202

Sir Isaac Newton’s BookII.

great, as the addition to the earth’s action by the fun in thequarters a ; so that upon the whole, the power of the earthupon the moon is diminished by the fun, and therefore ismost diminished, when the action of the fun is strongest: butas the earth by its approach to the fun has its influence lessen-ed, the moon being less attracted will gradually recede fromthe earth ; and as the earth in its recess from the fun recoversby degrees its former power, the orbit of the moon must a-gain contract. Two consequences follow from hence : themoon will be most remote from the earth, when the earth isnearest the fun ; and also will take up a longer time in per-forming its revolution through the dilated orbit, than throughthe more contracted.

2.1. These irregularities the fun would produce in themoon, if the moon, without being acted on unequally by thefun, would describe a perfect circle about the earth, and inthe plane of the earth’s motion ; but though neither of thesesuppositions obtain in the motion of the moon, yet the fore-mentioned inequalities will take place, only with some diffe-rence in respect to the degree of them; but the moon by notmoving in this manner is subject to some other inequalities al-so. For as the moon describes, instead of a circle concentri-cal to the earth, an ellipsis, with the earth in one focus, thatellipsis will be subjected to various changes. It can neitherpreserve constantly the same position, nor yet the same fi-gure ; and because the plane of this ellipsis is not the fame

» Newton. Princ. Lib.I. prop. 66. coroll. 7.

with