Uranus spins on its side, so each pole gets 42 years of daylight
Uranus is tipped over by 82 degrees, so it rolls around the Sun rather than spinning upright. Over its 84-year orbit, each pole spends about 42 years in unbroken sunlight followed by 42 years of darkness. Its atmosphere is also the coldest of any planet in the Solar System, dipping to 49 kelvin, about minus 224 degrees Celsius.
Although faintly visible without a telescope, it moves so slowly and shines so dimly that ancient observers never spotted it as a planet. Astronomers logged it repeatedly as a star: Hipparchus may have recorded it in 128 BC, and John Flamsteed catalogued it in 1690 as 34 Tauri. Pierre Charles Le Monnier saw it at least twelve times between 1750 and 1769 without realising what it was.
On 13 March 1781, William Herschel spotted it from his garden in Bath with a homemade 6.2-inch reflector. He reported a comet, noting that it grew larger at higher magnification while stars stayed points. The Astronomer Royal, Nevil Maskelyne, admitted he did not know what to call it, since it had no tail or coma. It proved to be the first planet discovered with a telescope, and because it orbits about twice as far out as Saturn, it roughly doubled the known size of the Solar System. Some seventy years passed before astronomers settled on naming it after Ouranos, the Greek primordial sky god.
The cyan ice giant is mostly water, ammonia and methane in a hot, dense fluid state astronomers call ices, wrapped around a rocky core under a thick hydrogen and helium atmosphere. It has the third-largest diameter among the planets and a retrograde day of 17 hours 14 minutes. Winds reach 900 km/h, and the planet gives off puzzlingly little internal heat compared with the other giants.
Its rings are extremely dark, reflecting only around 2 percent of the light that falls on them, perhaps darkened by charged particles in its lopsided magnetosphere. Of its 29 known moons, five are major, among them Miranda, Titania and Oberon, while ten irregular moons orbit far out. Only Voyager 2 has visited, in 1986, and a proposed orbiter and probe was named a top priority for 2023 to 2032.
Source: Uranus
