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Having reached, in the course of the preceding lec ture, to the outermost confines of the visible creation, let us now return home from this survey of the “island universes " which crowd the illimitable regions of space, to the stars which compose our own cluster, and learn how far the human mind has progressed in its examination of the millions of suns which constitute, in a more definite sense, our own Milky Way.

We have already seen that the parallax of 61 Cygni rewarded the laborious and extraordinary efforts of Bessel. The example set by this great astronomer encouraged those who followed him, and while his results in this particular case have been confirmed in the most astonishing manner, the distances of many other stars have been obtained, until a sufficient amount of data has been accumulated to determine the approximate distances of the spheres of the fixed stars of different magnitudes. Struve es timates the mean distance of stars of the first magni tude to be 986,000 times the radius of the earth's orbit, or so remote that their light reaches us only after a journey of fifteen years and a half. Stars of the second magnitude send us their light in twentyeight years, those of the third magnitude in fortythree years; while the light from stars of the ninth Z

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magnitude only reaches the eye of the observer after traversing space for five hundred and eighty-six years, at the rate of twelve millions of miles in every minute of time.

My range of investigation does not permit me to explain, at this time, how these extraordinary conclusions have been reached. The reasoning, however, is close and clear, and the results are no doubt approximately correct.

Such, then, are the distances separating man from the objects of his research. To have attained to a knowledge of these distances even, is sufficiently wonderful, but what we are about to reveal as the results of human investigation among these far distant orbs, cannot fail to fill the mind with astonishment, and demonstrate the great truth that "man has been made but a little lower than the angels.”

Before it became possible to examine with absolute certainty the places of the stars, with a view to ascertain their absolute fixity, many difficult preliminary preparations had to be accomplished. Instruments of the most perfect kind must be provided, not only in their optical performances, but in their space-di viding machinery. Moreover, the places of the stars as determined by the best telescopes, must be corrected for every possible instrumental error. The two points to which the stars are referred are the north pole and the vernal equinox. In case any motions belong to these points, their amounts and directions must be ascertained and allowed for.' Then the effects of refraction, and of the abberration of light, were indispensable to a perfect investigation of the absolute places of the stars.

All these and many other preliminary matters having been satisfactorily determined, it became possible to examine, in the most critical manner, the places of the stars, and to learn whether indeed, (as had been supposed for thousands of years), their configurations were eternal and unchangeable, or whether they moved among themselves with a motion rendered so slow by their immense distance, as hitherto to have escaped the most scrutinizing watch.

Fully armed with the necessary instruments, it did not require many years to determine the grand truth, that among the tens of thousands of stars which fill the heavens, not a solitary one, in all probability, is in a state of absolute rest. Many were found to move so swiftly, that their velocity was determined even in a single year; while others, in consequence of their enormous distance, may require centuries to detect any appreciable change. In the outset these extraordinary movements seemed to be directed by no law-some stars were sweeping in one direction, and some in another. Motion, ceaseless, eternal motion, seems to be stamped on the entire universe, and while the stars are pursuing their mighty orbits, we cannot resist the idea that our own sun, the centre of our great planetary system, itself a star, must participate in the general movement, and is, in all probability, urging its flight, accompanied by all its planets, satellites, and comets, to some unknown region of space.

The revolution of the stars, the organization of the grand cluster with which our sun is associated, the demonstration of the sun's absolute translation through space, its direction, velocity, and period, are the topics to which I invite your attention in the closing lectura of the present course.

When forced to acknowledge the rotation of our globe on its axis, and its swift orbitual motion, surrounded by wheeling planets and flying comets, the mind naturally retreats to the sun as the great immoveable centre, where it can rest and contemplate these circling worlds. But even here, as we shall presently see, there is no rest. The sun himself becomes a subordinate member of a grander combination of worlds, and, obedient to higher influence, sweeps around in its unmeasured orbit.

We shall present a rapid summary of the evidences of change among the fixed stars, and then proceed to develop the reasoning by which the direction and velocity of the sun's motion in space has been determined.

More than two thousand years ago, the celebrated Greek astronomer, Hipparchus, was astonished by the sudden bursting forth of a brilliant star in a region of the heavens where none had previously existed. Up to this time, no doubt of the immutability of the starry sphere seems to have been entertained, and while the philosopher gazed and wondered, he resolved to execute a work from which posterity might learn the changes of the celestial sphere. He undertook and completed his great catalogue of the places of a thousand stars, locating them with all the accuracy permitted by the rude instruments then in use. Subsequent observers, by comparing their own determined positions of the stars with their places as fixed on the catalogue of Hipparchus, could readily perceive any sensible change which might occur in

their configuration, the appearance of new stars, or the disappearance of those which had once existed.

The sudden breaking forth of a new star is a phenomenon of such wonderful character that we might well doubt the possibility of its occurrence, if we were obliged to rely on the historical account transmitted to us from the time of Hipparchus. But, fortunately, more than one brilliant example of the kind has occurred in modern times, presenting the most unequivocal evidence of the reality of this inexplicable wonder.

In 1572, a new star of great splendor appeared suddenly in the constellation Cassiopeia, occupying a position which had previously been blank. This star was first perceived by Schuler, of Wittemburg, on the 6th of August. It was detected by Tycho, the Danish astronomer, on the 11th of the followiny November, and the wonder produced by this most extraordinary phenomenon induced him to give to the star the most unremitting attention. Its magnitude increased until it is said to have surpassed even Jupiter in splendor, and finally became visible in the day time. It retained its greatest magnitude but for a very short time, when it commenced to diminish in brilliancy, changing from white to yellow, then to reddish, and finally it became faintly blue; and so diminishing by degrees, it vanished from the sight in March, 1574, and has never since been seen.

In the year 1604, while the scholars of Kepler were engaged in observations of Mars, Jupiter and Saturn, then in close proximity to each other, having been interrupted a day or two by clouds, on the return of fine weather, Maestlin was astonished to find

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