Page:Popular Astronomy - Airy - 1881.djvu/94

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80
POPULAR ASTRONOMY

LECTURE III.

Recapitulation of Lecture II.—Apparent motion of the planets.—Greek Theory of Planetary Motions.—Epicycles, Deferents, &c.—Copernican Theory.—Kepler's Elliptic Theory.—Theory of Central Forces.—Laws of Motion.—Composition and Resolution of Forces.—Motion of a Planet in its orbit deduced from these laws.—Measure of distance by Parallax.

The subject of the lecture of yesterday evening was the dimensions, the figure, and the rotation of the earth. I then thought it necessary to put before you some details of evidence relative to the rotation of the earth; and in again entering upon that subject, but not exactly in the same order in which I took it last night, I shall feel it necessary to remark that it is highly important, in beginning a subject like this, to divest yourselves as far as you possibly can, of notions acquired in the ordinary routine of education. Every person here has, without doubt, been brought up in the belief that the earth is in motion; and because they have had this belief instilled into their minds from their earliest infancy, they may have concluded that it is necessarily and obviously true. This is a thing most dangerous, and instances are not wanting to prove that in every branch of science, absurdities have arisen from it. I may mention one which just at this moment occurs to my mind, and which influenced the mind of one of the earliest philosophers of the Royal Society of