THE SATELLITES OF JUPITER.

331

one degree. For the 1st Satellite the inclination is only about,hn(l of one degree, or less than one minute.

It may he understood, by means of our previous explanationsin the case of the Moon (see Lecture IV., p. 90), and in thatof the Satellites of Mars (see Lecture X., p. 257), that Jupiter’sMoons, and especially the inner ones, when viewed from theplanet, would vary very considerably in apparent size accordingas they might he observed near to the horizon, or nearly over-head. In the latter case an observer, situated upon theplanet’s equator, would be not much less than 44,000 miles (i.e.the length of Jupiter ’s radius) nearer to the Satellite thanin the former case: which would reduce the distance of thenearest from about 258,000 miles to about 218,000 miles, orby nearly |th part. The apparent diameters in the twopositions would consequently be nearly as the numbers 5: 6;and the areas of the discs as the squares of these numbers, oras 25 : 36; so that the disc of this Satellite, which is the onewhose change of size would be the most remarkable, wouldappear to be larger by about one-half when seen in the zeniththan when seen in the horizon.

If we take Jupiter ’s day to be about ten of our own hoursin length, it may be easily calculated that its four Moons goround respectively in rather more than 4, rather more than 8,about 17, and about 40, of the planet’s days. In these com-paratively short periods they pass through all their variedphases, with the exception, as we shall presently show, thatthree of them are always eclipsed when they would otherwisebe full moons.

Many very remarkable combinations of their phases mustfrom time to time be visible in the Jovian skies ; the interestattaching to which is, however, greatly diminished when weremember that they always reflect so small an amount ofSolar light, that it must not for a moment be imagined, as thequotation at the head of this lecture may perhaps seem tosuggest, that they can be intended to compensate Jupiter bynight for the feebleness of the illumination which it enjoys byday in consequence of its great distance from the Sun. Theapparent areas of their four discs, if they could all be seen at