Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 2.djvu/472

 13 variation from the different force of grae vity of the Moon and Earth to the Sun, ariing from  different ditances of the Moon in its everal ai'pe6l:s. The mean gravity of the Moon to the Sun, he uppoes, is atified by the annual motion of the ¢Moon round the Sun; the gravity of the Moon to the Earth, he uppoes, is atified by a ref volution of the Moon about the Earth. But the difference of the Moon's gravity to the Sun more or leii; than the Earth's gravity, he uppoeé, produces two effects; for as this difference of force may be reolved into two forces, one acting in the Way, or contrary to the Way, of the Moon about the Earth, and the other acting in 'the line to or from the Earth: the firt: caues the Moon to decribe .a larger or maller area in the ame time about the Earth, according as it tends to accelerate or retard it; the other changes the form of the lunar orbit from what' it ought to be merely from the Moonfs gravity to the Earth, and both together make up that inequality which is called the variation. But ince the real motion of the Moon, Ii1D'a Iimaile motion, caued by a congnual dew élzion from a itreight line, by H  the l