What is Uranus like?
Uranus isn’t much to look at. When Voyager 2 first flew past the planet in 1986, it found a uniform ball that is tinted sky blue by methane in the atmosphere.
But looking beyond the calm exterior reveals a planet where winds blow at speeds in excess of 900km/h, in the wrong direction.
While most planets’ spin axes are at right angles to their orbits, Uranus has been knocked over onto its side.
No one is entirely certain why, but the lead theory is that an Earth-sized planetoid collided with Uranus early in the Solar System’s history, tipping it over.
However, Uranus isn’t always as calm as when Voyager 2 spied on it.
As the planet takes 85 years to orbit the Sun, the seasons last for decades and the probe passed by during the planet’s northern summer.
When Uranus passed into autumn in 2007, long-range observations found the planet seemed to wake up, with storms creating bright spots the size of North America in the planet’s atmosphere.
The planet is encircled by 13 thin rings.
Uranus is surrounded by a thin ring, seen here in infrared images from the Keck telescope. Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory
Diameter: 50,724km (3.98 times Earth)
Mass: 86.8 trillion trillion kg (14.5 times Earth)
Distance from the Sun: 2872 million km (19.2 AU)
Length of day: 17.2 hours
Length of year: 83.8 years
Number of moons: 27
Average temperature: -195ºC
No of spacecraft visitors: 1
Type of planet: Ice giant
How can I observe Uranus?
With a dark sky and good eyesight, it’s just be possible to make out Uranus with the naked eye when it is at opposition (closest approach to Earth).
For the best views, however, you should use a telescope.
Uranus should begin to become clear with an aperture of at least 3 to 4 inches and a magnification of 100x to 150x.
At these high magnifications, your observations may be affected by a phenomenon known as seeing, where the turbulence in Earth’s atmosphere causes the image to shimmer.
Experiment with different eyepieces, Barlow lenses and focal extenders to find the best magnification on any given night.
It is possible to take a photograph of the planet’s disc, but its distance from Earth makes doing so something of a challenge.
How was Uranus discovered?
While Uranus is just about visible with the naked eye, it is so dim and moves so slowly that ancient astronomers overlooked it.
After the invention of the telescope, several astronomers did spot the planet, but it was only when William Herschel observed it in 1781 that its true origin became apparent.
He first reported his sightings and later recognised it as a planet.
From where did Uranus gets its name?
The planet was initially named George, in honour of the English king at the time who’d given Herschel a grant in recognition of hid discovery. The name was understandably unpopular outside of Britain.
In 1782, German astronomer Johann Bode suggested Uranus after the roman god of the sky and father of the Titans, which would blend in better with the other planets.
The moons, meanwhile, were named by William Herschel’s son, John, who decided to name them after characters from English literature (most from Shakespeare).
The trend has continued as more moons are discovered.
Recent images of Uranus taken by Hubble show the ice giant now has light coloured bands, and a dark spot which could be a storm. NASA/Space Telescope Science Institute
How have we explored Uranus?
So far only one mission has visited Uranus, Voyager 2, which flew past the planet in 1986. The mission showed Uranus was an almost featureless green-blue ball.
In the years since, telescopes on Earth and in orbit have improved to the point where they can now take detailed images of the planet, allowing us to make out the changing weather on its surface.
It seems that the planet’s upper atmosphere has become much more varied since then, suggesting that we happened to fly past the planet during a particularly calm point in Uranus’s climate cycle.
Explored by: Voyager 2 (1986, NASA)