XVII THE SOLAR SYSTEM
Our sun is but a star traveling through the universe. It is accompanied in its journey to unknown parts of space, that lie in the general direction of the constellation Hercules, by an extensive family of minor bodies consisting of the eight planets and their encircling moons, one thousand or more asteroids, numerous comets, and meteors without number, all moving in prescribed paths around their ruler.
The most important members of the sun's family are the planets, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, named in the order of their position outward from the sun. We hear occasionally of the possibility of the existence of intra-Mercurial and trans-Neptunian planets and it is possible that some day an additional planet may be discovered within the orbit of Mercury or beyond the orbit of Neptune. The gravitational control of the sun extends far beyond the orbit of Neptune and there are reasons for believing in the existence of at least one or two additional planets on the outskirts of the solar system. The existence of a planet within the orbit of Mercury is now, after long continued and diligent search, believed to be very doubtful.
Were it possible to view the sun from the distance of the nearest star with the aid of the greatest telescope on earth all the members of his family would be hopelessly invisible. So, also, we cannot tell as we point our powerful telescopes at the stars whether these other suns are attended by planet families. We may only argue that it is very unlikely that there should be only one star among hundreds of millions that is attended by a group of comparatively small dark bodies that shine by the reflected light from the star they encircle.
With the exception of the two planets, Mercury and Venus, which are known as the inferior planets, since their paths lie between the earth and the sun, all the planets have moons or satellites of their own that encircle the planet just as the planet encircles the sun.
Our planet earth has one satellite, the moon, that has the distinction of being the largest of all the moons in proportion to the size of the planet it encircles. Jupiter and Saturn have moons that surpass our moon in actual size; in fact, two of the moons of the outer planets are actually larger than the smallest planet Mercury, but they are very small in proportion to the size of the planets around which they revolve. Mars, the next planet beyond the earth, the nearest of the superior or outer planets, has two tiny moons that bear the names of Deimos and Phobos, respectively. They are both less than twenty miles in diameter and revolve very near to the surface of Mars. They can only be seen with the aid of very powerful telescopes. The inner moon, Phobos, is unique in the solar system for it makes three trips around Mars while the planet is turning once on its axis.
Jupiter, the next planet outward from the sun, is almost a sun itself to its extensive family of nine moons. Four of these moons were first seen about three hundred years ago when Galileo pointed his first crude telescope at the heavens and any one can now see them with the aid of an opera glass. One of the four is equal in size to our own moon; the others surpass it in size. These moons are most interesting little bodies to observe. Their eclipses in the shadow of Jupiter, occultations or disappearances behind his disk, and the transits of the shadows as well as the moons themselves across the face of the planet can be easily seen even with the smallest telescope. The five remaining moons have all been discovered in modern times. They are extremely small bodies visible only in large telescopes. Satellite V is the nearest of all the moons to Jupiter. The other four are at great distances from the planet.
The planet Saturn has nine moons. Titan, the largest, is nearly equal in size to Jupiter's largest moon, and is larger than Mercury; four of the other moons have diameters between one thousand and two thousand miles in length. Since Saturn is nearly twice as far from the sun as Jupiter his moons are more difficult to observe, though the two largest are visible in small telescopes.
Saturn is unique in the solar system in possessing in addition to his nine satellites a most wonderful ring system, composed of swarms of minute moonlets, each pursuing its individual path around the planet. It is this unusual ring system that makes Saturn the most interesting to observe telescopically of all the planets.
The planet Uranus has four satellites and Neptune one. These planets and their satellites cannot be well observed on account of their great distances from the earth. The indistinctness of surface markings makes it impossible to determine the period of rotation of these two outer planets on their axes. It is believed that their rotation is very rapid, however, as is the case with the other planets Jupiter and Saturn.
All the planets in the solar system fall naturally into two groups. Jupiter, Saturn, Uranus, and Neptune, the members of the outer group, have on the average, diameters ten times as great and, therefore, volumes one thousand times as great as Mercury, Venus, Earth and Mars, the members of the inner or terrestrial group.