Page:Popular Science Monthly Volume 2.djvu/240

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Making leisure, in the summer of 1869, for entering upon such an examination, I was led to several results, which not only confirmed the above-mentioned theory, but suggested relations which I had not hitherto thought of. Some of these results are discussed in the article called "Are there any Fixed Stars?" already referred to; others are presented in an article called "Star-drift," in the Student for October, 1870. The special results on which Dr. Huggins's recent discoveries throw light, were first publicly announced in a paper read before the Royal Society, on January 20, 1870.

I had constructed a chart in which the proper motions of about 1,200 stars were pictured. To each star a minute arrow was affixed, the length of the arrow indicating the rate at which the star is moving on the celestial vault, while the direction in which the arrow pointed shows the direction of the star's apparent motion. This being done, it was possible to study the proper motions much more agreeably and satisfactorily than when they were simply presented in catalogues. And certain features, hitherto unrecognized, at once became apparent. Among these was the peculiarity which I have denominated "Star-drift;" the fact, namely, that certain groups of stars are travelling in a common direction.[1] This was indicated, in certain cases, in too significant a manner to be regarded as due merely to chance distribution in these stellar motions; and I was able to select certain instances in which I asserted that the drift was unmistakable and real.

Among these instances was one of a very remarkable kind. The "seven stars" of Ursa Major—the Septentriones of the ancients—are known to all. For convenience of reference, let us suppose these seven divided as when the group is compared to a wagon and horses. Thus, there are four wagon-wheels and three horses. Now, if we take the wagon-wheels in sequence round their quadrilateral (beginning with one of the pair farthest from the horses), so as to finish with the one which lies nearest to the horses, these are named by astronomers, in that order, Alpha, Beta, Gamma, and Delta, of the Great Bear. Thus, Alpha and Beta are the well-known pointers (Alpha nearest the pole), and Delta is the faintest star of the Septentrion set. The three horses are called in order Epsilon, Zeta, and Eta; Epsilon being nearest to Delta. Now, when the proper motions of these seven stars had been mapped, I found that, whereas Alpha and Eta are now moving much as they would if the sun's motion were alone in question, the other five are all moving at one and the same rate (on the star-sphere, that is) in almost the exactly opposite direction. Moreover, a small star close by

  1. I include this among "features hitherto unrecognized," though Michell had already noted the fact that the stars are arranged into systems. "We may conclude," he said, "that the stars are really collected together in clusters in some places, where they form a kind of systems; while in others there are few or none of them, to whatever cause this may be owing, whether to their mutual gravitation or to some other law or appointment of the Creator."