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Page:Popular Science Monthly Volume 57.djvu/525

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515
CHAPTERS ON THE STARS.

From all these results it would seem that the most likely apex of the solar motion is toward the point in

Right Ascension, 280°
Declination, 38° north.

This point is situated in the constellation Lyra, about 2° from the first magnitude star Vega. The uncertainty of the result is more than this difference, four or five degrees at least. We may therefore state the conclusion in this form:

The apex of the solar motion is in the general direction of the constellation Lyra, and probably very near the star Vega, the brightest of that constellation.

It must be admitted that the wide difference between the position of the apex from large and from small proper motions, as found by Porter, Boss and Stumpe, require explanation. Since the apparent motions of the stars are less the greater their distance, these results, if accepted as real, would lead to the conclusion that the position of the solar apex derived from stars near to us was much further south than when derived from more distant stars. This again would indicate that our sun is one of a cluster or group of stars, having, in the general average, a different proper motion from the more distant stars. But this conclusion is not to be accepted as real until the subject has been more exhaustively investigated. The result may depend on the selection of the stars; and there is, as yet, no general agreement among investigators as to the best way of making the determination.

The next question which arises is that of the velocity of the solar motion. The data for this determination are more meagre and doubtful than those for the direction of the motion. The most obvious and direct method is to determine the parallactic motion of the stars of known parallax. Regarding any star 90° from the apex of the solar motion as in a state of absolute rest, we have the obvious rule that the quotient of its parallactic motion during any period, say a century, divided by its parallax, gives the solar motion during that period, in units of the earth's distance from the sun. In fact, by a motion of the sun through one such unit, the star would have an apparent motion in the opposite direction equal to its annual parallax. If the star were not 90° from the apex we can easily reduce its observed parallactic motion by dividing it by the sign of its actual distance from the apex.

Since every star has, presumably, a proper motion of its own, we can draw no conclusion from the apparent motion of any one star, owing to the impossibility of distinguishing its actual from its parallactic motion. We should, therefore, base our conclusion on the mean result from a great number of stars, whose average position or center of mass we might assume to be at rest. Here we meet the difficulty that there are