74
moment. Thus the earth, in. order to prevent theescape of her satellite, has not to overcome the sun’spull upon the moon, but only the excess of that pullover the pull he exerts upon the earth herself. Thisexcess, as will presently appear, is always far less thanthe earth’s own influence on the moon.
But it may be noticed, that in considering the moon’scourse round the sun we recognize the inferiority ofthe earth’s influence in a very evident manner. Themoon seems well under the earth’s control when we con-sider only the nature of the lunar orbit round the earth;but if for a moment we forget the fact that the moonis circling round the earth, and consider only the factthat the moon travels as a planet round the sun,—withperturbations produced by the attractions of anotherplanet,—our own earth,—we can readily test the extentof these perturbations. Now let the circle M M' (fig.16,Plate V.) represent the moon’s path round the sun S,and let us suppose that at 0 the moon is between theearth and sun, and again similarly placed at 1, 2, 3 . .11, and 12,—being therefore on the side away from thesun at the intermediate stations marked with a smallline outside the circle MM'; then the moon’s-orbitalcourse is a serpentine or waved curve, having itsminima of distance from the sun at 0,1, 2, 3 .... 11,12jand its maxima of distance at the intermediate points.But on the scale of fig. 16, the whole of this serpentinecurve would lie within the breadth of the fine circularline MM'. Thus it will readily be understood thatthe curvature of the moon’s path remains throughout