60

THE MOUN.

positions the effect of the Earth ’s attraction will be of anintermediate character. The position of New Moon is thecritical one, and if the path is then (as we have proved to bethe case) concave towards the Sun, it must always be so.

The orbit of the Moon round the Earth is indeed so smallcompared with that of the Earth round the. Sun , that we mayeven speak of them as both going round together very nearlyin the same orbit,—the Earth at a mean distance of about93,000,000 miles and with a speed of about 66,000 miles perhour,—the Moon with a speed which is, periodically, about 2,300miles greater or less than the Earth ’s, and varies from about64,000 to 68,000 miles per hour; while its distance alternatelyexceeds or falls short of that of the Earth by about a quarterof a million of miles. This variation of distance is, however,so small in comparison with the radius of the Earth ’s orbit,that it is almost impossible to represent it upon any scale withwhich a diagram can conveniently be drawn. It is a compara-tively trifling quantity in contrast with the changes to whichthat radius is itself subject in the course of a year, as the resultof the ovalness of the Earth ’s orbit, by which in midwinter ofour northern hemisphere we find ourselves about 3,000,000miles nearer to the Sun than we are in midsummer.

To those who may find some difficulty in realizing how theMoon can really go round and round the Earth month bymonth while it is all the time moving in an orbit round theSun, which, like that of the Earth , is always concave to it, anadditional diagram may perhaps be of use. Let Mj, m 2 , m 3 , m 4 , m 5in Fig. XII. represent the Moon ’s orbit; e„ e 2 , e 3 , e 4 , e 6 that ofthe Earth. When the Earth is at e 4 , let us suppose the Moon to he at m 4 ; when the Earth is at e 2 , e 3 , e 4 , e 5 respectively, letthe corresponding places of the Moon be m 2 , m 3 , m 4 , m 5 . At m 3the Moon will be New; at m 4 and m 5 it will be Full; theportion of its path which we have drawn corresponding to aninterval of a month.* The curves are intended to representthe adjacent orbits of the Earth and Moon , both being concave

* The month in question is termed a synodic month, and exceedsa sidereal month (or the interval in which the Moon performs one circuitround the Barth) by rather more than 21 days. It may be seen from