On June 8, 2004 (my 24th birthday!), Venus will pass between the Earth and the Sun for the first time in over a century. Unfortunately, for you sorry suckers on the West Coast, the transit will occur during the night so you will not be able to see it at all. Luckily, the Exploratorium will be providing live web-coverage of the entire 6-hour event from Italy, so everyone can enjoy this rare occurrence.

This image shows the silhouette of venus against
the sun during its last transit in 1886.

Venus transits occur in pairs, eight-years apart, so the next one will be on June 6, 2012 (ahh... maybe you west-coasters will luck out in 8 years) and then there is not another one for 100 years. In the 1700's the Venus transit provided an opportunity for scientists to calculate the astronomical unit (which is very close to today's value) and provided evidence of an atmosphere on Venus. Today, planetary transits can be used to identify extrasolar planets (planets outside our solar system). Scientists analyze the change in the light emitted from stars and can detect the presence of a planet. Analysis of these extrasolar planetary transits provides information on planet's size, temperature, and atmospheric composition.

The Exploratorium has dedicated an entire website to the up-coming transit of Venus with many cool graphics and animations.

Check it out. http://www.exploratorium.edu/venus/index.html