The videos below give some background into the transit and the history of past transits.
Here is an article explaining the science and maths behind the calculation of the distance of the planets, using the transit of Venus.
As the videos explained transits occur in pairs 8 years apart, followed by breaks of either 105 years or 122 years. The current transit is the second in the latest pair, the previous one being in 2004.
The transit before the one in 2004 was in 1882 - yes, there wasn't one transit of Venus in the 20th century. Astronomer William Harkness, of the US Naval Observatory, wrote the following just before the 1882 transit:
We are now on the eve of the second transit of a pair, after which there will be no other until the 21st century of our era has dawned upon the Earth. When the last transit season occurred the intellectual world was awakening from the slumber of ages and that wonderous scientific activity, which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows.
Today I watched the transit on my computer as it was streamed, over the Internet, from telescopes in Hawaii. Although today's transit will provide useful information, it will not help in the major scientific objective of transits before this century, as the scale of the Solar System has been determined by other means. We now know the distances to the other objects in the Solar System by the use of powerful radar pulses sent from our radio telescopes. As well there are satellites circling Mercury and Saturn and others on the way to Mars and Pluto. Further the Opportunity rover is sitting on the edge of Endeavour crater waiting for its next season of exploring on Mars. No one in Harkness's time could have imagined anything like that.
Here are some photographs taken today of the transit (click on them for a larger view):
About half way through
The following two photos are oriented differently to those above - ie top to bottom.
Venus close to exititing (at the bottom) as a cloud crosses the sun
The transit almost over - Venus can be seen as a little bump at the bottom right
The videos below were constructed from still frames produced by the Solar Dynamics Observatory . Venus in the videos looks transparent, but in fact the light from the sun is being bent around the planet by its atmosphere.