinel outpost of the solar system. We are now 2,750 millions of miles from our sun. Yet we are apparently no nearer the fixed stars than when we first started. They twinkle as serenely there in the far-off sky as to us here on the earth. The heavens by night, with the exception of a few changes in the planets, look perfectly familiar. Between them and us there is a vast chasm which no imagination can bridge; a distance so immense that figures are meaningless, and we can only call it space,—so profound that to us it is limitless, though beyond we see other worlds twinkling like distant lights over a waste of waters.
We Never See The Stars.—This assertion seems almost paradoxical, yet it is strictly true. So far are the stars removed from us, that we see only the light they send, but not the surface of the worlds themselves. They are merely glittering points of light. The most powerful telescope fails to produce a sensible disk. This constitutes a marked point of difference between a planet and a fixed star.
The Annual Parallax Of The Fixed Stars.—When speaking of this subject on page 139, we said that 183,000,000 miles, or the diameter of the earth's orbit, is taken as the unit for measuring the parallax of the fixed stars. Yet when the stars are viewed from even these extreme points, they manifest so very slight a change of place, that to estimate it is one of the most delicate feats of astronomy.* At the. present time, it is considered that the star Alpha (a) Centauri in the southern heavens is the nearest to the earth. Its parallax is judged to be about 1". Its distance is more than 200,000 times that of the earth from the sun, or nineteen trillions of miles. This is probably by no means its extreme distance, but merely the limit within which it cannot be, but beyond which it must be. These figures convey to our mind no idea of distance. Our imagination fails to grasp the thought, or to picture the vast void across which we are gazing. We remember that light moves at the wonderful rate of 183,000 miles per second. A ray at this speed would plunge out into the abyss beyond Neptune, in one day, six times the distance of that planet
* Prof. Airy says the star which gives the greatest parallax of any, presents the same angle as that at which a circle sixtenths of an inch in diameter would be seen at the distance of a mile 1
from the sun. Yet it must sweep on at this prodigious speed, day and night, for three years and nine months to span the gulf and reach a stopping point at the nearest fixed star. "To a spectator standing at a Centauri, the entire diameter of the earth's orbit would be hidden by a thread °f an mch in diameter, held at a distance of 650 feet from the eye." That is to say, a line 183,000,000 miles long, looked at broadside, would shrink into a mere point. If our sun were removed to that distance, it would shine with a light only equal to that of the north polar star, and would take its place among the constellations as a fixed star.
This, we must remember, is the distance of the nearest fixed star. It has been estimated that the average time required for the light of the smallest stars which are visible to the naked eye to reach the earth is about 125 years. What, then, shall we say of those far-distant ones, whose faint light appears as a mere fleecy whiteness even in the most powerful telescopes? The conclusion is irresistible, that the light we receive set out on its sidereal journey far back in the past, perhaps before the creation of man!
Motion Of The Fixed Stars.—It will aid us still further in comprehending the immense distances of the stars, to learn that though they seem to be fixed, yet they are moving much more swiftly than any of the planets. Thus, Arcturus flies through space at the astonishing rate of about 200,000 miles per hour, or nearly twice that of Mercury, and more than three times that of the earth. Yet, through all our lifetime, we shall never be able to detect any change in its position. It requires three centuries for it to move over the starry vault a space equal to the moon's apparent diameter.
The Stars Are Suns.—The vast distance at which they are known to be, precludes the thought of their shining, like the planets or the moon, by reflecting back the light of our sun. They must be self-luminous, and are doubtless each the centre of a system of planets and satellites.
Our Sun A Star.—As we see only the suns of these distant systems, so their inhabitants see only the sun of ours, and that as a small star.
Our System Itself In Motion.—Like all the other stars, our sun is in motion. It is sweeping onward, with its retinue of worlds, 150,000,000 miles per year, toward a point in the constellation Hercules. The Pleiades are thought to be the centre around which this great movement is taking place, but the orbit is so vast and the centre so remote, that nothing definite is yet known.
The Number Of The Fixed Stars.—As we look at the heavens on a clear night, the stars seem almost innumerable. To count them, one would think almost as interminable a task as to number the leaves on the trees. It is, therefore, somewhat startling to learn that the entire number visible to the most piercing eyesight, does not exceed 6,000, while few