The Transit of Venus is the apparent movement – hence, “transit” – of the planet Venus across the face of the sun. It’s the rarest of eclipses: a pair of transits, spaced eight years apart, occurs about every 120 years. The last pair of transits occurred in 1874 and 1882.
The first transit of the current pair was in 2004, and the second will be visible locally on Tuesday afternoon, June 5, beginning around shortly after 3:00 pm and ending after 9:00 pm (although it won’t be visible once the sun goes down). Unless something happens to dramatically extend our expected life spans, this will be a once-in-a-lifetime event. The next transit will not take place until 2117.
As with any solar eclipse, don’t look directly at it: You won’t feel the burn, but you may do permanent damage to your eyes. There are safe ways to view the transit.
Transits of Venus have a long and important history in advancing our understanding of the solar system. Probably the best-known measurement to come out of observing the Transit of Venus is the astronomical unit (AU), roughly equal to the average distance between the Earth and the sun, which is typically used to measure distances within our solar system.
At its 1761 transit, scientists noticed a halo around Venus, visible only when the planet covered the edge of the sun – leading scientists to posit the existence of a Venusian atmosphere.
Eight years later, The Royal Academy in England sponsored a voyage by Lieutenant James Cook to Tahiti to observe the 1769 transit.
Today, transits of Venus are prized mainly for their beauty rather than their scientific value. But planetary transits in general are still of great interest, especially in locating planets outside our solar system (referred to as exoplanets). The Kepler Mission, launched in 2009 by NASA, studies more than 100,000 sun-like stars, seeking transits of their planets.
These exosolar planets block some small but measurable amount of light when they pass in front of their parent star. By observing and analyzing the light as it changes, astronomers can calculate a planet’s size, temperature and atmosphere.
From SLAC Today