Excerpt:
On June 6th Venus will transit the sun as seen from earth. The last transit was in 2004. But the next transit after this one is in 2117--no, that's not a typo. Venus transits of the sun are rare--they are among the rarest predictable events in astronomy--and past transits were very important to the history of science.
This page explains what they are, why they are rare, why they show up in pairs eight years apart, but with the time between pairs being over a century, and why they are so important in the history of science. For instance, we would not know how far away stars are if we had not observed Venus transits in the eighteenth and nineteenth centuries.
