THE ASTEROIDS. 237 



The orbits of these bodies have an average radius of not far from two 

 hundred and fifty millions of miles, with a corresponding jieriod of a little 

 less than five years. But individual orbits differ widely from these figures. 

 Thus Flora, the nearest to the sun, has a period of only 1193 days— a trifle 

 over three years and a quarter — and a mean distance ()f only two hundred 

 and two millions of miles; on the other hand Hilda, the most remote, has 

 a period of 2868 days, or very nearly eight years, and the radius of her 

 orbit is more than three hundred and sixty millions of miles. The orbits 

 of the large planets are all nearly circular; many of the asteroid orbits, on 

 the contrary, are Very eccentric, resembling those of comets. Thus ^Ethra 

 has an eccentricity of 0.38, which amounts to saying that her least distance 

 from the sun is considerably less than half her greatest. The inclinations 

 of some of their orbits are no less remarkable, that of Pallas being more 

 than 34°, while that of Mercy is only 7°, and even that is altogether excep- 

 tional among the older planets. 



As has been said before, the Asteroids are very minute, too small indeed 

 to have their diameters determined with any certainty by direct measure- 

 ment; we are limited to approximate results obtained by comparing their 

 apparent brightness with that of planets whose size and distance from the 

 sun are known. If we knew the reflecting power of their surface — their 

 albedo as it is called — we could thus arrive at reliable conclusions; but 

 wanting this element and being obliged to content ourselves with the mere 

 assumption that this albedo does not difi:er much from that of the planet 

 Mars, values inferred in this way must be accepted with a good deal of re- 

 serve. Littrow, Lespiault, and -others have investigated the matter, and 

 find that the diameters ot the larger ones range from three hundred to 

 one hundred and fifty miles, while the smaller ones lie between fifteen and 

 thirty. They are so small that a good walker could easily make the circuit 

 of one of these microscopic globes in a single day, and unless their density 

 is much greater than that of any of the other planets, the force of gravity 

 must be several hundred times less than that on the Earth's surface. A 

 stone thrown from a boy's sling would fly off into space, never to return. 

 "We have spoken of them as globes, but certain otherwise unexplained 

 variations in the brightness of some of them, especially Pallas, have suo-- 

 gested the idea that they may be irregular pieces of rock rather than 

 spheres. 



As to their origin two theories are held ; one that they are the frag- 

 ments of an exploded planet, the other that the ring of nebulous matter 

 which in different circumstances would, according to the nebular hypothesis, 

 have formed a single planet like the others in the system, was in this case 

 broken up, mainly by the action of the great planet, Jupiter, just outside. 

 If the first hypothesis be so modified from that proposed by Olbers as to 

 introduce the idea of a number of disruptions, first of the original planet 

 and afterwards of its fragments, it becomes perhaps as tenable as the second, 

 and there would seem to be at j^resent no means of deciding between them. 



