CH. XXX. ORBITS OF JUPITER AND SATURN. 269 



ellipse round the earth, and therefore goes at one time a 

 little faster, and at another a little slower, while her rotation 

 on her own axis does not vary, she doss not keep always 

 exactly the same face towards us, but we catch little glimpses 

 farther round her globe, sometimes on one side and some- 

 times on the other. This balancing movement is called the 

 libration of the moon. 



Laplace works out the Long Inequality of Jupiter and 

 Saturn, 1774-1783. — The next calculation about the planets 

 was made by Laplace, and is more difficult to understand. 

 You will remember that Newton showed that every planet 

 attracts every other planet, and has some effect upon its path 

 round the sun. Now it had' been found by comparing old 

 astronomical tables with later ones that these different 

 attractions had altered some of the ellipses in which the 

 planets move ; and both Lagrange and a celebrated mathe- 

 matician named Euler had tried to calculate these changes 

 and find out whether the planets would ever come back into 

 their old places. Laplace, however, carried the calculation 

 farther than either Lagrange or Euler had done, and he 

 showed that the whole machinery does work round in the 

 course of a long period. Only two planets, Jupiter and 

 Saturn, did not seem to follow this general law, but behaved 

 in a very eccentric manner; for it appeared that during 

 the seventeenth century Jupiter had been moving more 

 quickly every year and Saturn more slowly. If this went on 

 it was clear that Jupiter would draw nearer to the sun, and 

 at last fall into it, while Saturn would go farther off, and dis- 

 appear entirely from our system, and this would upset the 

 balance of our planets, and might lead eventually to our 

 being all drawn into the sun. 



This was a very serious question, and it was a grand step 



