Page:Popular Astronomy - Airy - 1881.djvu/213

LECTURE VI.

 

Recapitulation of Lecture V.—Velocity of Light deduced by Römer from observations of the Eclipses of Jupiter's Satellites.—Proper motion of Stars.—Motion of Solar System in space.—Theory of Gravitation.—Methods of computing Attraction.—Perturbations of the Moon.—Mutual Perturbations of the Planets.—Long Inequality of Jupiter and Saturn.—Calculation of Figure of the Earth from Pendulum Experiments.—Experiments on the Density of the Earth.—Schehallien Experiment.—Cavendish Experiment.—Weight of the Earth.—Weight and Density of the sun.—Weight of some Planets and of the Moon.—Conclusion.

IN the lecture of yesterday evening, the first subject to which I alluded was the Precession of the Equinoxes, in reference to its mechanical causes. This is a thing so important, partly in itself and partly in connection with the causes which produce it, that I have no hesitation in speaking of it again. The thing which I particularly intended to convey to you, was this: that if we consider the attraction of the sun upon the earth, and if we consider that the earth is not a sphere, but has a flattened turnip-like shape which we call an oblate spheroid; if we also remark the laws which apply to gravitation, namely, that the force which the sun exerts is greater the nearer the body is to it, and that the law of gravitation is to be understood as applying to every particle, not to the body as a mass; that it attracts the earth not as a whole, but as a number of parts separately; if we mark these things, we find that the sun attracts that