The shadows of the satellites transit the disk as dark spots larger than the satellites themselves, owing to the penumbral fringes. Before opposition these shadows precede the satellites; after opposition the latter come first. The shadow of II. appears to be much lighter than the others, and is usually of a pale chocolate-colour; and I saw it thus at the opening of the year 1885:—
Shadows of Jupiter’s Satellites II. and III. near an equatoreal white spot (Jan. 1, 1885, 7^h 20^m A.M.).]
Sat. II. is probably involved in an atmosphere sufficiently dense to enable it to present undue luminosity relatively to the others; and if so, the feeble shadow it transmits on Jupiter may be partly explained by the effects of refraction. On the day of opposition both satellites and shadows are projected on the same part of the disk, and the latter are occulted by the former. On Jan. 14, 1872, Mr. F. M. Newton saw I. centrally placed on its shadow; so that the satellite was apparently surrounded with a ring of shade. On May 13, 1876, Mr. G. D. Hirst saw Sat. I. partly occulting its own shadow; a black crescent was seen in the bright zone N. of the equator. On Feb. 18, 1885, Dr. R. Copeland, at Dun Echt, saw the shadow of I. “almost totally occulted by the satellite itself; as the satellite approached Jupiter’s limb it came out quite bright and large, with a mere crescent of the shadow showing on its southern edge.” This phenomenon was also observed at Bristol.
Occasionally all the satellites become invisible at the same time, being either eclipsed, occulted, or in transit. An instance of this kind was recorded by Molyneux on Nov. 2, 1681 (O.S.). Sir W. Herschel observed a similar occurrence on May 23, 1802; also Wallis on April 15, 1826, and Dawes and others on Sept. 27, 1843, and Aug. 21, 1867. A visible repetition of the event was narrowly avoided on the morning of Oct. 15, 1883. On this occasion the planet should, according to the ‘Nautical Almanack,’ have been denuded of his satellites for a period of 19 minutes; but this disappearance did not occur, for at no time were all the satellites included within the margin or shadow of Jupiter. I observed that Sat. III. entered upon the disk just as IV. released itself, and the two formed a curious configuration at 4^h A.M., hanging close upon the planet’s limbs.