Transits of Venus come in pairs, eight years apart. This one follows one in 2004. The next pair will occur more than 100 years from now, in 2117 and 2125. Transits of Venus show a repeating pattern over 243 years. Two transits separated by eight years are followed by another pair in 105.5 years. That pair is followed by another 121.5 years later. The pattern then repeats.
Transits of Venus are rare enough that only a handful of observations have been recorded. According to Wikipedia, there is no evidence that any of the great ancient civilizations were aware of the transits. The first documented scientific observation was in 1639, made by Jeremiah Horrocks, near Preston, England. Horrocks used his own corrections to Kepler's calculations of the orbit of Venus to estimate the timing of the event. Kepler had predicted the 1631 transit that preceded the one Horrocks observed. Horrocks appears to be the first to have understood that the transits would occur in pairs separated by eight years. Horrocks observed the transit the same way I did, by projecting an image onto a piece of paper using a simple telescope.
The next pair came in 1761 and 1769. Edmund Halley (after whom the comet is named) championed the idea of using the 1760s transits as a means of calculating the distance between the sun and the earth (known as the astronomical unit, or AU), having himself made calculations of the AU in 1676 using a transit of Mercury. He was unsatisfied with his results, however, because only one other scientist had made such an observation. The AU could be accurately measured, he reasoned, using triangulation, but that would require precisely simultaneous observations from widely separated points on Earth. He urged expeditions to make such observations. Halley died in 1742, but people listened. Among those who recorded the 1761 event were Captain James Cook, at Tahiti, on his first voyage around the world. Observations in 1761 from various locations around the globe yielded an estimate of 153mn kilometers for the AU--a significant improvement (Horrocks had estimated 95.6mn kilometers). Calculations from the 1874 and 1882 transits yielded a fairly accurate figure of 149.59mn kilometers. Today the AU is given as 149, 597, 870.700 kilometers--actually the mean distance, as the distance fluctuates slightly. Radio telemetry and radar have replaced the methods used in the past, but I found it satisfying to be able to do essentially what Horrocks did way back in 1639. I had the advantage, however, of being able to find the exact time of the event by using Google.
If you missed this event, there will be a transit of Mercury on May 9, 2016. Mark your calendar.