DISTANCE, SIZE, AND MASS.

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displacement can be ascertained, we can infer pre-cisely what proportion the distance of the earth’scentre from the centre of gravity of the earth andmoon bears to the earth’s mean diameter. We shallhave to make an assumption as to the value of thesolar parallax (that is, in effect, as to the sun’s dis-tance) ; but that is an element which has been deter-mined with a satisfactory degree of accuracy in manydifferent ways. Hence the moon’s mass can be deter-mined with a corresponding degree of accuracy, ifonly the observations of the sun’s displacement areaccurately made.

From a great number of observations of the moon,Delambre deduced for the sun’s maximum displace-ment (called the sun’s parallactic inequality), the value7" - 5. Hence Laplace deduced the value i for themoon’s mass. With the values at present adoptedfor the distances of the sun and moon, he would havededuced T \ as the value of the moon’s mass.

In recent times the meridional observations of thesun have been so numerous and exact, that the meansof determining the moon’s mass by this method aremuch more satisfactory. Thus we can place verygreat reliance on Leverrier ’s estimate of the parallacticinequality, viz. 6 ,,- 50. Professor Newcomb, of America,deduces from a yet wider range of observations thevalue 6" - 52. These values lie so close together as toshow that the observations on which they have beenbased suffice for the very accurate determination ofthis quantity.