136 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



Very faint satellites would not show on photographs made even 

 with a large telescope guided in the ordinary way because, during 

 the necessarily long exposures, they would move, and therefore fail 

 to register. By moving the telescope to follow Jupiter during the 

 exposure, images of the very faint satellites which move very little 

 relative to Jupiter in 1 hour, can be made to fall nearly at the same 

 spot on the photographic plate during the whole exposure, thus leav- 

 ing a visible image. On such a plate the brighter stars make elonga- 

 ted images easily distinguished from the round image left by a 

 satellite. 



One or more of the thousands of asteroids which revolve around 

 the Sun between the orbits of Mars and Jupiter registered on 

 almost every photograph taken in the search. The motions of these 

 tiny planets are, however, generally faster than that of Jupiter, and 

 their images are somewhat elongated, although not so much as the 

 images of stars. About 40 moving objects were found in the course 

 of the search. Among these were the known satellites VI, VII, 

 VIII, and IX, and 5 other objects which were moving along with 

 Jupiter. 



Satellites VI and VII were identified by their positions given in 

 the American Ephemeris and Nautical Almanac, and VIII by an 

 ephemeris computed by Hertz of Yale. No ephemeris of J IX was 

 computed until after the survey was finished so that the rediscovery 

 of that satellite served as a check on the completeness of the survey. 



The five unidentified objects were followed until their motions iden- 

 tified two of them as new satellites of Jupiter^ and the other three 

 as asteroids which happened to be nearly in line with Jupiter for a 

 few days. 



The new moons are very faint and anyone who wishes to observe 

 them should remember the advice Galileo gave to those who might 

 try to see the satellites he had just discovered. "I give them warn- 

 ing, however, again so that they may not approach such an inquiry 

 to no purpose, that they will need a very accurate telescope." The 

 new satellites can be photographed with a telescope smaller than 

 the 100-inch reflector if a sufficiently long exposure is given but it 

 would be difficult to photograph them with an aperture much smaller 

 than 36 inches. No effort has been made to see them that I know 

 of, and at the present time there is only one telescope with which 

 they could be seen, the 100-inch telescope of the Mount Wilson Ob- 

 servatory. All the previously discovered satellites of Jupiter have 

 been seen with that telescope, even IX, which ls just as faint as 

 XI and almost as faint as X.® An idea of the extreme f aintness of 



» Publ. Astron. Soc. Pacific, vol. 50, p. 292, 1938. 

 »PubL Astron. Soc. Pacific, vol. 50, p. 350. 1938. 



