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earth most displaced towards the sun), or very slightlysmaller (when the moon is “ new ” and the earth mostdisplaced from the sun). Both effects would be quiteinappreciable. But when the moon is at her firstquarter, the earth is displaced towards the side occu-pied by the moon at her third quarter; that is, she isat her maximum displacement in advance of her meanplace, and the sun also appears accordingly at hismaximum displacement in advance of his mean placein his apparent annual motion round the heavens. Inlike manner, when the moon is at -her third quarter,the sun appears at his maximum displacement behindhis mean place. It is easy to ascertain what the sun’sdisplacement should be, on any given assumptionas to the moon’s mass. Suppose the moon’s mass,for example, to be sVth of the earth’s, then the centreof gravity of the earth and moon lies eighty timesfarther from the moon’s centre than from the earth’s.Hence the distance of this centre of gravity from theearth is -gS-st part of 238,818 miles, or 2,949 miles.Thus the sun may be displaced from his mean placeby the angle which a line 2,949 miles long subtends atthe earth’s distance from the sun. Since the equa-torial diameter of the earth is 3,963 miles, this dis-placement of the sun is equal to about -|ths of thesmall arc called the solar parallax, or is rather morethan 6"’6, if we assume 8" - 9 to be the mean value ofthe solar parallax. This quantity is about * £- 0 -tli partof the sun’s apparent diameter.
But obviously if the exact amount of the maximum