BOUNDARIES OF OUR SOLAR WORLD 313 



it as Columbus saw America from the shores of Spain. Its 

 movements have been felt, trembling along the far-reaching line 

 of our analysis with a certainty hardly inferior to ocular de- 

 monstration." 



The discovery is often cited as one of the great feats of 

 calculation ; it was hardly less a witness to the marvellous 

 accuracy which astronomy had attained. The deviation of 

 Uranus from its appointed path, set by Kepler's law, never ex- 

 ceeded two astronomical minutes that is, the thirtieth part of a 

 degree. It lies just on the fringe of visibility ; Neptune far 

 beyond. The one would probably never have been recognised 

 as a planet but for the invention of the telescope ; without 

 its present-day perfection, the second would never have been 

 known. 



The last of the planets possessed, like its immediate fore- 

 runner, but few conspicuous traits. It is a little larger than 

 Uranus ; like the latter, it rotates in an opposite direction to 

 that of all the Qther planets. But its distance was a complete 

 anomaly. If the law of Bode held good, it would have been 

 found at something like twice the distance of Neptune that 

 is, at nearly forty times the distance of the earth from the sun. 

 Instead of that, it is something less than thirty times. It had 

 been difficult enough before to reconcile the observed distances 

 of the planets with any absolute order of arrangement. With 

 the discovery of Neptune's orbit, the idea of a simple " law " 

 of distance was given up. Until we know more of planetary 

 evolution than we do, it will be quite impossible to say why 

 it is that the planets are spaced as they are. 



It remained a natural question as to whether the new planet 

 was in reality the outermost member of the system, or whether 

 others lay yet beyond. Sixty years and more of observation, 

 scrupulous in its method, and with an ever-increasing delicacy 

 of observing instruments, has failed to present even a suspicion 

 of an ultra-Neptune. The orbit of the new planet showed no 

 such wobbling as that which, in the motion of Uranus, had 

 pointed to the existence of the other. We may conclude, there- 

 fore, not absolutely, but with a considerable degree of pro- 

 bability, that we now know the shape and size of our planetary 

 world, and practically all that is worth knowing of its contents. 



Our planetary system consists, then, of eight large bodies, 



