Venus transits occur approximately four times in 243 years, more precisely in pairs of events separated by about eight years and these pairs are separated by about 105 or 121 years.
The reason for these long intervals lies in the fact that the orbits of Venus and the Earth do not lie in the same plane and a transit can only occur if both planets and the Sun are situated exactly on one line.
This has happened only six times in the telescopic age: in 1631, 1639, 1761, 1769, 1874 and 1882. The next transit will now be in 2012, and the one after that will be in 2117.
Very slow turn
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.
Today their value is more in their uniqueness. "It's an extremely rare astronomical event," said Gordon Bromage, a professor of astronomy at UCLAN.
Observers watched the event at the Royal Observatory, Greenwich
The planet itself is soon to come under close scrutiny from Europe's Venus Express probe. The spacecraft, due to be launched next year, will study the planet's thick atmosphere and look for any volcanic activity that might still persist today.
It is an extraordinary world. 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.
And because Venus rotates so slowly on its axis, its year (225 Earth days) is actually shorter than its day (243 Earth days).
"In a lot of ways, Venus can be considered as Earth's hellish twin because it has evolved in a quite different way to Earth," said Dr Andrew Coates, a mission scientist on Venus Express.
"It has a runaway greenhouse effect which has caused the temperature on the surface to be one of the hottest in the Solar System at 460C."