Page:The Harvard Classics Vol. 51; Lectures.djvu/117

Rh Copernicus showed that all the difficulties which the movements of the planets present would become very much less if the moon were left the only satellite of the earth, and the earth itself and all the planets were assumed to move around the sun. He did not prove—in truth being wise and realizing his own limitations, he did not seek to prove—this hypothesis, but only to present the reasons why it must appear the most probable explanation of the principal astronomical phenomena.

The new doctrine made converts slowly. At first it was opposed by the professional astronomers, with whose time-honored habits it interfered, and who were, for the most part, not competent to understand it. Later the opposition of the great Tycho Brahe worked against it for many years. Still later the opposition of theologians effectually cut off many converts, most notably Descartes. But the discovery of Kepler's laws completely destroyed the Ptolemaic system, and must have convinced nearly all reasonable men of the correctness of that of Copernicus. These famous laws are as follows: The line joining the sun with a planet sweeps over equal areas in equal periods of time. Every planet moves in an ellipse with the sun at one focus. The squares of the times of the revolution of any two planets are in the same ratio as the cubes of their mean distances from the sun.

GALILEO AND NEWTON

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." 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