A capacity crowd gathered to hear Andrea Wulf talk about her latest book, Chasing Venus: The Race to Measure the Heavens, at the Royal Society library on Friday 4th May 2012. The podcast of Andrea Wulf’s lecture, which I highly recommend, will soon be available here.
A transit of Venus is a rare astronomical event in which the planet Venus crosses in front of the disc of the sun. Transits happen in pairs separated by eight years, and the transit pairs themselves occur over a century apart. Consequently, there have been only five Venus transits since the first one was recorded in 1639. The crucial measurements in any transit of Venus are:
- The external ingress (when Venus first touches the sun)
- The internal ingress (when the disc of Venus separates from the edge of the sun)
- The internal exgress (when Venus touches the other edge of the sun from the inside, at the end of the transit)
- The external exgress (when the disc of Venus and the disc of the Sun separate)
By combining the timing of transits observed from both the northern and southern hemisphere, trigonometry can be used to calculate the distance from the Earth to the sun. This distance is known as an Astronomical Unit.
Watch this explanatory video for a more detailed description of the phenomenon.
(thanks to www.transitofvenus.org for this video)
The first international collaboration to measure the transit of Venus occurred in 1761, in the midst of the Seven Years War. There were many colourful participants, each of whom encountered trials and tribulations in the field. French astronomer Guillaume Le Gentil (below) endured hurricanes, came under British attack, was forced to observe the transit from a rolling ship’s deck, and spent so long chasing astronomical observations in the southern hemisphere that his French relatives declared him legally dead and divided his estate.
The next transit of Venus, in 1769, was most famously observed by Captain James Cook and his scientific companions in Tahiti, on his first great voyage aboard the HMS Endeavour. A temporary observatory, Fort Venus, was erected for the purpose.
Just down the hall from the Royal Society library stands a pendulum clock, which is believed to be the one used by Captain Cook at Fort Venus in Tahiti.
I was lucky to observe the 2004 transit in a sunny Sydney backyard, but now that I’m living in cloudy Britain, my chances aren’t good for seeing the next one, on the 5th/6th of June 2012. This will be the last chance for any of us to see a transit of Venus, so if you haven’t yet made plans to do so, get cracking.
Andrea Wulf is an author, journalist and design historian. Her previous books include The Brother Gardeners: Botany, Empire and the Birth of an Obsession, and The Founding Gardeners: How the Revolutionary Generation Created an American Eden.