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The 1st, lid, and Hid 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 solar 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 IVth 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 1st 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 1st goes through all its phases in If days, and the IVth in less than twenty days. A spectator on Jupiter might witness, during the Jovian year, 4,500 ecbpses of the moon (moons), and about the same number of the sun.
Jupiter's belts.—These are dusky streaks of varying breadth and number, lying more or less parallel to the planet's equator, but terminating at a short distance from the edges of the disk. Between these a brighter, often rose-colored space, marks the equatorial regions. They are not permanent, but change sometimes very materially in the course of a few minutes. Occasionally only two or three broad belts are seen; at other times a dozen narrow ones appear. It is supposed that the planet is enveloped in dense masses of cloud, and that the belts are merely fissures, laying bare the solid body beneath. The parallel appearance is doubtless due to strong equatorial currents, analogous to our tradewinds.
Velocity Of Light.—By an attentive examination of the eclipses of Jupiter's moons, Homer (a Danish astronomer, in 1617) was led to discover the progressive motion of light. Before him, it had been considered instantaneous. He noticed that the observed times of the eclipses were sometimes earlier and sometimes later than the calculated times, according as Jupiter was nearest or furthest from the earth. His investigations convinced him that it requires about 16^ min. for light to traverse the orbit of the earth. Romer's conclusion has since been verified by the phenomena of aberration of light. The velocity of light is about 183,000 miles per second. (See New Physics, p. 150.)
The god of time. Sign }, an ancient scythe.
Description.—We now reach, in our outward journey from the sun, the most remote world known to the ancients. On account of its distance, it shines with a feeble but steady pale yellow light, which distinguishes it from the fixed stars. Its orbit is so vast that its movement among the constellations may be easily traced through one's lifetime. It requires two and a half years to pass through a single sign of the zodiac; hence, when once known, it may be easily found again. The earth leaves it at conjunction, makes a yearly revolution about the sun, comes to its starting point, and overtakes Saturn in about thirteen days thereafter.* On account of its slow, dreary pace, Saturn was chosen by the ancients as the symbol for lead. It is smaller than Jupiter, but much more gorgeously attended. Besides a retinue of eight satellites, it is surrounded by a system of rings, some shining with a golden light and others transparent—a spectacle which is as wonderful as it is unique.
Motion In Space.— Saturn revolves about the sun at a mean distance of 872,000,000 miles. The eccentricity of its orbit is a trifle more than that of Jupiter,
* Prom this the year of Saturn may be determined. As 13 : 378 days :: Earth's year : Saturn's year = 30 yr. nearly.
so that while it may at perihelion come fifty million miles nearer than its mean distance, at aphelion it swings off as much beyond. We can form some estimate of the size of its immense orbit, when we remember that it is moving along at the rate of 21,000 miles per hour, and yet as we look at it from night to night, we can scarcely detect any change of place. The Saturnian year is equal to about thirty of ours, and comprises nearly 25,000 Saturnian days, each of which is about ten and a half hours in length.
Distance From Earth.—This is found in the same manner as that of the other superior planets, being least in opposition and greatest at conjunction. As the earth and Saturn occupy different portions of their orbits, the distances between them at different times may vary 200,000,000 miles.
Dimensions.—Its diameter is about 72,000 miles. Its volume is nearly 750 times that of the earth. Its density is very low indeed, being much less than that of water, and about the same as that of pine wood. The Saturnian force of gravity is therefore scarcely greater than the terrestrial, so that a stone falls toward the surface of that immense globe only about seventeen feet the first second.
Seasons.—The light and heat of the sun at Saturn are only jfo that which we receive. The axis of Saturn is inclined from a perpendicular to the plane of its orbit about 31°. The seasons therefore are similar to those on the earth, but on a