Telescopic Work for Starlight Evenings

by William F. Denning

The satellites are just visible to the naked eye when the conditions favour their detection; but they are so much involved in the rays of the planet, and often so near to him, that it may be regarded as an exceptional feat to discern them without telescopic aid. When III. and IV. are near their max. elongation and on the same side of the planet, they have been occasionally observed separately. I. and II., though much closer to Jupiter and more within the influence of his glare and rays, have been similarly seen. When attempting such observations it is best to hide the bare disk of the planet behind some terrestrial object, as this will cut off the obnoxious rays and prevent the brilliant light from dazzling the eye. An opera-glass, or any small portable telescope, reveals the whole retinue of satellites, and enables them to be traced through their revolutions. The ‘Nautical Almanack’ gives diagrams of their diurnal positions, and with this work as a reference observers will find no difficulty in identifying them apart.

Sir W. Herschel, in the years 1794 to 1796, found that the satellites revolve on their axes in the same time that they revolve about the planet. He was led to this conclusion by a study of the variations in the light emitted by the satellites in different parts of their orbits, and described I. as “of a very intense bright, white, and shining light,—brighter than II. or IV. (not larger). IV. inclines to red, and nearly as bright as II. The latter is of a dull ash-colour. III. is very white.” Modern observers have selected II. as relatively the most highly reflective, while IV. is the least. Spots exist on the surfaces of these objects, and probably occasion many of the differences observed.

The eclipses, occultations, and transits of the satellites afford a very fertile and attractive series of phenomena for telescopic review. The exact times of occurrence are tabulated in the ‘Nautical Almanack’ and asterisks are affixed to such as are visible in this country. Prior to the date of opposition of Jupiter the eclipses occur of course on the W. side of the disk, while after opposition they take place on the E. side. The durations are as follow for the several satellites:—I. = 2^h 20^m, II. = 2^h 56^m, III. = 3^h 43^m, IV. = 4^h 56^m. In reference to III. and IV. the entire phenomenon may be generally observed; but this is not so in regard to II., as the emersions are frequently effected behind the planet. Only the immersions of I. are visible before opposition, from the same cause; for the satellite enters the cone of shadow close to the planet’s limb, and only comes out of it when the globe of the planet is interposed in the line of sight. In such cases the satellite emerges soon after from the limb of Jupiter; so that its obscuration has been compounded of two separate phenomena, viz. an eclipse and an occultation. After opposition this satellite is first occulted and then eclipsed. IV. sometimes escapes eclipse altogether, by passing above or below the shadow.

The motion of light was discovered, and its velocity determined, by means of the eclipses of Jupiter’s satellites. These phenomena are also useful in ascertaining longitudes. A spectator on Jupiter himself would see a vast number of solar and lunar eclipses—about 4500 of each—during the Jovian year of 4332·6 days, because the three inner satellites exhibit these phenomena at every revolution, their orbits being very slightly inclined to Jupiter’s equator, and the latter being but little inclined to the plane of the ecliptic.

The occultations of the satellites are comparatively frequent, and may be well observed in a good telescope. A tolerably high magnifier is required to witness these occurrences with the best effect, the disks of the satellites being small and not clearly traceable through the various stages of their disappearances unless much amplified. With considerable telescopic power the disks are well seen, and it then becomes feasible to watch the satellites, first as they come into contact with the limb, then as the globe of the planet overlaps more and more of their diminutive forms, and finally as they reach last contact and withdraw their narrow unobscured segments behind the expansive sphere of their primary. Both the beginning and end phase of these occultations is generally observable in regard to Sat. IV., and frequently also in the case of III. But with reference to II. and I. it often happens that only the disappearance or reappearance can be witnessed. These occultations have furnished some singular and unexplained facts of observation. On meeting the limb of Jupiter Sats. I. and II. have not always disappeared in a normal way. On April 26, 1863, Wray, with an 8-inch objective, saw II. distinctly projected within the limb for nearly 20^s. Other similar cases are recorded. The satellites have been seen apparently “through the edge of the disk.” One observer mentions that II. appeared and disappeared several times before occultation. The explanation appears to be that there is so much irradiation round the disk of Jupiter that it produces a false limb, and it is through this the satellites have been seen. A very tremulous image, in bad air, may also be responsible for some of the anomalies recorded.

The transits of the Jovian moons offer the most attractive phenomena of all, and they come well within the reach of small telescopes. On entering upon the planet they are visible as bright round spots projected on the dusky limb, and subsequently present some eccentric features. II. is invisible, except on the limbs; I. is often seen as a grey spot threading along the belts; III. appears as a large dark spot[41], nearly as black as its shadow; IV. seems to be black, and scarcely to be recognized from its shadow. The appearances are certainly to some extent variable. Mr. Stanley Williams has seen III. as a brilliant disk at mid-transit. I. sometimes crosses the whole disk as a white spot; at certain other times it is invisible; at others, again, it is seen as a faint grey spot. IV. is not always black, its aspect depending upon the chord it traverses. Thus, on the evening of Sept. 12, 1889, Mr. Williams, Mr. G. T. Davis of Reading, and myself were observing Jupiter when IV. was in transit on a belt in the N. hemisphere, but not a vestige of the satellite was seen by any of us. On the morning of May 23, 1890, at 3^h 30^m A.M., however, while observing the red spot on Jupiter, I noticed a black circular spot on the great N. equatoreal belt; and this proved to be IV. in transit. These peculiarities have been accounted for as partly due to contrast and partly to dusky spots on the surfaces of the satellites. Dr. Spitta has made a number of experiments to elucidate this subject, and concludes that “the perpetual whiteness of the second satellite, and the darkened tints of the others during transit, are due to differences in their relative albedo [reflective power] as compared with that of Jupiter, and are not dependent upon the relative quantity of light reflected by one or the other, or upon any physical peculiarities of the Jovian system.”

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:—

Fig. 37.

Shadows of Jupiter’s Satellites II. and III. near an equatoreal white spot (Jan. 1, 1885, 7^h 20^m A.M.).