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 4h A.M., hanging close upon the planet’s limbs.
Spots have been seen on the satellites both in transit and while shining on the dark sky. This particularly refers to III. and IV. II. has never given indications of such markings on its bright uniformly clear surface. Dawes, Lassell, and Secchi frequently observed and drew the spots. Secchi described III. as similar in aspect to the mottled disk of Mars as seen in a small telescope; his drawings exhibit no analogy, however, to those by Dawes of the same object. III. has been remarked of a curious shape, as if dark spots obliterated part of the limbs. Sat. I. was observed in transit on Sept. 8, 1890 by Barnard and Burnham, and it appeared to be double, being divided by a bright interval or belt. They used a 12-inch refractor, powers 500 and 700, and the seeing was very fine.
Many other curious points have been noticed in the various aspects and phenomena of the Jovian satellites. Further observations will doubtless throw new light on some of the puzzling records of the past.
Occultation of a Star.—An occultation of the 7th mag. star 4 Geminorum by Jupiter took place on Nov. 7, 1882, and it was observed by Prof. Pritchett, of Glasgow, Missouri, with a 12-1/4-inch equatoreal, power 200. “The images of both planet and star were very steady. The margin of Jupiter’s disk was very sharply defined. The immersion was very near the N. border of the broad S. equatoreal belt. At 11h 28m 10s·65 local mean time the star was apparently within the dark outline of the disk, apparent geometric contact having occurred at 11h 20m 24s·49. For a moment the star seemed to disappear, but a moment later was plainly seen, as if through a well-defined notch in the otherwise continuously even margin. This notch lasted 46s·26, and at 11h 28m 56s·91 it vanished, and the light of the star was entirely extinguished.” The emersion of the star could not be observed, as clouds supervened.
[CHAPTER XII.]
SATURN.
Apparent lustre.—Grand spectacle afforded by the Rings.—Period &c.—“Square-shouldered” aspect.—Early Observations.—Belts and Spots on the Planet.—Rotation-Period.—The Rings.—Divisions in the outer Ring.—The transparent or Crape-ring.—Discordant Observations.—Eccentric position of the Rings.—Aspect.—Further Observations required.—Occultations of Saturn.—The Satellites.—Occultations of Stars by Saturn.
“Muse, raise thy voice, mysterious truth to sing,
How o’er the copious orb a lucid ring,