Jupiter rightly wears the name of the "giant planet." His orbit is more nearly circular than most smaller planets. He could not turn short corners with facility. We know little of his surface. His spots and belts are [Page 165] changeable as clouds, which they probably are. Some spots may be slightly self-luminous, but not the part of the planet we see. It is covered with an enormous depth of atmosphere. Since the markings in the belts move about one hundred miles a day, the Jovian tempests are probably not violent. It is, however, a singular and unaccountable fact, as remarked by Arago, that its trade-winds move in an opposite direction from ours. Jupiter receives only one twenty-seventh as much light and heat from the sun as the earth receives. Its lighter density, being about that of water, indicates that it still has internal heat of its own. Indeed, it is likely that this planet has not yet cooled so as to have any solid crust, and if its dense vapors could be deposited on the surface, its appearance might be more suggestive of the sun than of the earth. _Satellites of Jupiter._ In one respect Jupiter seems like a minor sun--he is royally attended by a group of planets: we call them moons. This system is a favorite object of study to everyone possessing a telescope. Indeed, I have known a man who could see these moons with the naked eye, and give their various positions without mistake. Galileo first revealed them to ordinary men. We see their orbits so nearly on the edge that the moons seem to be sliding back and forth across and behind the disk, and to varying distances on either side. Fig. 64 is the representation of their appearance at successive observations in November, 1878. Their motion is so swift, and the means of comparison by one another and the planet so excellent, that they can be seen to change their places, [Page 166] be occulted, emerge from shadow, and eclipse the planet, in an hour's watching. [Illustration: Fig. 64.--_a._ Various Positions of Jupiter's Moons; _b._ Greatest Elongation of each Satellite.] ELEMENTS OF JUPITER'S SATELLITES. +-------------------------------------------------------------+ | | Mean Distance | | | | | from Jupiter. | Sidereal Period. | Diameter.| | |---------------+------------------+----------| | | Miles. | Days Hrs. Min. | Miles. | | I. Io | 260,000 | 1 18 28 | 2,352 | | II. Europa | 414,000 | 3 13 43 | 2,099 | | III. Ganymede | 661,000 | 7 3 59 | 3,436 | | IV. Callisto | 1,162,000 | 16 18 5 | 2,929 | +-------------------------------------------------------------+