[THE PLANET JUPITER]
PLATE IX
Jupiter, the giant of the planetary world, is the fifth in order of distance from the Sun, and is next to Mars, our ruddy neighbor. To the naked eye, Jupiter appears as a very brilliant star, whose magnitude, changing with the distance of this planet from the Earth, sometimes approaches that of Venus, our bright morning and evening star.
The mean distance of Jupiter from the Sun is 475,000,000 miles, but owing to the eccentricity of its orbit, its distance varies from 452 to 498 millions of miles. The distance of this planet from the Earth varies still more. When nearest to our globe, or in opposition, its distance is reduced to 384,000,000 miles, and its apparent diameter increased to 50"; while when it is farthest, or in conjunction, its distance is increased to 567,000,000 miles, and its apparent diameter reduced to 30"; Jupiter being thus 183,000,000 miles nearer our globe while in opposition than when it is in conjunction.
This planet revolves around the Sun in 11 years, 10 months and 17 days, or in only 50 days less than 12 terrestrial years. Such is the year of this planet. The plane of its orbit is inclined 1° 19' to the ecliptic. No planet, except Uranus, has an orbit exhibiting a smaller inclination. The planet advances in its orbit at the mean rate of 8 miles a second; which is a little less than half the orbital velocity of the Earth.
Jupiter is of enormous proportions. Its equatorial diameter measures 88,000 miles, and its circumference no less than 276,460 miles, these dimensions being 11 times greater than those of the Earth. This planet, notwithstanding its huge size, rotates on its axis in not far from 9h. 55m. 36s., which period constitutes its day. Owing, however, to the changeable appearance of its surface, this period cannot be ascertained with very great exactitude. In consequence of its rapid rotation, the planet is far from spherical, its polar diameter being shorter than the equatorial by about ¹⁄₁₆, or 5,500 miles. Its surface is 124 times the surface of the earth; while its volume is 1,387 times as great. If Jupiter occupied the place of our satellite in the sky, it would appear 40 times as large as the Moon appears to us, and would cover a surface of the heavens 1,600 times that covered by the full Moon, and would subtend an angle of 21°. Jupiter's mass does not correspond with its great bulk, and is only ¹⁄₁₀₄₇ of the mass of the Sun, and 310 times the mass of the earth; its density being only ¼ of that of our globe. The force of gravitation at the surface of this planet is over 2½ times what it is on the Earth, so that a terrestrial object carried to the surface of Jupiter would weigh over two and a half times as much as on our globe.
Observed with a telescope, even of moderate aperture, Jupiter, with its four attending satellites and its dazzling brilliancy, appears as one of the most magnificent objects in the sky. The general appearance of the disk is white; but unlike that of Mars, it is brightest towards its central parts, and a little darker around the limb, especially on the side opposite to the Sun. Although an exterior planet, and so far from us, Jupiter shows faint traces of phases when observed near its quadratures, but this gibbosity of its disk is very slight, and is indicated only by a kind of penumbral shadow on the limb.
When observed with adequate power, the disk of Jupiter is found to be highly diversified. The principal features consist of a series of alternate light and dark streaks or bands, disposed most of the time parallel with the Jovian equator. These bands differ from each other in intensity as well as in breadth; those near the equator being usually much more prominent than those situated in higher latitudes north and south.
The equatorial zone of Jupiter is occupied most of the time by a broad, prominent belt 20° or 30° wide, limited on each side by a very dark narrow streak. Between these two dark borders, but seldom occupying the whole space between them, appears an irregular white belt, apparently composed of dense masses of clouds strongly reflecting the Sun's light, some of these cloudy masses being very brilliant. The spaces left between the cloudy belt and the dark borders, usually exhibit a delicate pink or rosy color, which produces a very harmonious effect with the varying grayish and bluish shades of some of the belts and streaks seen on the disk. Quite often the cloudy belt is broken up, and consists of independent cloudy masses, separated by larger or smaller intervals, these intervals disclosing the rosy background of this zone.
On each side of the equatorial belt there is usually a broad whitish belt, succeeded by a narrow gray band; the space left on each hemisphere between these last bands and the limb being usually occupied by two or three alternate white and gray bands. A uniform gray segment usually forms a sort of polar cap to Jupiter.
When observed under very favorable conditions, all the lighter belts appear as if composed of masses of small cloudlets, resembling the white opaque clouds seen in our atmosphere. This, as already stated, is particularly noticeable on the equatorial belt. It is not unusual, when Jupiter is in quadrature, to see some of the most conspicuous white spots casting a shadow opposite to the Sun; a fact which sufficiently indicates that these spots are at different levels. They probably form the summits of vast banks of clouds floating high up in the atmosphere of Jupiter.