Page:The Solar System - Six Lectures - Lowell.djvu/56



to the disturbing force is a maximum at A — because the force has been adding increments to it up to this point — and a minimum at F and E. The particle, by traveling fast, lessens the curvature of its path about C, since the pull from C has less time to act; and reversely by traveling slowly it increases this curvature. In consequence, then, of this component, the path is flattened at A and bulged at E.

The normal component acts inward at F and is proportional to CP. It helps the central force at F, and curves the path the more. At a it vanishes, and is then reversed, acting outward or against the gravity of C. It thus lessens the curvature from a to b. It thus conspires completely with the tangential component; and the two together squeeze the orbit into an ellipse with its longer diameter at right angles to the line joining C to M.

The tidal action on a particle of the ocean is thus precisely the same, neglecting pressure and friction, as that of the Sun upon the Moon's orbit. This deformation of the Moon's orbit was detected, probably by Aboul Wefa, nine centuries ago. It is called the Moon's variation. Thus the tidal wave and the variation are analogous exhibitions of the same force.