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.