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Uranus. Image credit: Hubble

Uranus. Image credit: Hubble

Uranus is the 7th planet from the Sun, and the 3rd largest planet in the Solar System. Uranus cannot be seen with the naked eye and therefore was not found until after telescopes were created. There is some debate regarding who discovered the planet because more than one astronomer worked on calculating its position and claimed credit. Modern astronomers give most of the credit to the astronomer Sir William Herschel; the planet had also been discovered by John Flamsteed in 1690, but he mistakenly believed the planet was a star. William Herschel also discovered two of the planet’s moons, Titania and Oberon. He thought he also saw other moons around Uranus.

Orbiting the Sun at an average distance of 2.88 billion km, Uranus takes 84.3 years to complete one full revolution around the Sun. It takes just 17 hours and 14 minutes to spin once on its axis. Uranus has an equatorial radius of 25,559 km, which is about 4 times bigger than Earth.

One unique feature of Uranus is that it rotates on its side. All of the planets are tilted on their axes to some degree, but Uranus has the most extreme axial tilt of 98°. This leads to the radical seasons that the planet experiences. Additionally, it leads to unusual days at the poles. At the equator, Uranus experiences normal days and nights. However, because it rotates on its side at any given time one pole is pointed towards the Sun. This results in one pole experiencing 42 Earth years of day followed by 42 years of night. When the North Pole is in the Sun, the South Pole is in darkness and vice-versa.

Uranus is one of the 4 gas giants in the Solar System (with Jupiter, Saturn and Neptune), but it’s really an ice giant. The planet contains roughly 14.5 times the mass of the Earth, but it has a low density, just a little more than water at 1.27 g/cm3. This means that the planet is probably made up of water, ammonia, and methane ices. It’s believed to have a rocky core surrounded by an icy mantle and then an outer gaseous atmosphere of hydrogen and helium.

Uranus has 27 moons; although that is many, it is less than half the number that Jupiter has. Although many moons are also named after mythological figures, the moons of Uranus are all named after characters from Shakespeare and Alexander Pope’s literary works, such as Ariel, Miranda, and Oberon. At first, the moons were only named after fairies and sprites from literary works; it was similar to minor gods or goddesses attending the major gods the planets were named after. However, in 1949, Gerard Kuiper named the fifth moon of Uranus, which he had discovered, after a human character – Miranda – in a Shakespeare play. Some of Uranus’s moons were discovered as recently as during the past decade.

Uranus has only been visited once by a spacecraft: NASA’s Voyager 2 flew past the planet in 1986. On January 24, 1986, Voyager 2 passed within 81,500 km of the surface of the planet, sending back the only close up pictures ever taken of Uranus. Voyager 2 then continued on to make a close encounter with Neptune in 1989.



Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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