Early this week sky-watchers around the world will be able to witness a transit of Venus-a celestial event that won\'t be seen again for more than a century.
Visible either Tuesday or Wednesday, depending on where you live, the transit will offer astronomers a chance to refine our understanding of Venus as well as to tweak models for searching for planets around other stars.
Transits happen when a planet crosses between Earth and the sun. Only Mercury and Venus, which are closer to the sun than our planet, can undergo this unusual alignment.
With its relatively tight orbit, Mercury circles the sun fast enough that we see the innermost planet transit every 13 to 14 years. But transits of Venus are exceedingly rare, due to that world\'s tilted orbit: After the 2012 Venus transit, we won\'t see another until 2117.
http://news.nationalgeographic.com/news/2012/06/120604-transit-of-venus-2012-sun-planet-hubble-space-science-how-when/
