A Transit of Venus is when Venus crosses the face of the Sun as viewed from some vantage point - usually the Earth. This is a special form of an eclipse - an Annular Eclipse.

The 2004 Transit of Venus
Transits of Venus are comparatively rare. There will be one in June 2012, and there was one in June 2004. Here are some of the dates:

1518-06-05 1526-06-02
1631-12-07 1639-12-04 113.5 years later
1761-06-06 1769-06-03 129.5 years later
1874-12-09 1882-12-06 113.5 years later
2004-06-08 2012-06-06 129.5 years later
2117-12-11 2125-12-08 113.5 years later
2247-06-11 2255-06-09 129.5 years later

As you can see, they come in pairs, eight years apart. The 1631 transit was predicted by Johannes Kepler, but he predicted a near miss for 1639. Jeremiah Horrocks corrected the calculations and correctly predicted the transit. He then became the first person in recorded history to witness a transit of Venus.

The transits were once very important, as they provided a method of computing the actual distance to the Sun. Until then, only relative distances within the Solar System were known. Transits were so important that various expeditions were dispatched by the Royal Society to observe them. Captain James Cook was sent to Tahiti to observe the 1769 transit, and that was the voyage on which he discovered New Zealand, the East coast of Australia, and many other things.

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