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Sir John Herschel says he is without discernible spots, and yet in his tables he down the time of the planet's rotation (which could only be ascertained by the rotation of spots upon the planet's disc), at 91⁄2 hours. This time is probably given on the authority of Schroeter, and is marked as doubtful by Dr. Herschel.
512. The motion of Uranus, in longitude is still slower than that of Saturn. It moves over but one degree of its orbit in 85 days; hence he will be seven years in passing over one sign or constellation. His periodic time being 84 years 27 days, his eastward motion can amount to only about 4° 17' in a whole year. To detect this motion requires instruments and close observations. At this date (1855), Uranus has passed over about of his orbit, since his discovery in 1781; and in 1865 will have traversed the whole circuit of the heavens, and reached the point where Herschel found him 84 years before.
It is remarkable that this body was observed as far back as 1690. It was seen three times by Flamstead, once by Bradley, once by Mayer, and eleven times by Lemonnier, who registered it among the stars; but not one of them suspected it to be a planet.
513. The inequalities in the motions of Jupiter and Saturn, which could not be accounted for from the mutual attractions of these planets, led astronomers to suppose that there existed another planet beyond the orbit of Saturn, by whose action these irregularities were produced. This conjecture was confirmed March 13th, 1781, when Dr. Herschel discovered the motions of this body, and thus proved it to be a planet.
514. The mean distance of Uranus from the Sun is 1,828,000,000 of miles; more than twice the mean distance of Saturn. His sidereal revolution is performed in 84 years and 1 month, and his motion in his orbit is 15,600 miles an hour. He is supposed to have a rotation on his axis, in common with the other planets; but astronomers have not yet been able to obtain any ocular proof of such a motion
515. His diameter is estimated at 34,000 miles; which would make his volume more than 80 times larger than the Earth's. To his inhabitants, the Sun appears only the part as large as he does to us; and of course they receive from him only that small proportion of light and heat. It may be shown, however, that the part of the Sun's light exceeds the illuminating power of 800 full moons. This, added to the light they must receive from their six satellites, will render their days and nights far from cheerless.
512. His motion in longitude? Periodic time? Angular motion per year? How far has he been traced since his discovery? When complete his revolution? Was he ever seen previous to 1781? By whom? Why are they not the discoverers, then? 518. Was his existence suspected previous to 1781 ? What ground for the suspicion? How proved to be a planet? 514. Mean distance? Sidereal revolution? Hourly motion in orbit? Use of satellit-s?
Rotation on axis? 515. Diameter ? Volume? Light and heat?
516. Uranus is attended by six moons or satellites, which revolve about him in different periods, and at various distances. Four of them were discovered by Sir William Herschel, and two by his sister, Miss Caroline Herschel. It is possible that others remain yet to be discovered.
Sir William Herschel reckoned six, though no other observer has confirmed this opinion; and even his son, Sir John Herschel, seems to consider the existence of six satellites quite doubtful. After mentioning the two most conspicuous, he says: "Of the remaining four, whose existence, though announced with considerable confidence by their original discoverer, could hardly be regarded as fully demonstrated, two only have been hitherto re-observed."-Outlines of Astronomy, Article 551. The two that he regarded as doubtful he supposed would be found, if at all, in orbits exterior to those of the other four. The distance from the planet and periodic times of the satellites of Uranus, respectively, are as follows:
517. This is the most distant of the primary planets, and in some respects one of the most interesting. It is about 31,000 miles in diameter, is situated at the mean distance of 2,850,000,000 miles from the Sun, and revolves around him in 164 years. So remote is this newly-discovered member of the solar system, that for a body to reach it, moving at railroad speed, or 30 miles an hour, would require more than twenty thousand years!
518. The circumstances of the discovery of this planet are at once interesting and remarkable. Such is the regularity of the planetary motions, that astronomers are enabled to predict, with great accuracy, their future places in the heavens, and to construct tables, exhibiting their positions for ages to come. after the discovery of Uranus, in 1781, his orbit was computed, and a table constructed for determining his future positions in the heavens, but instead of following the prescribed path, or occupying his estimated positions, he was found to be yielding to some mysterious and unaccountable influence, under which he was gradually leaving his computed orbit, and failing to meet conditions of the tables.
516. Number of Moons? By whom discovered? Is it certain that Uranus has six satellites? Why doubtful? 517. Distance and diameter of Neptune? Period? How 518. What remarkable circumlong to pass from the Sun to it at railroad speed? stances respecting its discovery? Perturbation?
519. At first this discrepancy between the observed and the estimated places of Uranus, was charged upon the tables, and a new orbit and new tables were computed, which it was thought could not fail to represent the future places of the planet. But these also seemed to be erroneous, as it was soon discovered that the computed and observed places did not agree, and the difference was becoming greater and greater every year. This was an anomaly in the movements of a planetary body. It was not strange that it should be subject to perturbations, from the attractive influence of the large planets Jupiter and Saturn, as these were known to act upon him, as well as upon each other, and the smaller planets, producing perturbations in their orbits, but all this had been taken into the account in constructing the tables, and still the planet deviated from its prescribed path.
