20. This difference in the distance between the earth and the sun produces a farther effect upon the moon’s motion; causing the orbit to dilate when less remote from the sun, and become greater, than when at a farther distance. For it is proved by Sir ~ISAAC NEWTON~, that the action of the sun, by which it diminishes the earth’s power over the moon, in the conjunction or opposition, is about twice as great, as the addition to the earth’s action by the sun in the quarters[188]; so that upon the whole, the power of the earth upon the moon is diminished by the sun, and therefore is most diminished, when the action of the sun is strongest: but as the earth by its approach to the sun has its influence lessened, the moon being less attracted will gradually recede from the earth; and as the earth in its recess from the sun recovers by degrees its former power, the orbit of the moon must again contract. Two consequences follow from hence: the moon will be most remote from the earth, when the earth is nearest the sun; and also will take up a longer time in performing its revolution through the dilated orbit, than through the more contracted. 21. THESE irregularities the sun would produce in the moon, if the moon, without being acted on unequally by the sun, would describe a perfect circle about the earth, and in the plane of the earth’s motion; but though neither of these suppositions obtain in the motion of the moon, yet the forementioned inequalities will take place, only with some difference in respect to the degree of them; but the moon by not moving in this manner is subject to some other inequalities also. For as the moon describes, instead of a circle concentrical to the earth, an ellipsis, with the earth in one focus, that ellipsis will be subjected to various changes. It can neither preserve constantly the same position, nor yet the same figure; and because the plane of this ellipsis is not the same with that of the earth’s orbit, the situation of the plane, wherein the moon moves, will continually change; neither the line in which it intersects the plane of the earth’s orbit, nor the inclination of the planes to each other, will remain for any time the same. All these alterations offer themselves now to be explained.