86 The Realm of Nature CHAP. 



the Sun, suggesting the forming and melting of great areas 

 of snow. Until 1877 Mars was supposed to have no 

 satellites, but in that year Professor Hall of Washington 

 discovered two. One is very small, very near the planet, 

 and races round it, from west to east, in little more than 

 7 hours, making three complete revolutions whilst the planet 

 rotates once ; the other, farther away, revolves in 30 hours. 

 129. Asteroids. It had been observed even before 

 Kepler's time that there is a certain symmetry in the 

 placing of the planets. This relation was subsequently 

 formulated by the German astronomer Bode in the end of 

 the eighteenth century, and has since been termed Bode's 

 Law. It is as follows : If 4 be added to each member of 

 the numerical series 



o 3 6 12 24 48 96 

 we get- - 



4 7 10 16 28 52 100 



Mercury. Venus. Earth. Mars. Jupiter. Saturn. 



These figures represent very nearly the relative distance 

 of the planets from the Sun, e.g. Saturn is I o times farther 

 than the Earth. There is a gap between Mars and Jupiter, 

 and although no physical reason was, or is, known for this 

 arrangement, the whole system seemed so orderly that 

 Kepler supposed this gap to represent the place of a 

 missing planet. Bode and several other astronomers were 

 so impressed by the gap in this law that they agreed 

 to examine the sky very minutely for the missing planet. 

 While their search was in progress the Italian Piazzi (who 

 was not one of the number) discovered on the first night 

 of the nineteenth century a small planet occupying exactly 

 the position prescribed by this law, and gave it the name 

 of Ceres. Next year another little planet was discovered, 

 and when half the century had elapsed no less than fifteen 

 had been found. A more systematic search was then 

 commenced by many astronomers, and the small stars 

 made visible only by powerful telescopes were followed 

 individually night after night, with the result that a great 

 many were found to have no fixed place on the star-dome, 



