SATELLITES OF JUPITER — NICHOLSON 135 



the large satellites, I, II, III, and IV, discovered by Galileo, in 

 periods ranging from 1 day and 18 hours to 16 days and 16 hours, 

 but VI and VII, discovered by Perrine, are so far away from Jupiter 

 that 260 days are required for them to encircle the planet. 



In 1908 Melotte at Greenwich, England, while photographing the 

 sixth and seventh satellites, discovered an eighth.® It was still far- 

 ther from Jupiter and required 750 days to complete its journey 

 around the planet. Still more peculiar was the fact that it moved 

 in a retrograde direction, opposite to that of all the other satellites 

 of Jupiter. In 1914, while a graduate student at the Lick Observa- 

 tory, I was assigned the task of photographing the distant satellites 

 of Jupiter to see how closely they were following their calculated 

 paths. A ninth satellite was found '' on photographs of the eighth, 

 just as some years before, the eighth had been found by Melotte near 

 the sixth and seventh. The period of the ninth satellite is almost 

 the same as that of the eighth and it also revolves in a retrograde 

 direction. 



Satellites VIII and IX are so far from Jupiter that their motions 

 are greatly disturbed by the gravitational attraction of the Sun, and 

 their paths around Jupiter do not even approximate closed curves. 

 The computation of their positions is therefore a difficult task and 

 it has been necessary to observe them frequently to prevent their 

 being lost. In the past 20 years, whenever Jupiter has been near the 

 earth, they have, therefore, been photographed many times, and such 

 photographs have always been examined for additional satellites, 

 but none has been found. 



Since no systematic search for undiscovered satellites had ever been 

 made with a telescope larger than 36 inches in diameter, it seemed 

 worth while to make such a search with the 100-inch reflector, and 

 accordingly that project was made a part of the observing program 

 at the Mount Wilson Observatory in the summer of 1938. The 

 plan was to photograph the region around Jupiter at the Newtonian 

 focus of the 100-inch reflector on 8- by 10-inch plates with exposures 

 of 1 hour each. The survey covered about 10 square degrees ex- 

 tending 3 degrees east and wesBt and a degree and a quarter to the 

 north and south of Jupiter. The photographs, which partially over- 

 lapped, covered 54' by 68' each and reached magnitude 20 over 

 most of that area. The survey was completed from July 27 to 

 August 1 except for two fields, which were photographed on August 

 25. Six additional fields, three on each side of Jupiter had been 

 photographed on July 5 and 6 to record any satellites that would be 

 hidden in the glare near Jupiter at the time of the principal survey 

 3 weeks later. 



" Mon. Not, Tol. 68, p. 373, 1908. 



' Publ. AstroD. Soc. Pacific, vol. 26, p. 198. 1914. 



