256. _Jupiter's Four Moons._--Jupiter is accompanied by four moons, as shown in Fig. 278. The diameters of these moons range from about twenty-two hundred to thirty-seven hundred miles. The second from the planet is the smallest, and the third the largest. The smallest is about the size of our moon; the largest considerably exceeds Mercury, and almost rivals Mars, in bulk. The sizes of these moons, compared with those of the earth and its moon, are shown in Fig. 279. [Illustration: Fig. 279.] The names of these satellites, in the order of their distance from the planet, are _Io_, _Europa_, _Ganymede_, and _Callisto_. Their times of revolution range from about a day and three-fourths up to about sixteen days and a half. Their orbits are shown in Fig. 280. [Illustration: Fig. 280.] 257. _The Variability of Jupiter's Satellites._--Remarkable variations in the light of these moons have led to the supposition that violent changes are taking place on their surfaces. It was formerly believed, that, like our moon, they always present the same face to the planet, and that the changes in their brilliancy are due to differences in the luminosity of parts of their surface which are successively turned towards us during a revolution; but careful measurements of their light show that this hypothesis does not account for the changes, which are sometimes very sudden. The satellites are too distant for examination of their surfaces with the telescope: hence it is impossible to give any certain explanation of these phenomena. [Illustration: Fig. 281.] 258. _Eclipses of Jupiter's Satellites._--Jupiter, like the earth, casts a shadow away from the sun, as shown in Fig. 281; and, whenever one of his moons passes into this shadow, it becomes eclipsed. On the other hand, whenever one of the moons throws its shadow on Jupiter, the sun is eclipsed to that part of the planet which lies within the shadow. To the inhabitants of Jupiter (if there are any, and if they can see through the clouds) these eclipses must be very familiar affairs; for in consequence of the small inclinations of the orbits of the satellites to the planet's equator, and the small inclination of the latter to the plane of Jupiter's orbit, all the satellites, except the most distant one, are eclipsed in every revolution. A spectator on Jupiter might therefore witness during the planetary year forty-five hundred eclipses of the moons, and about the same number of the sun. [Illustration: Fig. 282.] 259. _Transits of Jupiter's Satellites._--Whenever one of Jupiter's moons passes in front of the planet, it is said to make a _transit_ across his disk. When a moon is making a transit, it presents its bright hemisphere towards the earth, as will be seen from Fig. 282: hence it is usually seen as a bright spot on the planet's disk; though sometimes, on the brighter central portions of the disk, it appears dark. [Illustration: Fig. 283.]