On June 5th and 6th, you will be able to witness a true once-in-a-lifetime event. Venus will pass across the face of the Sun - for about six hours, it will appear as a small black dot on the Sun's surface. It won't happen again until 2117.
Transits of Venus occur only when Venus and the Earth are in a line with the Sun. At other times Venus passes below or above the Sun because the two orbits are at a slight angle to each other. Transits occur in pairs separated by eight years (the last transit was in 2004) with the gap between pairs of transits alternating between 105.5 and 121.5 years.
Johannes Kepler successfully predicted that both planets would transit the Sun in 1631, part of which was verified with Mercury's transit of that year. But the first transit of Venus to actually be viewed was in 1639 – an event that had been predicted by the English astronomer Jeremiah Horrocks. He observed the transit in the village of Much Hoole in Lancashire – the only other person to see it being his correspondent, William Crabtree, in Manchester.
Later, in 1716, Edmond Halley proposed using a transit of Venus to predict the precise distance between the Earth and the Sun, known as the astronomical unit. As a result, hundreds of expeditions were sent all over the world to observe the 1761 and 1769 transits. A young James Cook took the Endeavour to the island of Tahiti, where he successfully observed the transit at a site that is still called Point Venus. And he and his crew had a ridiculous amount of sex with native girls, if that helps pique your interest in science.
Timed with the event, Andrea Wulf has written a book called "Chasing Venus: The Race to Measure the Heavens", which recounts Halley's effort to get 18th century astronomers to measure the exact time and duration of the event, because the Transit of Venus was the key to unlocking the distance between Earth and the sun - and by extension, the size of our solar system.
If you decide to try and look for yourself, visit http://www.transitofvenus.org and http://www.transitofvenus.nl for tips on how to protect. Or just visit the NASA website, they will be webcasting it live.
