DISTANCE, SIZE, AND MASS.

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is seen on the horizon, due east; and when the placehas been carried to o ', the moon is again on thehorizon, but due west. When the place is at B, mid-way between e and o', the moon (under the imaginedconditions) is immediately overhead. Thus, the moonrising due east, passes to the point overhead andonwards to the west, where she sets. But it is clearthat the moon’s apparent motion in passing across thesky, from the eastern to the western horizon, is notuniform, as seen from the globe E E'. The arc e E o'is obviously less than a semicircle; in other words,the moon, under the imagined conditions, completesher course athwart the heavens—a seeming half-circle—in less than half a day, while she is below thehorizon (completing the other seeming half of thecircle) in more than half a day.

But as some find a difficulty in forming a clearconception of the apparent motion of a body placed asM in fig. 2, while a point is carried round such acircle as E E', I will at this stage introduce a slightchange in the method of considering the matter. It isof course obvious that the apparent motion of themoon is precisely the same as though the moon wentround the earth, while the earth’s globe remainedat rest. Let us then suppose this to be the actualstate of things. In fig. 3 the earth is supposed to beat rest, P being the pole, as in fig. 2 ; and the moonis supposed to be carried round uniformly about theearth’s centre, in the direction shown by the arrow.Comparing figs. 2 and 3, the student will at once see