Uranus was the first planet discovered using a telescope. William Herschel (1738-1822) first observed it on March 13, 1781. Herschel was not looking for a planet but was compiling a catalog of double stars when he noticed a "non-stellar object" moving slowly among the stars.
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Initially, Herschel named his discovery "Georgium Sidus" (George's Star) in honor of King George III of England. The name "Uranus," proposed by astronomer Johann Bode (1747-1826) to follow Greco-Roman mythology like the other planets, only became widely accepted decades later.
Since antiquity, the number of planets (seven, including the Sun and Moon in geocentrism) was considered a perfect and immutable number. The discovery of Uranus shattered this dogma, proving that the solar system was not fixed and that new worlds could be discovered. It paved the way for the discovery of the asteroid belt, Neptune, and Pluto.
Uranus' orbit did not perfectly match the predictions based on Newton's laws, given the known gravitational perturbations. This discrepancy between observation and theory led astronomers to hypothesize that an eighth planet, still invisible, must be perturbing its motion. This intellectual process led to the theoretical calculation and then the observational discovery of Neptune, brilliantly validating Newtonian mechanics.
The only spacecraft to have visited Uranus is Voyager 2 in 1986. This mission revealed for the first time the complex structure of the rings, discovered new moons, and measured the planet's off-center magnetic field. No other mission is currently en route to Uranus.
Uranus' equatorial diameter is 51,118 km, about 4 times larger than Earth's (12,742 km). Uranus belongs to the category of ice giants, like Neptune. Its composition is mainly water, ammonia, and methane ices, surrounding a rocky core. CH4 strongly absorbs red light, giving Uranus its characteristic turquoise hue. The planet has a system of thin rings and 27 known moons.
With an average temperature of about \(-224\,°C\), Uranus is the coldest planet in the Solar System, even though Neptune is farther from the Sun. This paradox remains unexplained. Some models suggest that its tilted axis may have caused a rapid release of internal heat after the initial impact, or that its inner layers block heat flow by convection.
| Parameter | Value | Unit | Comment |
|---|---|---|---|
| Average distance from the Sun | 2.871 × 109 | km | About 19.2 AU |
| Duration of one revolution | 84.01 | years | Complete Uranian year |
| Duration of one rotation | 17.24 | hours | Retrograde rotation |
| Axial tilt | 97.77 | degrees | Rotation almost perpendicular to the ecliptic |
| Average temperature | -224 | °C | The coldest in the Solar System |
Source: NASA Planetary Data System, Astronoo.com.
Uranus, the seventh planet of the Solar System, is distinguished by a rotational axis tilted at 98°. This means it appears to roll on its orbit, lying on the plane of the ecliptic. Its retrograde rotation, in the opposite direction to Earth's, remains a major mystery for astrophysicists. This unique configuration profoundly alters the dynamics of its seasons, each lasting about 21 Earth years.
Current models suggest that a colossal impact with an Earth-sized body during the early days of the solar system could have tilted its axis. This shock may also have influenced the distribution of its internal mass and the relative absence of emitted heat, unlike Neptune, its twin in composition.
Due to its extreme tilt, Uranus experiences particularly long and intense seasons. Each pole experiences about 42 Earth years of continuous sunlight followed by 42 years of total darkness. This unique configuration deeply influences the planet's atmospheric circulation and climate.
Uranus' winds reach impressive speeds of 200 to 900 km/h, circulating in an apparently calm atmosphere. The meridional circulation shows an alternation of bands and retrograde currents, revealed by infrared observations.
Uranus' rings, discovered in 1977, are dark and narrow. They are composed of ice particles mixed with dark organic compounds. Uranus also has at least 27 known moons, named after characters from the works of William Shakespeare and Alexander Pope.
Its magnetic field is strongly offset: tilted by about 59° relative to its rotational axis and offset by a third of the planetary radius. This configuration, observed by the Voyager 2 probe, creates an asymmetric magnetosphere, generating complex polar auroras.
Uranus has a complex system of 27 confirmed natural satellites, each named after a character from the works of William Shakespeare (1564-1616) or the poet Alexander Pope (1688-1744). This literary tradition gives the Uranian system a unique cultural identity in the solar system.
| Name | Character and Work | Author | Diameter (km) | Mass (×1019 kg) | Discovery | Discoverer | Features |
|---|---|---|---|---|---|---|---|
| Titania | Queen of the Fairies - A Midsummer Night's Dream | William Shakespeare | 1,577 | 35.2 | 1787 | William Herschel | Largest moon, 1,500 km canyons |
| Oberon | King of the Fairies - A Midsummer Night's Dream | William Shakespeare | 1,523 | 30.1 | 1787 | William Herschel | Ancient cratered surface |
| Umbriel | Dark Spirit - The Rape of the Lock | Alexander Pope | 1,169 | 11.7 | 1851 | William Lassell | Darkest surface |
| Ariel | Air Spirit - The Tempest | William Shakespeare | 1,158 | 13.5 | 1851 | William Lassell | Young surface with valleys |
| Miranda | Prospero's Daughter - The Tempest | William Shakespeare | 472 | 0.66 | 1948 | Gerard Kuiper | 20 km cliffs (Verona Rupes) |
| Sycorax | Caliban's Mother - The Tempest | William Shakespeare | 165 | 0.034 | 1997 | Brett Gladman | Largest irregular moon |
| Puck | Mischievous Spirit - A Midsummer Night's Dream | William Shakespeare | 162 | 0.029 | 1985 | Voyager 2 | Largest of the inner moons |
| Portia | Rich and Intelligent Heroine - The Merchant of Venice | William Shakespeare | 135 | 0.017 | 1986 | Voyager 2 | Second largest inner moon |
| Caliban | Wild Son of Sycorax - The Tempest | William Shakespeare | 72 | 0.003 | 1997 | Brett Gladman | Irregular moon, retrograde orbit |
| Prospero | Duke of Milan - The Tempest | William Shakespeare | 50 | 0.00085 | 1999 | Matthew Holman | Retrograde orbit |
Source: NASA JPL Solar System Dynamics and NASA Solar System Exploration.
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