The Heavens Above: A Popular Handbook of Astronomy

by J. A. Gillet; W. J. Rolfe

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.

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.

Fig. 283.

It will be seen from Fig. 282 that the shadow of a moon does not fall upon the part of the planet's disk that is covered by the moon: hence we may observe the transit of both the moon and its shadow. The shadow appears as a small black spot, which will precede or follow the moon according to the position of the earth in its orbit. Fig. 283 shows two moons of Jupiter in transit.

260. Occultations of Jupiter's Satellites.—The eclipse of a moon of Jupiter must be carefully distinguished from the occultation of a moon by the planet. In the case of an eclipse, the moon ceases to be visible, because the mass of Jupiter is interposed between the sun and the moon, which ceases to be luminous, because the sun's light is cut off; but, in the case of an occupation, the moon gets into such a position that the body of Jupiter is interposed between it and the earth, thus rendering the moon invisible to us. The third satellite, m'' (Fig. 282), is invisible from the earth E, having become occulted when it passed behind the planet's disk; but it will not be eclipsed until it passes into the shadow of Jupiter.