The transit of Venus begins today, June 5. The next one is not for 105 years, in 2117. You can check where and when today's transit will be visible in the map below. Plus a live webcast of the transit at NASA tv.
There's a nifty citizen science effort underway via Astronomers Without Borders if you care to download the free app and contribute the data from your own sighting. Here's what they say about the history of the event:
Only six Venus transits have occurred since the invention of the telescope in the early 17th century. There were no observers of the first one in 1631 that we know of, and only two who we know saw the transit in 1639. In the 18th century, Sir Edmond Halley described a method for measuring the distance from the Earth to the Sun through observations of Venus transits from widely separated sites. The same had been attempted with transits of Mercury but Venus transits allow for much more precise measurements.Halley's publication led to expeditions sent around the world by many nations to view the pair of Venus transits later in the 17th century. The same took place with the 19th century pair of Venus transits. No Venus transits occurred in the 20th century. While 20th century methods eventually supplanted the Venus transit method in measuring distances in the solar system, the history of the event is an important link to our past.
As for what scientists are looking to learn from the 2012 transit, LiveScience notes a few interesting research questions, including:
The video below summarizes our modern understanding of the transit, including a brief history of its science and what we hope to learn today.
The next video recreates a high-tech presentation of the transit from 1769. The tools have changed but not the fascination.