The planet Venus has completed a very rare passage across the face of the sun - an event not witnessed since 1882.  | | Venus on its transit across the sun |
The so-called transit of Venus began at about 5:20 am BST with the planet's tiny black disc edging over the bright limb of our star. The phenomenon continued for about six hours, ending around 12:24 pm BST. The transits have had huge significance in the past, as they were used by scientists to work out the Sun-Earth distance - and hence to get a proper scale for the Solar System. Venus has been referred to as Earth's hellish counterpart - it's certainly an extraordinary place. Its thick, yellowish clouds contain sulphuric acid. The atmospheric pressure at the surface is equivalent to that at a depth of 900m in the Earth's oceans. Venus rotates so slowly on its axis, its year (225 Earth days) is actually longer than its day. They are so rare that it is a very significant event - the next one will be in 2012, but after that you'll have to wait until 2117. | 
