Transit of Venus 2012—Sun Show Will Be Last for a Century
How to see rare planetary lineup that may unlock puzzles of alien worlds.
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.
During the upcoming transit, Venus will look like a black dot gliding across the face of the sun over the course of about six hours.
"Venus's diameter will appear only about a 30th the diameter of the sun, so it will be ... like a pea in