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that if we describe circlesM/x and M'/Zabout E'as centre(fig. 11, Plate III.), and passing through the pointsM and M', then the circles M p and M' // represent thedimensions of the lunar disc when the moon is at M'or M respectively. In like manner we could comparethe dimensions of the lunar disc when the moon is inperigee and apogee, and the eccentricity has its leastvalue (i.e. the earth as at e, fig. 11); or when the eccen-tricity has its mean value (the earth as at E, fig. 10).*
It remains only that we should consider the subjectof the moon’s mass,—that is, of the quantity of mattercontained in her globe, whose volume or size is alreadyknown to us.
There are four different ways in which the moon’smass may be determined.
First, since we have already mentioned (and shallexplain further in the next chapter) that the moon’smotion under the earth’s attraction is calculable whenthe size of the earth, the value of terrestrial gravity,and the moon’s distance and mass are known, itfollows that as the size of the earth, the earth’s gravity,
* This is a very convenient method of comparing the apparentdimensions of the same orb seen at different distances. We takethese distances, and with them describe circles ; then these circlesrepresent the relative apparent dimensions,—the largest, of course,corresponding to the appearance of the globe as seen at the leastdistance, and vice versd. Thus suppose that we wish to comparethe size of the sun as seen from two planets, which we may call,for convenience, P and P', and that we have a chart of orbits in-cluding the orbits of these planets ; then if the orbit of P representthe size of the sun as seen from P', the orbit of P' represents thesize of the sun as seen from P.