ing him alive I was constantly obliged to feed him by putting the victuals by bits into his mouth." By day he ground mirrors and gave music lessons; in the evenings he conducted concerts and oratorios, running out at intervals to look through a telescope; at night he scanned the sky. Uranus swims After seven years spent in this way Uranus swam into his ken on March 13, 1781. He tells of the discovery into his ken. thus: 66 'On this night, in examining the small stars near Eta Geminorum I perceived one visibly larger than the rest. Struck with its uncommon appearance I compared it with Eta Geminorum and another star, and finding it so much larger than either, I suspected it to be a comet." Professional astronomers began to observe the new body, and later computations showed that its orbit was nearly a circle; it was therefore no comet, but a new planet. Herschel's The discovery aroused great enthusiasm, since all the other planets had been known from the earliest an- prosperity. tiquity. Herschel was at once brought into royal favor, received a pension, and was given all needed funds for constructing a twenty-foot reflecting telescope, which was much larger than any hitherto made. With this instrument and a forty-foot, built afterward, Herschel carried forward the wonderful series of observations which made him supreme among astronomical observers of all ages. His faithful sister Caroline was his indefatigable assistant, recording his observations at night, as he dictated them to her, and making tedious calculations by day. Herschel and Uranus were discovered simultaneously; the importance of the discovery of the man is a sufficient excuse for devoting so much attention to him. Of Uranus little is known which cannot be expressed Details about in cold figures. Its distance from the sun is 1,780,000,- east. Short and simple annals. Its discovery. NEPTUNE. More than 1,000,000,000 miles beyond Uranus plods slow-footed Neptune, the outpost of the solar system. Its mean distance from the sun is 2,792,000,000 miles, and its diameter is 35,000 miles. An opera-glass will render it visible; it exhibits in a large instrument a small greenish disc on which no details can be seen. Like Uranus it is enveloped in a dense atmosphere, through which struggles sunlight only as intense ast ours. Its one moon is a tiny speck of light, and is supposed to be about as big as ours. Like the moons of Uranus it revolves backward in its orbit. Neptune requires 165 years to complete a journey around the sun. The circumstances of its discovery are of high interest and involve one of the greatest triumphs of mathematicians. The discovery arose from the strange behavior of Uranus, which refused to follow the path which had been laid down for it by the mathematicians. After they had thought that it was securely ensnared it persisted in breaking the chains of their analysis, wandering into by and forbidden paths. Sixty years after its discovery it had gone so far astray that no one could doubt that something was wrong; to be sure, the theoretical and the actual planet were so close together that the unaided eye would see them as one body, but the discrepancy An intolerable was intolerable to a mathematical mind. discrepancy. So firmly convinced were astronomers of the accuracy and universality of Newton's law of gravitation that they became convinced that the observed irregularities must be due to the attraction of some other body, which pulled Uranus away from its proper path. It is a problem of no mean difficulty to compute the effect of one planet's pull on another, when the masses and relative positions of the bodies are known. How much A difficult greater the difficulty of discovering the mass and successive positions during a series of years of an unknown body, which, as the upshot showed, was more than 1,000,000,000 miles away from Uranus. Several eager minds attacked the problem, but found it too difficult for their powers. Mr. J. C. Adams, a student of the University of Cambridge, resolved to look into the matter as soon as his final examinations were over. In January, 1843, having graduated as senior wrangler, he set to work. In October, 1845, he communicated his results to the astronomer royal, who naturally thought it very improbable that a young and unknown student should have solved so profound a problem. He looked over the papers, and seeing that they gave evidence of careful research, wrote to their author concerning an obscure point in the investigation. Unfortunately Mr. Adams did not reply at once, and his communication was pigeon-holed. problem. Adams. Meanwhile a young Frenchman, Leverrier, had con- Leverrier centrated his marvelous powers upon the problem. In calculates. Challis hunts. Galle finds it. November, 1845, he sent a paper to the French Academy, in which he showed that no known causes of error would account for the wanderings of Uranus. A second paper in June of the next year assigned to the disturbing body a definite place in the zodiac. When this news reached England the astronomer royal was astonished to find that Adams and Leverrier were in substantial agreement. He at once wrote to Professor Challis of Cambridge, asking him to search for the suspected planet. Professor Challis was not very enthusiastic, but set about the work with due regard to thoroughness and to leisurely dignity. He began to take the positions of all visible stars in the suspected region, going over the same locality three times. It was his intention at some convenient season to prepare a map from each night's work, and by comparing them to find out if any one of the objects noted had moved. While he was engaged in manipulating his astronomical drag-net, Leverrier, who knew nothing of the work of the Englishmen, completed his investigations and requested Galle, director of the observatory at Berlin, who was already in possesssion of an excellent star chart, to look in a certain place; there he would find the planet. The letter was received on September 23, and on the same night Galle came upon the planet within a degree of the predicted place. When the news reached England Professor Challis bestirred himself, looked over his note-books, and found that he had observed the planet on August 4 and August 12. Had he been prompt in comparing his results, he would have detected the new body before Galle looked for it; but his burst of speed came after the race was over. Thus did confidence and energy win the victory over doubt and delay. CHAPTER XVI. COMETS AND METEORS. "Stranger of Heaven, I bid thee hail! Broad pennon of the King of Heaven." -Hogg. "And certain stars shot madly from their spheres, To hear the sea-maid's music." -Shakespeare. Few astronomers devote themselves to searching for comets; such work requires extreme patience, involves irregular hours of work, requires very little mathematical training, and is quite monotonous except at the supreme moment of discovery. If the moon is bright in the early evening the comet hunter waits till it has set. Night after night he shifts the pointer on his alarm clock and alters his hours for sleep. When once at his telescope he sweeps over a certain part of the sky, keeping his eye closely confined at the eyepiece, that nothing may escape. If a faint wisp of nebulous light comes into view he inspects it with care; if he does not recognize it he looks in his catalogue of nebula to see if it is described there. If not, he concludes that it ew, and watches it for an hour or so to see whether it appears to move among the surrounding stars. Any motion betrays its cometary nature; if it remains at rest it is a nebula. A comet may also be discovered by an astronomical photographer, who finds its image impressed upon one of his plates. Comet hunting. |