64 MASSES OF THE MOOX. [SECT. vin. 



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 terrestrial equator, which occasions the nuta- 

 tion in the axis of rotation ; from her horizontal parallax; 

 from an inequality 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 observed 

 values, give her mass respectively equal to the ^, 7 Vi, 

 and 53.2, part of that of the earth, which do not differ much 

 from each other. Dr. Brinkley, Bishop of Cloyne, has found 

 it to be ^j from the constant of lunar nutation : but, from 

 the moon's action in raising the tides, her mass appears to 

 be about the 7 J 5 part of that of the earth a value that can- 

 not 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 dia- 

 meter 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 apparent diameter of the sun is 1922", and 

 with the solar parallax 8"'5776, it will be found that the 

 diameter of the sun is about 886,877 miles. Therefore, if 

 the centre of the sun were to coincide with the centre of the 

 earth, his volume would not only include the orbit of the 

 moon, but would extend nearly as far again ; for the moon's 

 mean distance from the earth is about sixty times the earth's 

 mean radius, or 237,360 miles : so that twice the distance of 

 the moon is 474,720 miles, which differs but little from the 

 solar radius ; his equatorial radius is probably not much less 

 than the major axis of the lunar orbit. The diameter of the 

 moon is only 2160 miles ; and Jupiter's diameter of 87,000 

 miles is very much less than that of the sun ; the diameter of 

 Pallas does not much exceed 79 miles, so that an inhabitant 



