54 MASSES OF THE PLANETS. SECT. VHI. 



is. At such a distance not only the terrestrial orbit 

 shrinks to a point, but the whole solar system, seen in 

 the focus of the most powerful telescope, might be 

 eclipsed by the thickness of a spider's thread. Light, 

 flying at the rate of 190,000 miles in a second, would 

 take more than three years to travel over that space. 

 One of the nearest stars may therefore have been 

 kindled or extinguished more than three years, before 

 we could have been aware of so mighty an event. But 

 this distance must be small, when compared with that 

 of the most remote of the bodies which are visible in 

 the heavens. The fixed stars are undoubtedly luminous 

 like the sun ; it is therefore probable that they are not 

 nearer to one another than the sun is to the nearest of 

 them. In the milky way and the other stariy nebulae, 

 some of the stars that seem to us to be close to others, 

 may be far behind them in the boundless depths of 

 space; nay, may be rationally supposed to be situate 

 many thousand times farther off. Light would there- 

 fore require thousands of years to come to the earth 

 from those myriads of suns of which our own is but 

 "the remote companion." 



SECTION VIII. 



Masses of Planets that have no Satellites determined from their Perturba- 

 tions Masses of the others obtained from the Motions of their Satellites 

 Masses of the Sun, the Earth, of Jupiter, and of the Jovial System- 

 Mass of the Moon Real Diameters of Planets, how obtained Size of 

 Sun Densities of the Heavenly Bodies Formation of Astronomical 

 Tables Requisite Data and Means of obtaining- them. 



THE masses of such planets as have no satellites, are 

 known by comparing the inequalities they produce in 

 the motions of the earth and of each other, determined 

 theoretically, with the same inequalities given by ob- 

 servation ; for the disturbing cause must necessarily 

 be proportional to the effect it produces. The masses 

 of the satellites themselves may also be compared with 

 that of the sun by their perturbations. Thus, it is 

 found, from the comparison of a vast number of observa- 

 tions, with La Place's theory of Jupiter's satellites, 



