Jupiter's satellites may be seen with very low powers; indeed the outer ones have been seen with the naked eye, and all are visible in a good opera-glass. Their dimensions relatively to the disc are shown in Plate 7. Their greatest elongations are compared with the disc in the low-power view.
Jupiter's belts may also be well seen with moderate telescopic power. The outer parts of his disc are perceptibly less bright than the centre.
More difficult of observation are the transits of the satellites and of their shadows. Still the attentive observer can see the shadows with an aperture of two inches, and the satellites themselves with an aperture of three inches.
The minute at which the satellites enter on the disc, or pass off, is given in 'Dietrichsen's Almanac.' The 'Nautical Almanac' also gives the corresponding data for the shadows.
The eclipses of the satellites in Jupiter's shadow, and their occultations by his disc, are also given in 'Dietrichsen's Almanac.'
In the inverting telescope the satellites move from right to left in the nearer parts of their orbit, and therefore transit Jupiter's disc in that direction, and from left to right in the farther parts. Also note that before opposition, (i.) the shadows travel in front of the satellites in transiting the disc; (ii.) the satellites are eclipsed in Jupiter's shadow; (iii.) they reappear from behind his disc. On the other hand, after opposition, (i.) the shadows travel behind the satellites in transiting the disc; (ii.) the satellites are occulted by the disc; (iii.) they reappear from eclipse in Jupiter's shadow.
Conjunctions of the satellites are common phenomena, and may be waited for by the observer who sees the chance. An eclipse of one satellite by the shadow of another is not a common phenomenon; in fact, I have never heard of such an eclipse being seen. That a satellite should be quite extinguished by another's shadow is a phenomenon not absolutely impossible, but which cannot happen save at long intervals.
The shadows are not black spots as is erroneously stated in nearly all popular works on astronomy. The shadow of the fourth, for instance, is nearly all penumbra, the really black part being quite minute by comparison. The shadow of the third has a considerable penumbra, and even that of the first is not wholly black. These penumbras may not be perceptible, but they affect the appearance of the shadows. For instance, the shadow of the fourth is perceptibly larger but less black than that of the third, though the third is the larger satellite.
In transit the first satellite moves fastest, the fourth slowest, the others in their order. The shadow moves just as fast (appreciably) as the satellite it belongs to. Sometimes the shadow of the satellite may be seen to overtake (apparently) the disc of another. In such a case the shadow does not pass over the disc, but the disc conceals the shadow. This is explained by the fact that the shadow, if visible throughout its length, would be a line reaching slantwise from the satellite it belongs to, and the end of the shadow (that is, the point where it meets the disc) is not the point where the shadow crosses the orbit of any inner satellite. Thus the latter may be interposed between the end of the shadow—the only part of the shadow really visible—and the eye; but the end of the shadow cannot be interposed between the satellite and the eye. If a satellite on the disc were eclipsed by another satellite, the black spot thus formed would be in another place from the black spot on the planet's body. I mention all this because, simple as the question may seem, I have known careful observers to make mistakes on this subject. A shadow is seen crossing the disc and overtaking, apparently, a satellite in transit. It seems therefore, on a first view, that the shadow will hide the satellite, and observers have even said that they have seen this happen. But they are deceived. It is obvious that if one satellite eclipse another, the shadows of both must occupy the same point on Jupiter's body. Thus it is the overtaking of one shadow by another on the disc, and not the overtaking of a satellite by a shadow, which determines the occurrence of that as yet unrecorded phenomenon, the eclipse of one satellite by another.[13]
The satellites when far from Jupiter seem to lie in a straight line through his centre. But as a matter of fact they do not in general lie in an exact straight line. If their orbits could be seen as lines of light, they would appear, in general, as very long ellipses. The orbit of the fourth would frequently be seen to be quite clear of Jupiter's disc, and the orbit of the third might in some very exceptional instances pass just clear of the disc. The satellites move most nearly in a straight line (apparently) when Jupiter comes to opposition in the beginning of February or August, and they appear to depart most from rectilinear motion when opposition occurs in the beginning of May and November. At these epochs the fourth satellite may be seen to pass above and below Jupiter's disc at a distance equal to about one-sixth of the disc's radius.