Jupiter, the Giant.
Jupiter has been well named the Giant planet, since his diameter is eleven times greater, and he is thirteen hundred times larger than our planet. His inclination is very small, and you now know that under such circumstances he enjoys very small changes of seasons. Jupiter has four moons, or satellites, and an illustration of the “Jovian System” is herewith given.
Fig. 586.—The Jovian System.
Jupiter himself was well known to the ancients, but Galileo was the discoverer of the “moons.” His telescope was, of course, a very imperfect instrument, and while he was gazing at the planet he noticed three stars close by the bright disc, two on one side, but next day Galileo perceived them all at the same side. Next time he looked there were only two, and after many anxious observations he found out, not only that Jupiter had three attendant stars, but four!
These moons were found to revolve round Jupiter in times varying from nearly two days to nearly sixteen days, according as they were at a less or greater distance from him. They were found to have their times of eclipses and transits, etc., also. These moons act with respect to Jupiter very much as the inner planets act with respect to the sun, for observation showed Galileo that the satellites sometimes appeared on one side of the planet, and at other times on the opposite side.
Fig. 587.—Satellite in Transit.
From the diagram of the Jovian System we shall understand the orbits of the moons, which are all of nearly equal size,—two thousand miles in diameter,—and cause eclipses of the sun to Jupiter. If the earth be in the same direction as the sun the moons are lost to view. The satellites disappear into the shadow, and are eclipsed at 1′″, 2′″, 3′″, 4′″, respectively, but they do not always come into view again immediately they have passed through the planet’s shadow, because the earth is a little at one side of the sun. So when the satellite gets behind the edge of Jupiter, his shadow being on the opposite side to the satellite’s, it is said that the “moon” is in “occultation”; when it disappears in the shadow it is “eclipsed.” Cassini discovered the “transit” of Jupiter’s moons. The annexed diagram illustrates the eclipses, etc., very clearly. At the four points, A B C D, we have the earth; J is Jupiter with his moons; 1 2 3 4 is their orbits. At a moon No. 1 enters his shadow, and emerges at b. From the earth at D a will be visible, but not b, because Jupiter is in the way. So at B, the coming out, or emersion, will be visible, but not the entrance into the shadow, or immersion. At A the satellite is in transit d, on the disc of the planet, J.
