Uranus
The planet Uranus is 19 astronomical units from the Sun is 2.87 billion kilometers. At that distance, it takes 84 Earth years to make one revolution around our star. It is the third largest planet in the solar system with a diameter of 51 800 km.
The atmosphere of Uranus
Uranus is mainly composed of hydrogen and helium, with also some methane and trace amounts of other compounds.
When flying over Voyager 2 in 1986, the planet appeared uniformly blue green, with no visible detail. Because of its mass, Uranus has less internal energy release than Jupiter and Saturn and convection in the atmosphere is more limited, resulting in a lack bands and a much more uniform appearance.
Note however that more recent observations by the Space Telescope revealed a more pronounced banded structure, possibly due to climate change during the revolution of the planet around the sun.
The blue-green color of Uranus comes from the methane in the upper atmosphere. The sun's rays are reflected from the highest clouds in the world but must cross the methane layer that lies above. But mostly methane absorbs red fraction of the sunlight and the blue portion is the only one able to escape.
The rotation of the planet is relatively fast, with a slightly higher 17 hours period.
Uranus is distinguished from other bodies in the solar system that its rotation axis is almost in the plane of the orbit. Thus, when flying by Voyager 2 in 1986, the South Pole of the planet facing the sun. In 2007, after a quarter of an orbit, it will be the equator of the planet that will be the most lit area.
These variations could cause slow climate changes observed on the planet. Note again that the unusual position of the axis of rotation is likely the result of a collision with another protoplanet in the distant past.
Satellites and rings of Uranus
Uranus has at least 27 satellites, of which the top five are, in order of increasing distance to the planet Miranda, Ariel, Umbriel, Titania and Oberon. The largest of these satellites is Titania, with a diameter of 1580 kilometers, less than half that of our Moon.
The planet is also surrounded by several thin rings discovered from Earth during the observation of a star occultation in 1977. Their existence was then confirmed when flying by Voyager 2 and other rings yet later discovered, particularly by the Hubble space telescope.
These rings are formed of dark ice balls and little reflective, the largest up to several meters in diameter.
The discovery of Uranus
Uranus was observed repeatedly in the seventeenth and eighteenth centuries, but these early observations mingled with the star. It was not until 1781 that English astronomer William Herschel recognized it for what it was, a planet, the first not to have been identified in antiquity.
Various names were proposed, especially Georgium Sidus in honor of King George III or Herschel to greet its discoverer. But it was the German Johann Elert Bode who finally proposed the winning name, Uranus, inspired by Uranus, the Greek god of the sky.
The two largest satellites, Titania and Oberon, were discovered by William Herschel in 1787. Ariel and Umbriel were discovered by the English in 1851 and William Lassel Miranda by American Gerard Kuiper in 1948. Their names come from characters in parts William Shakespeare.
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