BOUNDARIES OF OUR SOLAR WORLD 307 



exact ; the distance grows less and less as you journey outwards. 

 Thus, the earth is only two and a half times the distance of 

 Mercury ; Mars is only about one and a half times the distance 

 of the earth. But Jupiter is five times. Between Mars and the 

 great planet there is a broad space that, even for Herschel's 

 wonderful telescopes, seemed empty. Kepler had speculated 

 upon this gap, had dreamed of a missing planet. There was no 

 mistaking the fact that there was some sort of order in the 

 distribution of distances. Saturn was very closely twice the 

 distance of Jupiter. All sorts of ratios were tried in the en- 

 deavour to find an exact law. 



Finally, taking a hint from Titius, in 1772 the astronomer 

 Bode of Berlin noted that if by taking a little different geomet- 

 rical progression than that of simple doubling, and by adding 

 a constant each time, a very close approximation could be 

 found. Thus, if the series be taken as o, 3, 6, 12, 24, and so 

 on, and the number 4 be added at each step, the result is a 

 series which expresses fairly well the relative distances. This 

 is what is known as Bode's law. The discovery of Uranus, 

 twice the distance of Saturn, a short time after, seemed to 

 indicate that it was a veritable law. It deepened the mystery 

 of the extra-Martian gap. 



The upshot of it was that a little committee of astronomers 

 was formed to search for Kepler's hypothetical planet. The 

 zodiac and its adjacent regions was divided into twenty- four 

 parts ; each was portioned off to an observer. Undoubtedly 

 the discovery that followed would have been made very shortly ; 

 as a matter of fact, it was an accident. Quite unknown to 

 this special committee, Piazzi in Sicily, engaged in making a 

 catalogue of the stars, noted one evening a small object which 

 had no place on his map. It was the first night of the nine- 

 teenth century. The next evening it had shifted a little ; so 

 on through a fortnight or more, until he was convinced that 

 it could not be a star. Like Herschel with Uranus, he thought 

 it might be a comet. It was quite invisible to the eye ; if 

 it were a star it could not be above the eighth magnitude. In 

 his puzzlement he wrote to two of his friends; one of these 

 was Bode. 



The supreme test of any natural law is the element of pre- 

 dictability. If after some striking relationship, some succession 

 in the order of events, some apparently methodical arrangement, 



