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Dimensions.—Its diameter is about 88,000 miles, or one-tenth of the sun. Its volume is 1,400 times that of the earth, and much exceeds that of all the other planets combined. Seen at the distance of the moon, this immense globe would embrace 1,200 times the space of the full moon. Jupiter's density is only one-fifth that JDPITEB
of the earth; moreover, its rapid rotation upon its axis, whereby a particle on the equator revolves with a velocity of 467 miles per minute against the earth's 17 miles per minute, must produce a powerful centrifugal force which materially diminishes the weight of all objects near its equator. Consequently a stone let fall on Jupiter would pass through but about thirty-nine feet the first second. As a result of this rapid rotation, the planet is one of the most flattened of any in the solar system—the equatorial diameter exceeding the polar by about 5,000 miles.
Seasons.—As the axis of Jupiter is but slightly inclined from a perpendicular to the plane of its orbit, there is but little difference in the length of its days and nights, which are each of about five
hours' duration. At the poles the sun is visible foi nearly six years, and then remains set for the same length of time. The seasons also are but slightly varied. Summer reigns near the equator, while the temperate regions enjoy perpetual spring. The light and heat of the sun are only ^ of that which we receive; yet peculiarities of soil or atmosphere may compensate this difference. The evening sky on Jupiter must be inexpressibly magnificent; besides the glittering stars which adorn our heavens, four moons, waxing and waning, each with its diverse phase, illuminate its night. All the starry exhibition sweeps through the sky in five hours.
Telescopic Features.—Jupiterh moons.—Under the telescope Jupiter presents a beautiful Copernican system in miniature. Four small stars—moons—are seen to accompany it in its twelve-yearly revolutions. From hour to hour their positions vary, and they seem to oscillate from one side to the other of the planet. At one time there will be two on each side, and again, three on one side; while the remaining star is left alone. They are also frequently found to disappear, one, two, or even three at a time, and, more rarely, all four at once. There are wellauthenticated instances on record of their having been seen by the naked eye. Among others, the following singular case is mentioned. Wrangle, the celebrated Russian traveller, states, that when in Siberia, he once met a hunter, who said, pointing to Jupiter, "I have just seen that star swallow a small one and then vomit it up again." These moons are called by the ordinal numbers, reckoning outward from the planet. With an ordinary glass, there is nothing to distinguish them from small stars. The Hid, however, being the largest and brightest, will generally be identified easiest. The Ist satellite appears to the inhabitants of the planet almost as large as our moon to us; the lid and Illd about half as large. Their real size and density are indicated in the following table. It will be seen that the IVth is about the weight of cork, and the Ist and lid are still lighter.
Satellites Of Jupiter.
It is noticeable that here are four satellites revolving about Jupiter, one of them larger than the planei Mercury, and each far surpassing in size the minor planets between Mars and Jupiter. The moons are not only thus distinguished by their various dimensions, but also by the variety of their color. The Ist and lid have a bluish tint, the Hid a yellow, and the IVth a reddish shade. The total space occupied by this miniature system is about two and a half million miles in diameter.
Edipseof the moons.—Jupiter, like all celestial bodies not self-luminous, casts into space a cone of shade.
The Ist, lid, and IHd satellites revolve in orbits but very little inclined to the plane of the planet's orbit. During each revolution they pass
between the Sun and Jupiter, producing a solai eclipse; and also by passing through the shadow of the planet itself, cause to themselves an eclipse of the sun, and to Jupiter an eclipse of a moon. The IYth passes through a path more inclined, and therefore its eclipses are less frequent: instead of being fully eclipsed, it sometimes just grazes the shadow, as it were, and so its light is much diminished. Through a telescope we can distinctly watch the disappearance or immersion of the satellites in the planet's shadow, their reappearance or emersion, and also their transits, as a round black dot or shadow moving across the disk of Jupiter. In the cut, we see represented the various positions of the moons: the Ist is eclipsed; the lid is passing across the disk of the planet on which its shadow is also thrown; the Illd is just behind the planet, and so occulted or concealed, while it has not yet entered the shadow; the IVth is in view from the earth. These satellites revolve with great rapidity, as is necessary in order to overcome the superior attraction of the planet and prevent being drawn to its surface. The Ist goes through all its phases in 1| days, and the IYth in less than twenty days. A spectator on Jupiter might witness, during the Jovian year, 4,500 eclipses of the moon (moons), and about the same number of the sun.