THE SATELLITES OF JUPITER.

341

the same order of sequence in the two transits, wherever theyare visible, will occur, until Jupiter is so near to Conjunctionwith the Sun that we are unable to watch it. When it againbecomes visible after Conjunction, and until Opposition takesplace, the Satellite will be seen to follow instead of to precedeits shadow as it passes across the disc.

From our previous comparison of the occurrence to that ofa Solar eclipse, it will, of course, be understood that, whereversuch a shadow falls upon Jupiter, an eclipse of the Sun is inprogress. But we have met with no attempt, in any elementarywork, to calculate the size of these shadows, or the durations ofthe total eclipses of the Sun which they produce upon Jupiter ;nor can we here devote much space to any such calculation.

We may, however, mention that, in some of our lecturesdelivered at G-resham College in which we have consideredthese Satellites more minutely, we have worked out a fewapproximate calculations, without any attempt at strict accu-racy, which show that the shadow of the 4th (or furthest)Satellite is about 950 miles in diameter when it falls uponthe planet. We also found that, at a place upon Jupiter ’sequator, if this Satellite were in the zenith, a total Solar eclipsewould be caused of about 6 minutes’ duration. There would,however, be one remarkable difference between the circum-stances involved in such a Solar eclipse, and those of one seenupon the Earth ; viz., that, instead of the rotation of the place ofobservation round the axis of Jupiter helping to diminish thespeed of the shadow passing across it (as is the case with therotation of the Earth and the shadow of the Moon upon it), thespeed of the shadow (viz., about 18,500 miles per hour) wouldhelp to diminish the greater speed of about 28,000 miles perhour, with which the axial rotation of the planet wouldotherwise carry the observer through it.*

For the 3rd and 2nd Satellites, we found the speed of therespective shadows, under like circumstances, to be about

* The difference of the two speeds being 9,500 miles per hour, thewidth of the shadow (950 miles) would pass over the supposed point ofobservation in about r \th of an hour, or in about 6 minutes, as statedabove.