A small black dot passing across the face of the sun next week will be watched with interest, wonder and excitement by experts and amateurs around the world.
The event on Tuesday and Wednesday will be the last opportunity for anyone this century to witness the transit of Venus.
The dot is our closest planetary neighbour, moving between the earth and the sun.
For amateur skywatchers viewing projected images of the phenomenon with their telescopes, it will be something to marvel at and remember. But it will also provide valuable data for professional astronomers and planetary scientists studying worlds orbiting distant stars.
Venus transits occur in repeating pairs. The last was seen in 2004 and the next two will not be until 2117 and 2125. The previous transit to the one in 2004 was on December 6 1882.
This year’s transit will last slightly under seven hours but only the final hour or so will be visible in the UK at dawn on Wednesday June 6.
The transit starts at 11.04pm on Tuesday, after sunset in the UK. The planet will take a curved path across the northern part of the sun, reaching a half-way point at about 2.30am.
Venus will then begin to move away from the sun at about 5.37am on Wednesday. Since the sun will not rise until about 4.50am, only a short period of time is available for transit watching in the UK and a good view requires an unobstructed horizon.
During the transit Venus is visible as a dark disc covering 1/32nd of the sun’s surface and blocking out about one per cent of its light.
People are warned not to look directly at the sun, which risks blindness. The event can be watched on via the US space agency Nasa’s live webcast http://venustransit.nasa.gov/transitofvenus/.