The next important step in the growth of knowledge of the solar system was Galileo’s study of the laws of fall and the composition of two kinds of motion, like fall and projection, as in the case of a projectile. This was followed by Newton’s magnificent extension of gravity from the earth to the whole of space, with the assumption and proof that the intensity of gravitational attraction varies inversely as the square of the distance.

These ideas, combined with Kepler’s laws, led at once to the theory of planetary motion and its proof, in Newton’s “Principia.”〖H. C., xxxix, 150, and see General Index in vol. l, under Newton.〗 The motion of the planets appeared as the resultant of their tendency to go on in the direction in which they were moving (inertia), and their tendency to fall to the sun (gravitation). The problem 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.

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