Page:The Harvard Classics Vol. 51; Lectures.djvu/118

 108 yielded completely, so far as two bodies are concerned, to the mathematical genius of Newton.

Still the revolution of the earth about the sun was not, by many astronomers, considered to be proved, while some even denied it. For if the earth really revolved about the sun, the relative positions of the stars ought not to appear the same to us from different parts of the orbit. Yet no difference in their places at the two solstices could be detected, although the stands of the observer were separated by a hundred and eighty million miles in the two instances.

James Bradley was the first person to obtain important results from the investigation of this problem of parallax. He found, not, to be sure, a periodic change of the apparent position of the stars that could be explained as parallax, but a different change of position, quite unexpected. This he called aberration, and recognized that it was due to a composition of the motion of the earth and of the light from the star itself, which is analogous to the entry of rain falling straight down, yet into the open front of a moving carriage. Here, nevertheless, was a proof, the more valuable because unexpected, of the earth's motion. It was not until 1837 that Bessel finally measured the parallax of a fixed star, and this finally ended the problem. The whole difficulty had been due merely to the enormous distance which separates us from the nearest of the stars.

SPECTRUM ANALYSIS

A new period in the history of astronomy followed upon the discovery of spectrum analysis by Bunsen and Kirchhoff. At the outset the chemical composition of the sun revealed itself. Later that of the stars became known; still later it became possible to classify the stars on the basis of their spectra, and at length it has become evident that variations in spectra are at least largely due to differences in the age of suns (the length of time during which cooling has gone on), that all stars are probably very much alike both chemically and physically, and that our sun is probably very much like all other stars. The geological doctrine of uniformity has been extended to astronomy.

This results in renewed interest in the nebular hypothesis and in novel speculations regarding the origin of the solar system. In like