562 Mr. Arthur Stanley Eddington [March 26, 



the winter of 1904-5. Since then it has been our practice at 

 Greenwich to photograph them at frequent intervals, for the study of 

 their motions presents many points of astronomical interest ; the 

 seventh satellite in particular is very faint and requires powerful 

 instruments, long exposures, and, may we add, a favourable climate 

 for its observation. The third round speck was something new, 

 which could not have been expected ; it was an eighth satellite of 

 Jupiter. 



But it was a long while before its real nature could be determined 

 definitely, though it was suspected from the very first. Just previous 

 to its discovery, a number of plates of the sixth and seventh satel- 

 lites had been taken, and by looking back at these, Melotte was able 

 to trace the new object on seven of them. Thus there could be no 

 doubt that it was a real body and not a defect of the plate. The 

 fact that it moved along at almost the same rate with Jupiter was 

 suspicious, but still it was by no means certain that it belonged to 

 Jupiter ; it might only be a new comet, or, still worse — a new minor 

 planet, one of the " small vermin of the heavens." One thing, how- 

 ever, was quickly established : Mr. Crommelin calculated that, whether 

 it belonged to Jupiter or not, it was at all events near Jupiter, and 

 not merely in the same line of sight. That in itself was sufficient to 

 make it an interesting body. I cannot give you any idea of the 

 calculations which ultimately showed that it belonged permanently to 

 Jupiter ; perhaps the next slide will show that the hesitancy in coming 

 to a definite decision as to its real nature was not unjustified ; it may 

 even seem somewhat daring to have acknowledged it as probable. 

 [The slide showed the observations of the three outer satellites made 

 at Greenwich during the opposition of 1908.] There is no difficulty 

 in seeing YI and VII belong to Jupiter, they are clearly circulating 

 round it : but the eighth satellite looks, at first sight, to be going on 

 its way regardless of Jupiter. Its path is apparently convex to the 

 planet instead of concave to it as would be expected. It is not easy 

 to see how it will get round it at all. That difficulty, however, 

 disappears when the change of position of the Earth and Jupiter, 

 altering the point of view from day to day, is taken into account. 

 The record of the diagram breaks off in April ; after that Jupiter 

 became unfavourably placed for observation, and the satellite could 

 only be followed by calculation through its course during the summer 

 and autumn. At the first opportunity, however, it was searched for, 

 and in January it was found again at Greenwich, very near to 

 the predicted spot. [One of this year's photographs was thrown on 

 the screen.] It has now moved far away from Jupiter, which no 

 longer comes on to the plate ; but it is still and will be under the 

 control of the giant planet, and we can say as confidently as in the 

 case of the other satellites, that it is really circling round Jupiter. 



The satellites of Jupiter now form a very remarkable family, and 

 before we go on to say more about the new sateUite it seems worth 



