Jupiter. By far the largest of the planets is second in brilliancy to Venus, unlike which, however, it is a “superior” planet, having its orbit outside that of the earth. It is about five times as brilliant as Sirius, the brightest of the fixed stars.
The planet is a beautiful object when viewed with a telescope; it is probable that the markings are entirely due to its atmosphere, and that the actual surface of the planet is rarely visible. Jupiter has hardly yet cooled from the condition of incandescence, and it is only slightly solidified. It possesses eight satellites, four of which were discovered by Galileo when he applied the telescope first to the investigation of the heavens. By means of these satellites the first observations of the velocity of light were made. A fifth was discovered in 1892 at the Lick Observatory.
Saturn was recognized as a planet by the ancients, and was the outside member of the solar system as known by them. His diameters at the equator and poles differ considerably, the protuberance at the equator giving him there a diameter of 74,000 miles, while at the poles it is only 68,000. In size Saturn is the largest of the planets except Jupiter, being in fact seven hundred times larger than our earth, but his density is so small that he would be able to float on water far more easily than an iceberg. From this it follows that he cannot consist of solid or liquid matter, and in fact we can only view a mass of clouds intensely heated within, the whole being probably a planet in the early stage of development—younger even than Jupiter.
The most remarkable characteristic of Saturn, which makes him an object of such interest in the sky, is his possession of a luminous ring. The ring is only luminous on account of its reflection of the sun’s light; hence is invisible to us when, for instance, we are endeavoring to look at the ring from below while the sun is shining above. It also sometimes happens that the plane of the rings passes through the sun or through the center of the earth, in which case only the thin edge of the rings can be seen. The ring is divided into two parts, the inner being the wider, while another faint division appears to divide the outer part into two smaller rings. In 1850 another ring was discovered; this is quite different from the outer rings, being dark, and generally known as the dusky ring of Saturn. The outer ones, though far from solid, can receive a shadow of Saturn, and themselves cast one on his disc. The rings are not continuous masses of matter, but consist of countless myriads of tiny satellites, so close together that to the observer they appear as one body. The planet has eight satellites which seldom pass behind or in front of the planet’s disc, and therefore are not objects of great interest.
Uranus is the next planet beyond Saturn. His mass is about fifteen times as much as that of the earth, an amount which makes him more than outweigh Mercury, Venus, the Earth, and Mars combined. All astronomers do not agree in their estimation of these numbers, Uranus being too far away for measurements to be more than approximate. Gravity on his surface is only three-quarters of what it is here. Uranus has four satellites, and possibly faint rings like those which encircle Saturn.
Neptune is farthest from the sun, the distance between the two bodies being about 2,750,000,000 miles. At this immense distance it will, according to Kepler’s laws, take a long time to travel once around its orbit, and this time has been found to be one hundred and sixty-five of our years. Although it is ninety-seven times as large as the earth, yet, on account of its enormous distance from us it can only just be seen, even with a powerful telescope. Neptune possesses one satellite, which moves around the planet in rather less than six days.
Mercury is the smallest planet, except the planetoids, in the solar system, and the one nearest the sun. It is never seen for more than two hours before sunrise or after sunset, and is not always visible then; but when it does appear, it is extremely brilliant. Even when it is most distant the sun appears four and a half times as big to it as it does to us, and when the two are at their nearest, this small planet gets ten times as much light and heat as we do. It is, however, so small and difficult to observe, that comparatively little is known of it.
Venus appears to us as the most brilliant of all the planets, sometimes heralding the sun’s approach in the morning and sometimes following him at night. Hence she has been called the “morning” and the “evening” star; and the ancient Greeks, believing her to be two bodies, and not one, called her Hesperus (Vesper) when she appeared at night, but Phosphorus when she preceded the dawn, this last name having been translated in the Latin, Lucifer. We know very little of the actual surface of Venus, for her envelope of clouds remains constantly in front of us to baffle curiosity, and never lifts to give us a glimpse of the planet beneath. These clouds send on to us the light they borrow from the sun, and shine to us with a brilliant silvery lustre interrupted here and there with shadowy markings of short duration. But when Venus shines to us in crescent-form, certain spots near the ends of the horns can be seen more definitely, and the effects of light and shadow round these points suggest that they are lofty peaks, reaching above the clouds.
The Minor Planets or Asteroids. The space between Mars and Jupiter is occupied by a strange and numerous swarm of minor planets or asteroids. The first of these singular bodies was discovered by an Italian astronomer, Piazzi, on the first night of the nineteenth century. Three others were discovered within the course of the next seven years, and the number now known is upward of 600, most of which have been recognized by the record of their motion on photographs of the sky. The four asteroids first discovered, Ceres, Pallas, Juno, and Vesta, are naturally the largest, ranging in diameter from four hundred to one hundred and eighteen miles.
Vesta, though not the largest, is considerably the brightest of the minor planets, and is occasionally visible to the naked eye. None of the other asteroids has a diameter so great as one hundred miles, and probably the majority of them are only ten or twenty miles in diameter.