THE ASTEROIDS. 527 



him, being so different, vary in apparent size. The outer one is twelve 

 thousand, the inner one about three thousand five hundred miles from the 

 planet, so the former would revolve in about thirty hours in a direction from 

 west to east, and the inner moon goes round in the same way in about seven 

 hours and a half. Mars revolves in twenty-four and a half hours from west 

 to east. So the outer moon rises for him in the east, and the inner one in 

 the west. This is accounted for by the fact that one travelling slower than 

 Mars rises in the east, the other outruns him, and comes up in the west. 



But if we suppose ourselves upon Mars we shall find that, after all, we 

 have only one moon properly so called. The outer satellite is very small 

 and very far away, so it is useless to give light at most, it is no bigger 

 than Mars appears to us on earth. So the Martians do not see two moons 

 passing each other in the sky that is, unless their eyes are of greater range 

 and power than ours. Thus they have one moon rising in the west, appearing 

 in all its phases every night, while our moon takes twenty-eight days to pass 

 through her phases; for we must remember that Mars' moon takes only 

 seven hours and forty minutes to pass through its orbit, and therefore each 

 quarter will not occupy quite two hours. 



THE MINOR PLANETS, OR ASTEROIDS. 



Passing onward from Mars towards Jupiter we arrive at a number of 

 smaller planets, which will not concern us very much, as they are very small 

 and scarcely visible without a good telescope. But a very interesting chapter 

 in the history of astronomy was commenced when the discovery of these 

 bodies was begun. In old times astronomers noticed a very considerable 

 gap between Mars and Jupiter, which was remarkable when the regular 



Dec. 8th. Dec. gth. 



Fig 584. Field of view showing motion of minor planets amongst the stars. 



progression of the distances between the planets was remembered. So Kepler 

 was of opinion that some planet would be discovered having its orbit in that 

 space between Mars and Jupiter. It is, however, to Piazzi, the Italian, that 

 the discovery of the zone of asteroids is due. 



Piazzi was surveying the constellation Taurus, where he fancied he had 

 discovered a change of place in a star which he had observed on the ist of 



