If you missed it you’ll have to wait another 105.5 years for the next ‘transit’ of Venus across the face of the sun. Prior to today’s event the last one was in 2004 and before that it was 1882. In astronomical history the transit was a huge deal as it provided a rare opportunity to define the Astronomical Unit (AU), or the distance from the sun to Earth. Dubbed a celestial yardstick, the AU helps astronomers measure distances among the planets and to link them to the stars beyond.
In 1716, the astronomer Edmond Halley calculated that the AU could be worked out if observers around the world timed the passage of Venus across the sun. Halley predicted the best spots around the world where the transits could be seen and throughout the 1700s and 1800s explorers fanned out across the globe to fulfill his quest. According to a website dedicated to the phenomenon, the mounting of several expeditions to view the transit in 1761, in the midst of the Seven Years War, marked one of the first times several countries co-operated to answer one of the main scientific questions of the day.
Thanks to my son Greg, who helped me rig up binoculars and a piece of card to record the event in California this afternoon, here is a shot of Venus on its way across the sun. Note the sunspots in the image.