50 MASSES OF THE PLANETS. SECT. VIII. 



theory and compared with their observed values, give her mass 

 respectively equal to the ^, T \. 2 , and ^. 5 , part of that of the earth, 

 which do not differ much from each other. Dr. Brinkley has 

 found it to be ^5 from the constant of lunar nutation : but, from 

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

 about the 7 ! 5 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 7926 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 Bessel, the mean apparent diameter of 

 the sun is 1923" '64, and with the solar parallax 8"-5776, it 

 will be found that the diameter of the sus 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 equatorial radius, or 238,793 miles : so that twice the dis- 

 tance of the moon is 477,586 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 88,200 miles 

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

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

 planet, in one of our steam carriages, might go round his world 

 in a few hours. The diameters- of Lutetia and Atalanta are only 

 8 and 4 miles respectively ; but the whole of the 55 telescopic 

 planets are so small, that their united mass is probably not more 

 than the fifth or sixth part of that of the moon. 



The densities of bodies are proportional to their masses, divided 

 by their volumes. Hence, if the sun and planets be assumed to 

 be spheres, their volumes will be as the cubes of their diameters. 

 Now, the apparent diameters of the sun and earth, at their mean 

 distance, are 1923"'6 and 17"'1552, and the mass of the earth is 

 the 354,936th part of that of the sun taken as the unit. It 

 follows, therefore, that the earth is four times as dense as 

 the sun. But the sun is so large that his attractive force would 



