Page:Astronomy for Everybody.djvu/265

moon it falls behind again to the average place. Thus a slight swing goes on in unison with the moon's motion around the earth. The amount of this swing is inversely proportional to the distance of the sun. Hence, by measuring this amount, its distance may be determined. As in other astronomical measurements, the difficulty of the determination is very great. A swing like this is very hard to measure without error; moreover, the problem of determining just how much swing the sun would produce at a given distance is one of the difficult problems of celestial mechanics, which has not yet been solved so satisfactorily as to leave no doubt whatever on the result.

The fourth method also rests on gravitation. If we only knew the exact relation between the mass of the earth and that of the sun; that is to say, if we could determine precisely how many times heavier the sun is than the earth, we could compute at what distance the earth must be placed from the sun in order to revolve around it in one year. The only difficulty, therefore, is to weigh the earth against the sun. This is most exactly done by finding the change in the position of the orbit of Venus produced by the earth's attraction. By comparing the positions of the orbit of Venus by its transits in 1761, 1769, 1874, and 1882, it is found that the orbit has a progressive motion, indicating that the mass of the sun is 332,600 times that of the earth and moon combined. Thus we are enabled to compute the distance of the sun by still another method.