Uranus

[[File:Uranus Voyager2 color calibrated.png|thumb|Uranus on 1986-01-23, taken by NASA's Voyager 2 probe. This color image was composed of three frames, orange, green, and blue, taken by Voyager 2's imaging system. This color image has been calibrated to best represent Uranus's true color and appearance. Based on: Irwin, Patrick G J (2023-12-23).

Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a supercritical phase of matter, which in astronomy is called 'ice' or volatiles. The planet's atmosphere has a complex layered cloud structure and has the lowest minimum temperature of 49 K (−224 °C; −371 °F) out of all the Solar System's planets. It has a marked axial tilt of 82.23° with a retrograde rotation rate of 17 hours. This means that in an 84-Earth-year orbital period around the Sun, its poles get around 42 years of continuous sunlight, followed by 42 years of continuous darkness.

Uranus has the third-largest diameter and fourth-largest mass among the Solar System's planets. Based on current models, inside its volatile mantle layer is a rocky core, and surrounding it is a thick hydrogen and helium atmosphere. The planet also has very low internal heat compared to other giant planets, the cause of which remains unclear.

Like the other giant planets, Uranus has a ring system, a large number of orbiting natural satellites, and a magnetosphere. Its ring system is extremely dark, with only about 2% of the incoming light reflected, and its satellite system contains 18 known regular moons, of which 13 are small inner moons. Further out are the larger five major moons of the planet: Miranda, Ariel, Umbriel, Titania, and Oberon. Orbiting at much greater distance from Uranus are the nine known irregular moons. The planet's magnetosphere is highly asymmetric and has many charged particles, which may be the cause the darkening of its rings and moons.

Uranus is visible to the naked eye, but it is very dim and was not classified as a planet until 1781, when it was first observed by William Herschel. About seven decades after its discovery, consensus was reached that the planet be named after the Greek god Uranus (Ouranos), one of the Greek primordial deities. As of 2024, it had been visited up close only once when in 1986 the Voyager 2 probe flew by the planet.

Orbit and rotation

Uranus orbits the Sun once every 84 years. As viewed against the background of stars, since being discovered in 1781, the planet has returned to the point of its discovery northeast of the binary star Zeta Tauri twice, in March 1865 and March 1949, and will return to this location again in April 2033.

Axial tilt

The Uranian axis of rotation is approximately parallel to the plane of the Solar System, with an axial tilt of 82.23°.

History

Like the classical planets, Uranus is visible to the naked eye, but it was never recognised as a planet by ancient observers because of its dimness and slow orbit.Sir William Herschel first observed Uranus on 13 March 1781, leading to its discovery as a planet, expanding the known boundaries of the Solar System for the first time in history and making Uranus the first planet classified as such with the aid of a telescope. The discovery of Uranus also effectively doubled the size of the known Solar System because Uranus is around twice the distance from the Sun as the planet Saturn.

Moons of Uranus

Montage of five largest moons of Uranus, in true color, ordered by their distance from Uranus, calibrated by albedo.

Major moons of Uranus in order of increasing distance (left to right), at their proper relative sizes and albedos. From left to right, they are Miranda, Ariel, Umbriel, Titania, and Oberon.

Uranus has 27 known natural satellites. The names of these satellites are chosen from characters in the works of Shakespeare and Alexander Pope. The five main satellites are Miranda, Ariel, Umbriel, Titania, and Oberon. The Uranian satellite system is the least massive among those of the giant planets; the combined mass of the five major satellites would be less than half that of Triton (largest moon of Neptune) alone. The largest of Uranus's satellites, Titania, has a radius of only 788.9 km (490.2 mi), or less than half that of the Moon, but slightly more than Rhea, the second-largest satellite of Saturn, making Titania the eighth-largest moon in the Solar System.

Name

The name Uranus references the ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός), known as Caelus in Roman mythology, the father of Cronus (Saturn), grandfather of Zeus (Jupiter) and the great-grandfather of Ares (Mars), which was rendered as Uranus in Latin (IPA: [ˈuːranʊs]). It is the only one of the eight planets whose English name derives from a figure of Greek mythology.

Uranus is called by a variety of names in other languages. Uranus's name is literally translated as the "sky king star" in Chinese (天王星), Japanese (天王星), Korean (천왕성), and Vietnamese (sao Thiên Vương). In Thai, its official name is Dao Yurenat (ดาวยูเรนัส), as in English. Its other name in Thai is Dao Maruettayu (ดาวมฤตยู, Star of Mṛtyu), after the Sanskrit word for 'death', Mrtyu (मृत्यु). In Mongolian, its name is Tengeriin Van (Тэнгэрийн ван), translated as 'King of the Sky', reflecting its namesake god's role as the ruler of the heavens. In Hawaiian, its name is Heleʻekala, the Hawaiian rendering of the name 'Herschel'. In Māori, its name is Whērangi.

In culture

• In astrology, the planet Uranus (symbol Uranus's astrological symbol) is the ruling planet of Aquarius. Because Uranus is cyan and Uranus is associated with electricity, the colour electric blue, which is close to cyan, is associated with the sign Aquarius.
• The chemical element uranium, discovered in 1789 by the German chemist Martin Heinrich Klaproth, was named after the then-newly discovered Uranus.
• Lydia Sigourney included her poem The Georgian Planet in her 1827 collection of poetry.
• "Uranus, the Magician" is a movement in Gustav Holst's orchestral suite The Planets, written between 1914 and 1916.
• Operation Uranus was the successful military operation in World War II by the Red Army to take back Stalingrad and marked the turning point in the land war against the Wehrmacht.
• The lines "Then felt I like some watcher of the skies/When a new planet swims into his ken", from John Keats's "On First Looking into Chapman's Homer", are a reference to Herschel's discovery of Uranus.