According to good friend of this blog Stuart Clark, writing in The Guardian, the 1761 transit of Venus was a watershed moment in the history of astronomy. It was, he says, the first time astronomers would have the opportunity to measure accurately the size of the solar system. The distance between the Earth and the Sun had been estimated, with varying degrees of success, since the Greeks, but this was different…the endeavour was the 18th century equivalent of the Large Hadron Collider. It was the first global scientific collaboration and took place in the midst and despite a global war at the time.
“The combined results from all the various missions were within about 4% of the modern accepted value of 150 million kilometres). At the next pair of transits, in 1874 and 1882, the accuracy was improved to 1%,” says Clark.
A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth. The “star” (actually a planet, obviously) seems to turn black against the solar disk (we’re simply seeing the side of the planet facing directly away from the sun and it is obviously not illuminated.
A transit lasts mere hours (the transit of 2004 lasted 6h). A transit is similar to a solar eclipse by the Moon. While the diameter of Venus is almost four times that of the Moon, Venus appears smaller, and travels more slowly across the face of the Sun, because it is much farther away from Earth.
The next transit of Venus will occur on 5 and 6 June 2012, and will be the last Venus transit this century; they come in pairs the previous one of the pair being on 8 June 2004. (Before that December 1874 and December 1882). The next pair of transits will not be until December 2117 and December 2125…so we’ve got a while to wait after Tuesday.