Astronomy for Everybody/Part 4/Chapter 8

Uranus is the seventh of the major planets in the order of distance from the sun. It is commonly considered a telescopic planet; but one having good eyesight can easily see Uranus without artificial help, if he only knows exactly where to look for it, so as to distinguish it from the numerous small stars having the same appearance. Had any of the ancient astronomers made so thorough an examination of the sky from night to night as Dr. Gould did of the southern heavens after he founded the Cordoba Observatory, they would have upset the notion that there were only seven planets.

Uranus was discovered in 1782 by Sir William Herschel, who at first supposed it to be the nucleus of a comet. But its motion soon showed that this could not be the case, and before long the discoverer found that it was a new addition to the solar system. In gratitude to his royal benefactor, George III, he proposed to call the planet Georgium Sidus, a name which was continued in England for some seventy years. Some continental astronomers proposed that it should be called after its discoverer, and the name Herschel was often assigned to it. But by 1850 the name Uranus, originally proposed by Bode (author of the "Law"), and always used in Germany, became universal.

​When the orbit of the planet was determined, so that its course in former years could be mapped out, the curious fact was brought to light that it had been seen and recorded nearly a century before, as well as a few years previously. Flamsteed, Astronomer Royal of England, while engaged in cataloguing the stars, had marked it down as a star on five occasions between 1690 and 1715. What was yet more singular, Lemonnier, at the Paris Observatory, had recorded it eight times in the course of two months, December, 1768, and January, 1769. But he had never reduced and compared his observations, and not till Herschel announced the planet did Lemonnier know how great a prize had lain for ten years within his grasp.

The period of revolution of Uranus is eighty-four years, so that its position in the sky changes but slowly from year to year. During the first ten years of our century it will be in or near the region of the Milky Way, which we see in summer and autumn, low down in the southern sky. This will make it difficult of detection by the naked eye.

The distance of Uranus is about twice that of Saturn. In astronomical units it is 19.2; in our familiar measures 1,790,000,000 miles, or 2,870,000,000 kilometres.

Owing to this great distance, it is hard to see with certainty any features on its surface. In a good telescope it appears as a pale disk with a greenish hue. Some observers have fancied that they saw faintly marked features on its surface, but this is probably an illusion. We may regard it as certain that it rotates on its axis; but ​no ocular evidence of this has ever been obtained, and of course the period is unknown. But the measures of Barnard showed a slight ellipticity of the disk which, if real, would prove a rapid rotation.

The spectroscope shows that the constitution of Uranus is materially different from that of any of the six planets which revolve between it and the sun. None of the latter gives a spectrum which is strikingly different from that of ordinary sunlight. But when the light of Uranus is spread out into a spectrum, a number of more or less shaded bands are seen, totally unlike the lines of an ordinary spectrum. Whether these bands are really what they appear, or whether they are composed of a multitude of fine dark lines invisible singly, owing to the faintness of the light, has not yet been ascertained; but the probabilities are that such is the case. Whether it is or not, the spectrum indicates that the light reflected from the planet has passed through a dense medium of a constitution quite different from that of our atmosphere. But it is as yet impossible to determine the nature of this medium.

There are four of these bodies moving round Uranus as he travels in his orbit. The two outer ones can be seen in a telescope of twelve inches aperture or more; the inner ones only in the most powerful telescopes of the world. The difficulty of seeing them does not arise from their small size, for they are probably nearly or quite as large as the others, but from their being blotted out by the glare of the planet. ​The history of these bodies is somewhat peculiar. Besides the two brighter ones, Herschel, before 1800, thought he caught glimpses from time to time of four others, and thus it happened that for more than half a century Uranus was credited with six satellites. This was because during all that time no telescope was made which could claim superiority over Herschel's.

Then about 1845, Lassell, of England, undertook the making of reflecting telescopes, and produced his two great instruments, one of two, the other of four feet aperture. The latter he afterwards took to the Island of Malta, in order to make observations under the fine sky of the Mediterranean. Here he and his assistant entered upon a careful examination of Uranus, and reached the conclusion that none of the additional satellites supposed by Herschel had any existence. But, on the other hand, two new ones were found so near the planet that they could not have been seen by any previous observer. During the next twenty years these newly found bodies were looked for in vain with the best telescopes then in use in Europe, and some astronomers professed to doubt their existence. But in the winter of 1878 they were found with the twenty-six-inch Washington telescope, which had just been completed, and were shown to move in exact accordance with the observations of Lassell.

The most remarkable feature of these bodies is that their orbits are nearly perpendicular to the orbit of the planet. The result is that there are two opposite points of the latter orbit where that of the satellite is seen edge​wise. When Uranus is near either of these points, we, from the earth, see the satellites moving as if swinging up and down in a north and south direction on each side of the planet, like the bob of a pendulum. Then, as the planet moves on, the apparent orbits slowly open out. At the end of twenty years we see them perpendicularly. They then seem to us almost circular, but appear to close up again year after year as the planet moves on its course. The orbits were last seen edgewise in 1882, and will be again so seen about 1924. For several years to come the orbits are seen from a nearly perpendicular standpoint, which is the most favourable condition for observing the satellites.

It is quite possible that continued observations of these bodies will yet enable the astronomer to reach some conclusion to the hitherto unsolved problem of the rotation of Uranus on its axis. In the cases of Mars, Jupiter, and Saturn, the satellites revolve very nearly in the plane of the equators of the several planets to which they belong. If this is true of Uranus, it would follow that the equator of the planet was nearly perpendicular to its orbit, and that its north pole, at two opposite points in its orbit, would point almost exactly to the sun. Such being the case, the seasons would be vastly more marked than they are on our earth. Only on or near the equator of Uranus would a denizen of the planet see the sun every day. If he lived in middle latitudes there would be a period equal in length to five or ten of our years during which the sun would never reach his horizon. Then, moving rapidly upwards, it would rise and set, giving him day and night, ​but in time it would get so far up toward the north pole that it would never set during a period equal to that at which it never rose.

The fact that all the satellites revolve in almost exactly the same plane gives some colour to this view, but does not quite prove it, because it is not impossible that their planes are kept together by their mutual action. If, however, this is the case, and if the equator of Uranus does not coincide with the orbits, the latter will, in the course of centuries, undergo a change which our successors will be able to determine. In this way they will be enabled to learn something of the equator and poles of Uranus, even if their telescopes are not powerful enough to afford any visual evidence on the subject.