Uranus, as seen by Voyager 2
|Discovered by||William Herschel|
|Discovery date||March 13, 1781|
20.083 305 26 AU
18.375 518 63 AU
|Semi-major axis||2,876,679,082 km
19.229 411 95 AU
|Eccentricity||0.044 405 586|
|Orbital period||30,799.095 days
84.323 326 yr
42,718 Uranus solar days
|Synodic period||369.66 days|
|Average orbital speed||6.81 km/s|
|Mean anomaly||142.955 717°|
|Inclination||0.772 556° to Ecliptic
6.48° to Sun's equator
1.02° to Invariable plane
|Longitude of ascending node||73.989 821°|
|Argument of perihelion||96.541 318°|
|Equatorial radius||25,559 ± 4 km
|Polar radius||24,973 ± 20 km
|Flattening||0.022 9 ± 0.000 8[b]|
|Surface area||8.115 6×10 km²[c]
|Mass||(8.6810 ± 0.0013)×10 kg
GM=5 793 939 ± 13 km³/s²
|Mean density||1.27 g/cm³[c]|
|Equatorial surface gravity||8.69 m/s²[c]
|Escape velocity||21.3 km/s[c]|
|–0.718 33 day
17 h 14 min 24 s
|Equatorial rotation velocity||2.59 km/s
|North pole right ascension||17 h 9 min 15 s
|North pole declination||–15.175°|
1 bar level
|Apparent magnitude||5.9 to 5.32|
|Scale height||27.7 km|
|Composition||(Below 1.3 bar)
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός) the father of Cronus (Saturn) and grandfather of Zeus (Jupiter). Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers because of its dimness and slow orbit. Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the Solar System for the first time in modern history. Uranus was also the first planet discovered with a telescope.
Uranus is similar in composition to Neptune, and both are of different chemical composition than the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Uranus's atmosphere, while similar to Jupiter and Saturn's in its primary composition of hydrogen and helium, contains more "ices" such as water, ammonia and methane, along with traces of hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (–224 °C). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds. In contrast the interior of Uranus is mainly composed of ices and rock.
Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. As such, its north and south poles lie where most other planets have their equators. Seen from Earth, Uranus's rings can sometimes appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock, though in 2007 and 2008 the rings appeared edge-on. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants. However, terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph).
Uranus had been observed on many occasions before its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed observed the planet at least six times, cataloging it as 34 Tauri. The French astronomer Pierre Lemonnier observed Uranus at least twelve times between 1750 and 1769, including on four consecutive nights.
Sir William Herschel observed the planet on 13 March 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset (now the Herschel Museum of Astronomy), but initially reported it (on 26 April 1781) as a "comet". Herschel "engaged in a series of observations on the parallax of the fixed stars", using a telescope of his own design.
He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet". On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place". When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:
|“||The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.||”|
Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it".
While Herschel continued to cautiously describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell was the first to compute the orbit of the new object and its nearly circular orbit led him to a conclusion that it was a planet rather than a comet. Berlin astronomer Johann Elert Bode described Herschel's discovery as "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn". Bode concluded that its near-circular orbit was more like a planet than a comet.
The object was soon universally accepted as a new planet. By 1783, Herschel himself acknowledged this fact to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System." In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on the condition that he move to Windsor so that the Royal Family could have a chance to look through his telescopes.
Maskelyne asked Herschel to "do the astronomical world the faver [sic] to give a name to your planet, which is entirely your own, [and] which we are so much obliged to you for the discovery of." In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus (George's Star), or the "Georgian Planet" in honour of his new patron, King George III. He explained this decision in a letter to Joseph Banks:
|“||In the fabulous ages of ancient times the appellations of Mercury, Venus, Mars, Jupiter and Saturn were given to the Planets, as being the names of their principal heroes and divinities. In the present more philosophical era it would hardly be allowable to have recourse to the same method and call it Juno, Pallas, Apollo or Minerva, for a name to our new heavenly body. The first consideration of any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, when this last-found Planet was discovered? It would be a very satisfactory answer to say, 'In the reign of King George the Third.||”|
Herschel's proposed name was not popular outside of Britain, and alternatives were soon proposed. Astronomer Jérôme Lalande proposed the planet be named Herschel in honour of its discoverer. Swedish astronomer Erik Prosperin proposed the name Neptune which was supported by other astronomers who liked the idea to commemorate the victories of the British Royal Naval fleet in the course of the American Revolutionary War by calling the new planet even Neptune George III or Neptune Great Britain. Bode, however, opted for Uranus, the Latinized version of the Greek god of the sky, Ouranos. Bode argued that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn. In 1789, Bode's Royal Academy colleague Martin Klaproth named his newly discovered element "uranium" in support of Bode's choice. Ultimately, Bode's suggestion became the most widely used, and became universal in 1850 when HM Nautical Almanac Office, the final holdout, switched from using Georgium Sidus to Uranus.
The pronunciation of the name Uranus preferred among astronomers is , with stress on the first syllable as in Latin Ūranus; in contrast to the colloquial , with stress on the second syllable and a long a, though both are considered acceptable. Because, in the English-speaking world, ū·rā′·nəs sounds like "your anus", the former pronunciation also saves embarrassment: as Dr. Pamela Gay, an astronomer at Southern Illinois University, noted on her podcast, to avoid "being made fun of by any small schoolchildren ... when in doubt, don't emphasise anything and just say ūr′·ə·nəs. And then run, quickly."
Uranus is the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology: the Greek "Οὐρανός" arrived in English by way of the Latin "Ūranus". The adjective of Uranus is "Uranian". Its astronomical symbol is . It is a hybrid of the symbols for Mars and the Sun because Uranus was the Sky in Greek mythology, which was thought to be dominated by the combined powers of the Sun and Mars. Its astrological symbol is , suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your name"). In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star (天王星).