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Odd Uranus is no joke

Tomorrow is Albert Einstein's birthday, but today marks the anniversary of the discovery of the first new planet since antiquity by William Herschel, in 1781 - Uranus.

Stop smirking, I've heard all the jokes about its name. Astronomers can only blame themselves however; in first converting it to Latin and then anglicising, the original pronunciation was lost. It's the only planet to be named after a Greek mythological deity: Ouranos, god of the sky, pronounced ooh-rah-nos. The others are all taken from Roman gods. The Americans tried to take the giggle out of it when Voyager 2 got there in the 1980s by changing the emphasis to the first syllable, but, to my mind, all they succeeded in doing was going from one bodily function to another.

Uranus is a mighty planet, third-largest in the solar system, and, at 51,120 kilometres, more than four times the diameter of the Earth. Unlike our world, though, it is a gas giant like Jupiter and Saturn. But despite the presence of hydrogen and helium, Uranus and Neptune also contain much larger quantities of frozen water, methane and ammonia and as such are now in a sub-group termed ''ice giants''.

Being more than 19 times Earth's distance from the sun, things are very cold at Uranus, in fact minus 224 degrees, and it takes 84 years to complete an orbit. Winds on the planet reach speeds of 900km/h, and its current tally of natural satellites is 27.

The most astounding fact about Uranus is that it does not spin vertically on its axis as the other planets do.

Perhaps owing to some enormous collision in the distant past, its north-south axis is perpendicular to the plane of the solar system. This makes it appear to roll along its orbit like a barrel.

Consequently, each pole experiences 42 years of daylight, followed by 42 in darkness. Even so, its equator continues to be warmer than the pole facing the sun, though the mechanism for this is not yet understood.