1911 Encyclopædia Britannica/Uranus (planet)

by Simon Newcomb

URANUS, in astronomy, the seventh major planet in the order of distance from the sun, and denoted by the symbol ⛢ or ♅. It was discovered by the elder Herschel on the 13th of March 1781. He saw it as a round nebulous disk, slowly moving among the stars, and at first supposed it to be a comet, and announced it as such to the Royal Society. But a few weeks' observation showed it to be moving in a nearly circular orbit at a distance from the sun about nineteen times that of the earth. Its planetary character was thus established, and Herschel named it the Georgium Sidus in honour of his royal patron. This name was long recognized in England, and “ the Georgian ” was officially used in the Nautical Almanac up to 1850. But it was never received with favour on the continent of Europe, nor was that of the discoverer, which was proposed by Lalande. The name Uranus was proposed by Bode, and adopted everywhere outside of England.

As seen in a telescope of the highest power, Uranus presents to the eye the appearance of a disk about four seconds in diameter of a faint sea-green tint. No trace of a marking can be seen on the surface, and, so far as measures have yet been made on it, no deviation of the disk from a circular form has been established. Nothing is therefore known as to its axial rotation. Although the planet is commonly considered a telescopic one, it is really of the sixth magnitude, and therefore faintly visible to the naked eye if one knows precisely where to look for it. Long before its discovery it had been observed as a fixed star by J. Flamsteed. P. C. Lemonnier also made eight observations of it during the opposition of 1768–69, which would have revealed its planetary character had he reduced and compared them. For other particulars relating to Uranus, its spectrum, &c., see Planet.

Satellites of Uranus.—In January 1787 Herschel detected two satellites of Uranus of which the inner one, now known as Titania, had a period of 9 days, the outer, Oberon, of 131/2 days. He also on other occasions saw what he supposed to be two additional satellites, but careful investigation of his observations has shown that the supposed objects could not have been of this character. But in 1851–52 William Lassell at Malta, in conjunction with his assistant A. Marth, observed two satellites yet nearer the planet than those of Herschel. These are now known as Ariel and Umbriel. Their periodic times are about 21/2 and 4 days respectively. Lassell’s telescopes, which were reflectors, were superior to others of his time in light-power, and these inner satellites were not seen by other astronomers for more than twenty years after their discovery. Indeed, doubts of their reality sometimes found expression until, in 1873, they were observed with the Washington 26-inch telescope, and observations upon them showed their identity with the objects discovered by Lassell. The greater difficulty in seeing the inner than the outer satellites arises from their proximity to the planet. There is no very great difference in the actual brightness of the four objects. It is found that Umbriel, though less easy to see than Titania, actually exceeds it in light. But none of them has been seen except in a few of the most powerful telescopes. The most remarkable feature of these bodies is that, instead of the planes of their orbits being near that of the ecliptic, they are actually inclined to it nearly 90° The result is that, as the planet performs its orbital revolution, there are two opposite points near which the orbits are seen edgewise, and the satellites seem to us to swing north and south on each side of the planet. This was the case in 1882, and will be the case again in 1924. At the points midway between these two, through which the planet passed in 1861 and 1903, and will pass again in 1945, the orbits are seen almost perpendicularly, so that the apparent orbit, like the real one, is nearly circular.

Orbits of the Satellites of Uranus.—So far as has yet been determined, the four satellites all revolve in the same plane, the position of which, referred to the Earth’s equator and equinox, is—

          R.A. of ascending node, 166°·05+0°·01420t.
          Inclination of orbit, 75°·28–0°·0013t.

None of the orbits seems to have a measurable eccentricity. The positions of the satellites in the orbits at any time may be found from the following elements, where u is the angular distance from the node upon a plane parallel to that of the Earth’s equator, and the motion is that in a Julian year.

Satellite.	u at     Epoch.	Annual Motion.	Daily     Motion.	Mean     Dist.
Ariel	22°·61	579 rev.+242°·64	142°·836	13″·78
Umbriel	136°·49	352 rev.+195°·31	86°·869	19″·20
Titania	229°·93	167 rev.+294°·20	41°·351	31″·48
Oberon	154°·90	108 rev.+186°·27	26°·739	42″·10

The epoch force is 1872, January 0, Washington mean noon. The mean distance is the angle subtended by the radius of the orbit as seen at the mean distance of Uranus from the Sun (log a=1·28310).

Bibliography.—Details as to Uranus are found in Chambers’s Descriptive Astronomy, and all the current treatises on popular astronomy. For researches on the spectrum of the planet see Sir William Huggins in Proceedings of the Royal Society, vol. xix., (1871); H. C. Vogel, Astrophysical Journal, vol. i.; and P. Lowell, Bulletin of the Lowell Observatory, No. 13. Tables of the motions of this planet were published by Alexis Bouvard in 1813, S. Newcomb in 1873 (Smithsonian Contributions to Knowledge, No. 262), Leverrier in 1877 (Annales de l’observatoire de Paris, Mémoires, tome xiv.), and Newcomb again in Astronomical Papers of the American Ephemeris, vol. vii. Tables of the four satellites are found in Newcomb, Uranian and Neptunian Systems (Appendix I. to Washington Observations for 1873). Observations are found in the Bulletins of the Lick Observatory and elsewhere.  (S. N.)