DISTANCE, SIZE, AND MASS.

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mean place in longitude. In fact we know that themoon, circling around the same centre of gravity, hutin a much wider orbit, is sometimes in advance of theearth and sometimes behind the earth,—regardingthese orbs as two planets severally pursuing theircourses round the sun; and if we look upon the earth’smotion as representing very nearly the motion of aplanet, at her distance and undisturbed by a satellite(which is not far from being the case), then we seethat the moon, owing to her motion in an orbit477,600 miles in diameter round the earth, is alter-nately 238,800 miles in advance of, and as manybehind, her mean place in longitude. So that, sincethe earth circuits round the common centre of gravityof the two bodies, in a smaller orbit, she will bealternately in advance of and behind her mean place *by the radius of that orbit. Obviously the effect ofthis will be that the sun, round which the earth isthus moving, will seem to be alternately in advanceof and behind the mean place due to his apparentannual motion round the heavens. His apparentplace will obviously not be affected at all when themoon is on a line with the sun and earth, or in syzygy,as it is called (that is, when it is either new moon orfull), for then the earth’s displacement is on the sameline, and the only effect is that the sun appears eithervery slightly larger (when the moon is “ full ” and the

* The mean place here referred to is that place which the earthwould have if she were travelling alone round the sun,—not, asis actually the case, under the perturbing influence of a satellite.