Page images
PDF

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 like 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 Dread th, 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 to the middle one. They differ in their brightness

the exterior ring is of a grayish tint; the middle one is the most brilliant and is more luminous than Saturn itself; the interior is dusky and has a purple tinge. The exterior and middle rings are both opaque and cast on the planet a distinct shadow; while the interior one is so transparent that it appears upon the globe of Saturn as a dark band through which the surface of the planet is readily seen. The dimensions of the rings are given in the following table (Guillemin):

Miles.

Diameter of exterior ring.............................. 173,500
Breadth of exterior ring............................... 10,000
Diameter of middle ring.............................. 150,000
Breadth of middle ring................................ 18,300
Distance between exterior and middle ring........... 1,750
Diameter of interior ring.............................. 113,400
Breadth of interior ring............................... 9,000
Distance of interior ring from planet.................. 10,150
Entire breadth of ring system......................... 39,050
Thickness of rings not more than..................... 100

The rings revolve around Saturn in about 103 hours, in the same direction as the planet revolves on its axis. The globe of Saturn is not exactly at the centre of the rings. This fact, combined with the rotary motion, is essential to the stability of the rings, preventing them from being precipitated in an overwhelming ruin and devastation upon the body of the planet.

Phases of the rings.—The plane of the rings is inclined 28° to the ecliptic. In its revolution about the sun, the axis of Saturn remaining parallel to itself, the sun sometimes illumines the northern and sometimes the southern face of the rings. At Saturn's equinoxes the edge only receives the light, and the rings are invisible to us, except with the

[ocr errors][graphic][merged small]

most powerful telescopes, and then only as a line of light. The body of the planet constantly cuts off the sun's rays from a portion of the rings, and also serves to conceal from our view some of the luminous part. By a careful study of the cut these various positions of the planet and rings, with the most favorable times for observation, may be understood.

Belts.—The surface of Saturn is traversed by dusky belts of a less distinct and definite appearance than those upon Jupiter. The equatorial regions are brighter than the other parts of the disk; the poles especially are less luminous. Satellites.—Saturn has eight satellites, named—

1. Mimas. 3. Tethys. 5. Rhea. 7. Hyperion.

2. Enceladus. 4. Dione. 6. Titan. 8. Iapetm

Iapetus is the largest of these, and in size exceeds Mars. Enceladus and Mimas are the faintest of twinklers, and can only be seen with a powerful telescope, and under most favorable circumstances. They were first detected by Herschel, "threading like pearls the silver line of light," to which the ring, then seen edgewise, was reduced,—advancing off it at either end, returning, and then hiding themselves behind the planet. The first four of these moons are nearer to Saturn than our moon to the earth, but Iapetus is nearly ten times as distant: so that the diameter of the Saturnian system is nearly four and a half million miles. The movements are extremely rapid. Mimas traverses a space equal to the diameter of our moon in two minutes, passing from new to full in twelve hours,—a little more than a Saturnian day.

Saturnian Scenery.—The grandeur and magnificence of the scenery upon Saturn undoubtedly far surpass anything with which we are familiar. In the cut is given an ideal view of a landscape located upon the planet at a latitude of about 28°, taken about midnight. The rings form an immense arch, which spans the sky and sheds a soft radiance around; while to add to the strange beauty of the

[merged small][graphic][merged small]

Saturnian night, eight moons in all their different phases, full, new, crescent, or gibbous, light up the starry vault.

UKANUS.

"Heaven," the most ancient of the gods. Sign IJI; H, the initial letter of Herschcl, with a planet suspended from the cross-bar.

Description.—On the 13th of March, 1781, between 10 and 11 p. m., Sir William Herschel was engaged in examining with his great telescope some stars in the constellation Gemini. One small star attracted his attention, which he accordingly observed with a higher magnifying power, when, unlike the

« PreviousContinue »