294 On the velvciffi of /Ig/'f. [July 



had occurred 24o0 cclipiises — ^diicli gives for the mean 

 interval between the echpses of Jupiter's first satellite, 

 Id. 18h, 28m. 35s. 77 a result which is probably very 

 accurate, for in selecting the above observations care W'as 

 taken to obtain two which should be made when Jupiter 

 was at very nearly the same part of his orbit, by which 

 means the eflt^ct of Jupiter's unequal motion in his orbit is 

 totally avoided — for the remaining causes (of which we 

 will speak presently,) it must be recollected that our result 

 is only alfected by l-2450ths of its amount, so that an 

 uncertainty of one hour v/ould only affect our interval by 

 a little more than one second of time, but since in the 

 end it wall appear that 5 or 6 minutes is the extent to 

 which we are liable, we may state that the mean inter- 

 val between the consecutive eclipses of Jupiter's first 

 satellite, or speaking in Astronomical language, the Sy- 

 7iodic revolution is performed in Id. 18h. 28m. 35s. 77. 

 In selecting the first satellite we are led to give it the 

 preference in the first instance in consequence of the ra- 

 pidity of its motion, which enables us to observe the 

 phenomena to much greater accuracy than can be at- 

 tained with the other more distant satellites, but more 

 particularly does the first satellite suit our present pur- 

 pose from the fact that the plane of its orbit is very 

 nearly accordant with the plane of Jupiter's orbit, and 

 further, that the deviation of the orbit of Jupiter's first 

 satellite from a circle is so small as not materially to af- 

 fect a result like the present one,wl)ich lays claim to but 

 a very limited degree of accuracy : under these circum- 

 stances the above result may safely be assumed as the 

 mean time of the Synodic revolution ; (an interval which 

 we ought to obtain from any two observations made when 

 Jupiter v/as situated at his mean distance from the Sun.) 

 Now it is plain that in the period of a Synodic revoluti- 

 on, the satellite has performed more than one complete, 

 or a Sidereal revolution ; to compute the time of a Side- 



