THE SATELLITES OF JUPITER.

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Barth has passed beyond a position in which Jupiter is seen inOpposition, and is proceeding towards one in which it will seeit in Conjunction, the phenomena to be observed in the case ofthe 1st and (in general) of the 2nd will consist of a Disappear-ance by occultation and an Egress from eclipse, the end of theoccultation and the beginning of the eclipse being invisible.

But, in addition to the eclipses and occultations of theSatellites , we frequently see that they pass between us and theglobe of Jupiter . At such times a transit of a Satellite is saidto occur, as it then appears to travel, or to transit, across thedisc of the planet. Nor is this all, for we are able (and withgreater facility) to watch the shadow which the Satellite castsupon Jupiter , as it also sweeps across its surface. This is the casein Mr. De la Rue’s beautiful drawing in Fig . LXVII., page 304,in which one Satellite is faintly visible in transit, while itsshadow is clearly seen at some distance from it against thebackground of the planet. Again, in the second drawing by Mr.G. D. Hirst, in Plate VIII., p. 304, the 4th Satellite appears asa small, darkish spot on the lower, or northern, half of theview, while somewhat above it and to the left of it the shadowof the 3rd is shown as a darker and much larger spot.

These two kinds of transits correspond respectively to thosewhich might be seen by an observer upon the Sun if the Moon were passing between him and the Earth . He might not onlysee it apparently crossing the Earth ’s disc, but he might alsosee its shadow sweeping over a zone of the Earth ’s surface,upon which a Solar eclipse would at the time take place.The occurrence of the various phenomena which we havethus described is well illustrated in Fig . LXXVIII., which isalmost exactly copied, by the kind permission of Mr. Bentley,from one in the English edition of M. Guillemin’s very popularwork, “ The Heavens.” In this figure a Satellite is shown intransit upon Jupiter at t, while its shadow is at the same timeseen upon the disc at s. Of the other three Satellites one isundergoing eclipse, and another is just about to do so.

It requires but little consideration, to see that, if a straightline joining the Earth and a Satellite meets the planet’s disc,it will in general do so, as in the figure, at a different point from