they are fragments that should, in the remote
ages of the beginning of the solar system, have
coalesced into a planet of ordinary size. If such
has been the manner of formation of the larger
planets, we cannot know the exceptional causes
that operated to prevent the more usual result
in the present instance. Our knowledge of the
existence of the asteroids dates back only to the
beginning of the Nineteenth Century, the first
of them, Ceres, having been discovered in 1801.
Jupiter, tlie giant planet of our system, is nearly eleven times as large in diameter as the earth. So very far away is it that the earth's orbit shrinks almost into insignificance by com- parison. It follows that there is never any sensi- ble foreshortening in our view of Jupiter, nor does it exhibit any phases like the moon or closer planets. Jupiter"lias no less than five satellites. ]'"our of these are of quite conspicuous magni- tude, and visible easily in the smallest telescope. Indeed, they were discovered, as we have already seen, by Galileo, the first time he ever turned to the heavens the telescope he had invented. The fifth is a very tiny one, so close to the planet that it can be distinguished only with the greatest difiiculty. It was discovered by i3arnard in 1892 at the Lick Observatory. These satellites present a great variety of interesting phenomena. At times one or another is eclipsed, on account of the planet being between the satellite and the sun. They then rather suddenly disappear from our view. At other times, they become invisible by going behind the planet as seen from the earth. Again, sometimes they pass between the planet and the sun, so that we can see quite plainly the satellite's shadow projected as a black dot on the planet's visible disk. In the telescope, Jupiter shows truly astonishing surface mark- ings. It would appear as though it possesses a great atmosphere with floating clouds of varying shapes and colors. It has an axial rotation which is completed once in about ten hours, so that the rotation alone is rapid enough to give ever changeable views of the visible surface to the observer.
Saturn, the last of the planets known to the ancients, is still farther away from the sun. It has no less than eight moons, all of which were discovered before 1850. But decidedly the most interesting feature of its system is the bright detached ring that encircles the ball of the planet. It was first discovered by Huygens in 1G55, but only in our own day has it been shown by the mathematical researches of Maxwell in England and the spectroscopic observations of Keeler in our own country that the ring is really a mass of small detached satellites, not without analogy to the belt of asteroids encircling the sun.
Uranus and Neptune are the two remaining planets. Uranus was discovered by William Herschel in 1781. It is an inconspicuous object on account of its immense distance, though it has four small satellites that just admit of being seen in a telescope of good size. The most inter- esting thing about Uranus is that it led to the discovery in 1840 of the new planet Neptune, from a mathematical consideration of the small perturbations of the orbit of Uranus by the latter planet. This discovery is regarded as the great- est triumph ever attained by mathematical science. The honor was shared equally by Le- verrier of France and Adams of England. Comets. In addition to the planets, the solar system also contains certain somewhat erratic bodies called comets (q.v. ). Nothing definite is known as to their origin or physical constitu- tion ; but it is certain that they move, imder the influence of gravitation, in orbits that admit of calculation by the aid of Newton's and Kepler's laws. In appearance the}' vary greatly. At times the comet is but a trace of thin, filmy haze on the sky, so tenuous that it can barely be glimpsed in the telescope, and allowing the stars to be seen easily through it on the background of the sky. At other times, the comet is a magni- ficent object, very conspicuous to the naked eye, and perhaps spreading its tail over a large part of the celestial vault. Such large comets in ancient times were usually looked upon with superstitious fear. In our day little of this re- mains, except that the possibility of actual col- lision with the earth is often the subject of in- quiry when new comets appear. It may be said with confidence that the inherent probability of such a collision is so small as to be practically nil. And even if a collision were imminent, we might look forward to it without apprehension, so far as our present knowledge of comets enables us to form an opinion. Sometimes comets are seen to divide into parts, and there is good rea- son to suppose that they occasionally become ^ completely disintegrated, subsequently changing i into swarms of small particles, perhaps in most cases no bigger than the head of a pin. When the earth passes through such a swarm, friction with our atmosphere heats the particles until they become incandescent. They are then plainly visible as meteors, and even quite large ones have at times been found actually to fall upon the earth's surface. See Meteors.
The Stabs. Far beyond the confines of the solar system, and, indeed, at a distance almost inconceivably great, is situated the universe of stars ( q.v. ) . Aecordingto the accepted theory these stars, which to us appear merely as points of light, are really great blazing suns, in many cases, doubtless, attended by systems of planets analogous to our own. From the earliest ages it has been the custom, for purposes of classi- fication, to divide the stars into the so-called constellations (q.v.). These are purely arbitrary subdivisions of the surface of the celestial vault, and are supposed to represent usually some mythological personage or fantastic animal. Exact science has substituted for the constella- tions, carefully made catalogues of the stars, in which their precise positions are set down much in the same manner as, on the earth, latitude and longitude are used to define the positions of various places. The stars are usually called 'fixed stars.' It is known, however, that they are not really inmiovable. but are in continuous motion through space in orbits so vast that we have as yet been unable to do more than giiess at their dimensions. Even the sun, regarded as a star, is known to be moving through space toward a point in the constellation Hercules with a velocity of about 10 miles per second. I'he whole solar system of course partakes of this motion.
Many attempts have been made to measure the distances of the stars. But as we can actually observe only their directions in space, the sole method open to us is to note whether such directions suffer any change when the earth is situ-
