Page:Popular Science Monthly Volume 2.djvu/241

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Zeta (the middle horse), a star known to the Arabian astronomers as the "Test," because to see this star was held a proof of good eyesight, is moving in the same direction and at the same rate as Zeta and the rest of this set. And besides this star (which has also been called Jack by the middle horse), Zeta has a telescopic companion which also accompanies him in his motion on the celestial sphere.

After a careful consideration of these circumstances, and an analysis of the probabilities in favor of and against the theory that the concurrence of apparent motion was merely accidental, I came to the conclusion that the five large stars and the two smaller ones form a true drifting set. I found, on a moderate computation, that the odds were upward of half a million to one against the concurrence being accidental; and, since I had recognized other instances of concurrence not less striking, I felt that it was morally certain that these stars belong to one star-family.

The reader will perhaps not be surprised to learn, however, that before publishing this conclusion I submitted it (in July, 1869) to one who was, of all men, the best able to pronounce upon its significance—the late Sir John Herschel. I have the letter (dated August 1, 1869), which he sent in reply, before me as I write. The part relating to my discovery runs as follows: "The considerations you adduce relative to the proper motions of the stars are exceedingly curious and interesting. Of late years catalogues have gone into much detail, and with such accuracy that these motions are of course much better known to us than some twenty or thirty years ago. The community of proper motion over large regions (of which you give a picture in Gemini and Cancer) is most remarkable, and the coincidence of proper motion in Beta, Gamma, Delta, Epsilon, and Zeta Ursæ Majoris, most striking. Your promised paper on this subject cannot fail to be highly interesting."[1]

In a letter written on May 11, 1870, and referring not to another letter of mine, but to my "Other Worlds," Sir John Herschel remarked, "The cases of star-drift such as that in Ursa Major are very striking, and richly merit further careful examination."

My first public expression of opinion respecting the star-drift in Ursa Major was conveyed in the following terms: "If these five stars indeed form a system (and I can see no other reasonable explanation

  1. He proceeds as follows (the passage is removed from the main text, as relating to a different branch of the subject): "I cannot say that I am at all surprised at its being found that the average proper motions of stars of small magnitudes are not less than those of large, considering (as I have always done) that the range of individual magnitude (i. e., lustre) must be so enormous that multitudes of very minute stars may in fact be our very near neighbors." Compare my paper on "The Sun's Journey through Space," above referred to, which paper also deals with the point touched on in the next sentence of Sir John Herschel's letter: "Your remark on the conclusion I have been led to draw, relative to the small effect of the correction due to the sun's proper motion, will require to be very carefully considered, and I shall of course give it every attention."