Page:A short history of astronomy(1898).djvu/281

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§§ 174, 175]
The Motion of the Moon, and of the Planets

substantially defective, is possible, but by no means certain; whatever the cause may have been, he laid the subject aside for some years without publishing anything on it, and devoted himself chiefly to optics and mathematics.

174. Meanwhile the problem of the planetary motions was one of the numerous subjects of discussion among the remarkable group of men who were the leading spirits of the Royal Society, founded in 1662. Robert Hooke (1635–1703), who claimed credit for most of the scientific discoveries of the time, suggested with some distinctness, not later than 1674, that the motions of the planets might be accounted for by attraction between them and the sun, and referred also to the possibility of the earth's attraction on bodies varying according to the law of the inverse square. Christopher Wren (1632–1723), better known as an architect than as a man of science, discussed some questions of this sort with Newton in 1677, and appears also to have thought of a law of attraction of this kind. A letter of Hooke's to Newton, written at the end of 1679, dealing amongst other things with the curve which a falling body would describe, the rotation of the earth being taken into account, stimulated Newton, who professed that at this time his "affection to philosophy" was "worn out," to go on with his study of the celestial motions. Picard's more accurate measurement of the earth (chapter viii., § 159) was now well known, and Newton repeated his former calculation of the moon's motion, using Picard's improved measurement, and found the result more satisfactory than before.

175. At the same time (1679) Newton made a further discovery of the utmost importance by overcoming some of the difficulties connected with motion in a path other than a circle.

He shewed that if a body moved round a central body, in such a way that the line joining the two bodies sweeps out equal areas in equal times, as in Kepler's Second Law of planetary motion (chapter vii., § 141), then the moving body is acted on by an attraction directed exactly towards the central body; and further that if the path is an ellipse, with the central body in one focus, as in Kepler's First Law of planetary motion, then this attraction must vary in different parts of the path as the inverse square of the