Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 2.djvu/473

 19 the joint force of its gravity to the Sun and Earth, thereby decribing an orbit, which inclofes not the Earth but the Sun, is yet conidered as a compound motion, made from two motions, one about the Sun, and the other about the Earth; becaue two uch motions are requiite to anwer the two forces of its gravity, if eparately conidered: For the very ame reaon, the Moon's motion ought to be reolved into a third motion of revolution, ince there remains a third force to be atified, and that is the force ariing from the altes ration of the Moon's gravity to the Sun. And this when conidered, will require a motion in a mall ellipis, in the manner here decribed. T H E circle ADFH repreents t.he Fig; orbit of the Moon about the Earth in the center if, as it would be at a mean ditance, ' uppoing the Moon had n'o gravity to any, other body but the Earth. The diameter AT F divides that part of the orbit which is towards the Sun, uppoe ADF, from the part oppoite to the Sun, uppoe AH E The diameter at. right angles H ST D, is the line of the Moon's conjunction with or oppoition to the Sun. The figure PQLK is an Ellipis, whoe cen-f ICI