One of the rarest celestial events, the transit of Venus is happening on Tuesday. The next transit of Venus, when the planet Venus will appear as a small, dark disk moving across the face of the Sun, will begin at 22:09 Coordinated Universal time (UTC) on June 5, 2012 and will finish at 04:49 UTC on June 6. Depending on the position of the observer, the exact times can vary by ±7 minutes.
For the Americas the transit is be visible on the evening of Tuesday, June 5. For Eurasia and Africa, the latter part of the transit will be seen the morning of June 6. Transits of Venus are among the rarest of predictable celestial phenomena and occur in pairs eight years apart: the previous transit was in June of 2004. If you miss seeing the far-off planet crossing in front of the sun, you might be out of luck: it won't happen again until December 2117 and December 2125.