III.

THE SIDEREAL SYSTEM.

"He telleth the number of the stars; He calleth them all by their

names.

PSALM CXlvii. 4.

THE SIDEREAL SYSTEM.

I. THE STARS.

N our celestial journey we have reached Neptune, the sentinel outpost of the solar system. We are now nearly 2,800,000,000 miles from our sun. Yet we are apparently no nearer the fixed stars than when we 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 familiar. Between them and us there is still 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 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 difference between a planet and a fixed star.

The Annual Parallax of the Fixed Stars.—When speaking of this subject on page 121, we said that 186,000,000 miles, or the diameter of the earth's orbit, is the unit for measuring the parallax of the fixed stars. Yet when the stars are viewed from even these extreme points, they manifest so 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 twenty trillions of miles. This is probably by no means its actual 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 rate of 186,000 miles per second. A ray at that speed would, in one day, plunge out into the abyss beyond Neptune six times the distance of that planet from the sun. Yet it must sweep on at this prodigious speed, day and night, for over 3 years to span the gulf

* David Gill, the Royal Astronomer at the Cape of Good Hope, has recently deter mined the parallax of a Centauri to be 0.75. This would make its distance 275,000 astronomical units. 275,000 × 93,000,000 miles = over 25 trillion miles. Light would require about 4 years to travel this enormous distance. Vega's parallax is placed at not far from 0.2, which indicates a distance of about 1,000,000 astronomical units. Hence, Vega shines upon us from the inconceivable distance of ninety-three trillion miles! The parallax of Sirius has been variously estimated at from 0.16 to 0.38. Newcomb places this star at more than a million radii of the earth's orbit away from us, yet its light is four times as brilliant as that of any other star. The difficulty of measuring the stellar parallax may be judged from the fact that 1′′ measures the angle at which a globe three-tenths of an inch in diameter would be seen when a mile away.

and reach a stopping point at 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 fardistant 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, they are moving much more swiftly than any of the planets. Thus, Arcturus flies through space at the astonishing rate of 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 the stars 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 selfluminous, and are doubtless each the center 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 our system, and that as a small star.

Our System in Motion-Like all the other stars,