520. To charge the discrepancy to the tables, was no longer reasonable, though it was thought perhaps sufficient allowance had not been made, in their computation, for the disturbing influence of Jupiter and Saturn. To determine this question, M. Leverrier, of Paris, undertook a thorough discussion of the subject, and soon ascertained that the disturbing influence upon Uranus of all the known planets, was not sufficient to account for the anomalous perturbations already described, and that they were probably caused by some unknown planet, revolving beyond the orbit of Uranus. From the amount and effect of this disturbing influence from an unknown source, the distance, magnitude, and position of the imaginary planet were computed.
521. At this stage of the investigation, Leverrier wrote to his friend, Dr. Galle, of Berlin, requesting him to direct his telescope to that part of the heavens in which his calculations had located the new planet, when lo! there he lay, a thousand millions of miles beyond the orbit of Uranus, and yet within less than one degree of the place pointed out by Leverrier! This was on the 1st of September, 1846.
522. While M. Leverrier was engaged in his calculations at Paris, Mr. Adams, a young mathematician of Cambridge, England, was discussing the same great problem, and had arrived at similar results even before M. Leverrier, though entirely ignorant of each other's labors or conclusions. This seems to estab
519. To what attributed at first? What done to correct? What then? 520. What next undertaken, and by whom? What result and conclusion? 521. What remarkable computation and letter? Result of Dr. Galle's search? 522. Who else investigating the subject at the same time? His conclusions? What fact does this establish? Why not Adams the discoverer?
lish the fact, that the new planet was discovered by calculation, though the failure of Mr. Adams to publish his conclusions, cut off his right to the honor of the discovery.
523. Since the discovery of this planet, it has been ascertained that it was seen as far back as 1795, though supposed to be a fixed star, and catalogued as such; and that all the irregularities of Uranus, with which astronomers were so much perplexed, are perfectly accounted for by the influence of the new planet.
524. On the 12th of October, 1846, Mr. Lassell, of Starfield, near Liverpool, discovered a satellite attendant upon Leverrier, and also, as he supposes, one or more rings similar to those of Saturn; but though the secondary has often been seen by others since, and has been made the basis of elaborate calculations respecting the mass of the primary, no further discovery of the rings has been made by any other observer.
COMETS-THEIR NATURE, MOTIONS, ORBITS, &o.
525. COMETS, whether viewed as ephemeral meteors, or as substantial bodies, forming a part of the solar system, are objects of no ordinary interest. When, with uninstructed gaze, we look upwards, to the clear sky of evening, and behold, among the multitudes of heavenly bodies, one, blazing with its long train of light, and rushing onward towards the center of our system, we insensibly shrink back as if in the presence of a supernatural being. But when, with the eye of astronomy, we follow it through its perihelion, and trace it far off, beyond the utmost verge of the solar system, till it is lost in the infinity of space, not to return for centuries, we are deeply impressed with a sense of that power which could create and set in motion such bodies.
526. Comets are distinguished from the other heavenly bodies, by their appearance and motion. The appearance of the planets
523. Has Neptune ever been seen prior to 1846? What supposed to be? Does it account for the perturbation of Uranus? 524. Has Neptune a satellite? When, and by whom discovered? What said of rings? 525. Subject of this chapter? How comets regarded by the uninstructed? By the astronomer? 526. How distinguished
is globular, and their motion around the Sun is nearly in the same plane, and from west to east; but the comets have variety of forms, and their orbits are not confined to any particular part of the heavens; nor do they observe any one general direction.
The orbits of the planets approach nearly to circles, while those of the comets are very elongated ellipses. A wire hoop, for example, will represent the orbit of a planet. If two opposite sides of the same hoop be extended, so that it shall be long and narrow it will then represent the orbit of a comet. The Sun is always in one of the foci of the comet's orbit.
Here it will be seen that the orbit is very eccentric, that the perihelion point is very near the Sun, and the aphelion point very remote.
There is, however, a practical difficulty of a peculiar nature which embarrasses the solution of the question as to the form of the cometary orbits. It so happens that the only part of the course of a comet which can ever be visible, is a portion throughout which the ellipse, the parabola, and hyperbola, so closely resemble each other, that no observations can be obtained with sufficient accuracy to enable us to distinguish them. In fact, the observed path of any comet, while visible, may belong either to an ellipse, parabola, or hyperbola.
527, That part which is usually brighter, or more opaque, than the other portions of the comet, is called the nucleus. This is surrounded by an envelope, which has a cloudy, or hairy appearance. These two parts constitute the body, and, in many instances, the whole of the comet. Most of them, however, are attended by a long train, called the tail; though some are without this appendage, and as seen by the naked eye, are not easily distinguished from the planets. Others again, have no apparent nucleus, and seem to be only globular masses of vapor.
Nothing is known with certainty of the composition of these bodies. The envelope appears to be nothing more than vapor, becoming more luminous and transparent when from other bodies? Form? Orbits? What practical difficulty mentioned? 527. What is the nucleus of a comet? The envelope? The tail? Have all comets these three parts? Do we understand of what they are composed? What evidence of their extreme tenuity?