VIII. MASS OP THE MOON. 55 



that the mass of the sun is no less than 65,000,000 

 times greater than the least of these moons. But as 

 the quantities of matter in any two primary planets are 

 directly as the cubes of the mean distances at which 

 their satellites revolve, and inversely as the squares of 

 their periodic times (N. 133), the mass of the sun and 

 of any planets which have satellites may be compared 

 with the mass of the earth. In this manner it is com- 

 puted that the mass of the sun is 354,936 times that 

 of the earth ; whence the great perturbations of the 

 moon, and the rapid motion of the perigee and nodes of 

 her orbit (N. 134). Even Jupiter, the largest of the 

 planets, has recently been found by Professor Airy to 

 be 1048-7 times less than the sun; and, indeed, the 

 mass of the whole Jovial System is not more than the 

 1046-77th part of that of the sun. So that the mass of 

 the satellites bears a very small proportion to that of 

 their primary. The mass of the moon is determined 

 from several sources from her action on the terres- 

 trial equator, which occasions the nutation in the axis of 

 rotation; from her horizontal parallax; from an in- 

 equality she produces in the sun's longitude ; and from 

 her action on the tides. The three first quantities, 

 computed from theory and compared with their ob- 

 served values, give her mass respectively equal to the 

 T _ t ? |.-, and, -^.o- part of that of the earth, which do 

 not differ much from each other. Dr. Brinkley, Bishop 

 of Cloyne, has found it to be ^ from the constant of 

 lunar nutation; but from the moon's action in raising 

 the tides, her mass appears to be about the Jj part of 

 that of the earth a value that cannot differ much from 

 the truth. 



The apparent diameters of the sun, moon, and planets 

 are determined by measurement ; therefore, their real 

 diameters may be compared with that of the earth ; for 

 the real diameter of a planet is to the real diameter of 

 the earth, or 7916 miles, as the apparent diameter of 

 the planet to the apparent diameter of the earth as seen 

 from the planet, that is, to twice the parallax of the 

 planet. According to Professor Bessel, the mean ap- 

 parent diameter of the sun is 1922", and with the solar 

 parallax 8"-5776, it will be found thatHhe diameter of 



