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constant. Owing to the perturbations which themoon undergoes (as explained in the next chapter),her path changes in shape, the mean distance remain-ing throughout nearly constant. The shape of herpath when it is most eccentric, as well as when itis least eccentric, would not differ appreciably fromfig. 10, and therefore, so far as this relation is con-cerned, no new figure is required. But for anotherpurpose, presently to be explained, it is convenientto have a picture exhibiting the moon’s path aroundthe earth when the eccentricity is a maximum. It istherefore shown in fig. 11, Plate II., the centre beingat C and the earth at E', and M M' the moon’s path.The point e shows the position occupied by theearth’s centre when the eccentricity is a minimum.The distance E' C is 15,760 miles, while e C is 10,510miles. Thus the difference, E' e, is 5,250 miles, orabout two-thirds of the earth’s diameter. Owing to thepeculiarities of the lunar perturbations, however, thesenumbers are not to be strictly applied in dealing with
the lunar orbit. In fact, her distance from the earthfW . ’ . , .
is somewhat mere increased, owing to perturbations,
than it is reduced—when the maximum effects either
way are compared.
The apparent diameter of the moon when she isat her mean distance is found by telescopic observa-tion (at night) to be 3P 9", or 1,869" (when reducedto correspond to the distance of the earth’s centre;or, approximately, when supposed to be made on themoon in the horizon).- But this value is partly in-