. Astronomy for high schools and colleges . rth with a common telescope, andthe times of observation are all found predicted in theNautical Almanac. In this way we are sure that the blackspots which we see moving across the disk of Jupiter arereally the shadows of the satellites themselves, and not phe-nomena to be otherwise explained. These shadows beingseen black upon Jupiters surface, show that this planetshines by reflecting the light of the sun. SATELLITES OF JUPITER. 347 Telescopic Appearance of the Satellites.—Under ordi-nary circumstances, the satellites of Jupiter are seen tohave disk
. Astronomy for high schools and colleges . rth with a common telescope, andthe times of observation are all found predicted in theNautical Almanac. In this way we are sure that the blackspots which we see moving across the disk of Jupiter arereally the shadows of the satellites themselves, and not phe-nomena to be otherwise explained. These shadows beingseen black upon Jupiters surface, show that this planetshines by reflecting the light of the sun. SATELLITES OF JUPITER. 347 Telescopic Appearance of the Satellites.—Under ordi-nary circumstances, the satellites of Jupiter are seen tohave disks—that is, not to be mere points of light. Un-der very favorable conditions, markings have beeen seenon these disks, and it is very curious that the anomalousappearances given in Fig. 93 (by Dr. Hastings; have beenseen at various times by other good observers, as Secchi,Dawes, and Eutherfurd. Satellite III, which is muchthe largest, has decided markings on its face ; lY some-times appears, as in the ligure, to have its circular outline. W W 1 Fig. 93.—telescopic appearance of jupiters satellites. cut off by right lines, and satellite I sometimes appearsgibbous. The opportunities for observing these appear-ances are so rare that nothing is known beyond the barefact of their existence, and no plausible explanation of theligure shown in lY has been given. Phenomena of the Satellites.—The phenomena of the satel-lites are ilhistratcd in Fig. 94. Here -8 represents the sun, ^4 Tthe orbit of the earth (the earth itself being at T), the outer circlethe orbit of Jupiter, and tlie four small circles upon the latter fourdifierent positions of the orbit of a satellite. In the centre of eachof the satellite orbits \\\\\ be seen a small white circle designed torepresent the planet Jupiter itself. The dotted lines drawn fromeach edge of the sun to the corresponding edges of the planet andcontinued until they meet in a point show the outlines of theshadow of Jupiter. Let us first consi
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