Page images
PDF
EPUB

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, Romer (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 161 min. for light to traverse the orbit of the earth. Komer's conclusion has since been verified by the phenomena of aberration of light. The velocity of light is about 183,000 miles pel second. (See 14 Weeks in Philosophy, p. 189.) ^Jl

SATUEN.

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, 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.

[graphic]

* From this the year of Saturn may be determined. As 13 : 378 days :: Earth's year : Saturn's year = 30 yr. nearly

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 ^ 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 larger scale. The sun climbs in summer about 8° higher above the horizon, and sinks correspondingly lower in winter. The tropics are 16° further apart, and the arctic and antarctic circles 8° further from the poles. Each of Saturn's seasons lasts more than seven of our years. There is about fifteen years interval between the autumn and spring equinoxes, and between the summer and winter solstices. For fifteen years the sun shines on the north pole, and a night of the same length envelops the south pole. The atmosphere is doubtless very dense, as the belts would seem to indicate.

Telescopic Features.Saturn's Rings. Galileo first noticed something peculiar in the shape of Saturn. Through his imperfect telescope it seemed to have on each side a small planet lite a supporter, to help old Saturn on his way. He therefore announced to his friend Kepler his curious discovery, that "Saturn is threefold." As the planet, however, approached its equinoxes, these attendants vanished altogether from his simple instrument. This was a great perplexity to Galileo, and he never solved the mystery. When the rings were afterward seen, their real form was not known. They were supposed to be a kind of handle attached to the planet, but for what purpose was not explained.

The series consists of three rings of unequal Dreadth, surrounding the planet at the equator. The exterior ring is separated from the middle one by a distinct break, while the interior one seems joined 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 7.2,000 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 y^ 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

« PreviousContinue »