THE STAR-DEPTHS.

THE awe with which the thoughtful student of astronomy in our day contemplates the star-depths can scarcely exceed the simple wonder of the Chaldæan herdsman, who gazed on the mysterious vault of heaven and watched the constellations as they passed with stately motion along their nocturnal arcs. Brought up from our youth to regard the fixed stars as the peers of our own sun, and the sun as an orb exceeding more than a million times in volume this earth on which we live, the grandeur of these conceptions is yet in part marred by their familiarity. The ancient astronomer, even though he might believe, with Aristarchus of Samos, that the stars are golden studs upon the crystal dome of heaven, and the sun scarce larger than the Peloponnesus, must yet have been penetrated with a profound sense of the mystery surrounding all he saw. We have learned so much, that we are apt to feel as though all knowledge were within our grasp. The orbs of heaven have been weighed and analyzed, they have been tracked on orbital paths around each other, they have been counted and gauged and charted, until it would seem as though their domain had been completely ex

plored and mastered. They might guess and theorize, but they knew scarce anything of the stars. And out of their want of knowledge sprang a sense of awe, which probably surpassed in intensity the feeling with which even the most thoughtful astronomers of our own day regard the orbs tenanting the depths of heaven.

It was not so with the ancients.

It is my purpose in the present paper to attempt to restore to the sidereal system something of that mystery which pervaded it of old. I wish to shew that some, at least, of those views which had seemed most thoroughly established, have but a slight foundation, if any, on which to rest; and that so far from having penetrated the secret of the star-depths, we stand as yet but on the threshold of that mighty domain which belongs to the astronomy of the future.

Since the establishment of the Copernican theory, the extension of the sidereal system as recognized by astronomers, has been progressively increasing. I do not refer here merely to the increase of telescopic power, and the corresponding increase of the range of astronomical vision. That increase of range could only tell us what might readily have been guessed without it—namely, that the vast spaces which lie beyond the range of any given telescopic power are not untenanted by stars. But the feature to which I would especially invite attention is the increase of the estimated scale on which the sidereal system is built, the increase in our estimate of the size and brightness of individual orbs, and the yet more surprising

increase in our estimate of the distances which separate orb from orb.

The first step in this progress was the most remarkable of all. It was very justly urged by Tycho Brahé, that, if the Copernican theory were true, the stars must be regarded as immensely more distant than astronomers had ever ventured to imagine. Tycho Brahé did not, indeed, know how far the Earth actually is from the Sun; but he knew that the extent of her orbit or of the Sun's orbit, according as the Sun or the Earth is fixed, must be measured by millions of miles. "Is it credible," he asked, "that although moving in an orbit so enormous in extent, the Earth as seen from the nearest star would seem absolutely unchanged in position?" Yet this must be the case if the Copernican theory be true. For not one of the stars seems to move, as the Earth completes the circuit spoken of by Copernicus; and if no star moves as seen from the Earth, the Earth must appear at rest as seen from each star in the heavens. Each star must therefore lie at so enormous a distance, that the wide extent of the Earth's orbit is reduced to a mere point. "Such a conception," Tycho reasoned, "seems wholly inadmissible; and therefore the Copernican theory must be erroneous."

This reasoning was valid, although the conclusion was incorrect. We must not class the objections urged by Tycho Brahé against the Copernican system, with those unmeaning arguments by means of which the Ptolemaists had long defended their position. Undoubtedly the conclusion that the stars are suns comparable in splendour

with our own, was not one to be lightly accepted. And yet no other conclusion could be adopted, if the motion of the Earth around the Sun were once admitted. We have just seen that the Earth's orbit, viewed from each of the fixed stars, would be reduced to a mere point. The Sun, then, which lies within that orbit, and whose relative dimensions were perfectly well known even in Tycho's time, would à fortiori be but a point as seen from even the nearest of the stars. If he were visible at all, it would merely be on account of the enormous intrinsic brilliancy of his light. But the stars are points of light, and the intrinsic brilliancy of their light also must be enormous, in order that they may be barely visible. Their seeming minuteness is at once seen to be no proof of real minuteness, when the fact is recognized that the Sun would appear at least as minute if viewed from the neighbourhood of a fixed star.

The mistake of Tycho Brahé consisted in his failing to consider that the whole question was one of evidence. If Copernicus and his followers could prove their case, any conclusions legitimately deducible were to be accepted, without any reference to the startling character of the views they might point to. When Tycho began to see the heavens opening out before him, and all the stars taking rank as suns, the blaze of splendour was too fierce-his mental vision was unprepared to contemplate so glorious a display, and he would fain have dropped a veil over that unbearable effulgence.

But the men who followed him were more daring.

Boldly grasping the weapons which Tycho Brahé had collected for an attack upon the Copernican theory, they turned those weapons against the Ptolemaists. Seizing the only available vantage-ground-that one peculiarity of the Solar System, without which the theories of Newton himself would never, in all probability, have had existence —the great astronomer Kepler found in the seemingly capricious motions of the planet Mars the means of abolishing at once and for ever the 'cycles and epicycles,' the 'centrics and eccentrics,' in which astronomers had so long put faith. Then Newton pushed the attack yet farther, setting forth the real significance of those laws which, in Kepler's hands, had seemed empirical. And lastly, one proof followed after another, until the new theories had become so firmly established that no one who comprehends their position has since ventured to attack them.

But in the meantime, while the confirmation of the Copernican theory was demonstrating as real those wonders at which Tycho Brahé had stood appalled, fresh light was thrown on the real dimensions of the universe of stars. For it was found, as research after research was directed to the point, that the Sun's distance had been hugely underrated, and that therefore even Tycho's estimate of the stars' distances and dimensions, according to the Copernican theory, fell far short of the truth. More and more scrutinizingly astronomers searched the evidence bearing on the subject of the Sun's distance; wider and wider grew the limits beyond which they proved