On June 5, the world will have a chance to observe the transit of Venus, an astronomical event that happens only twice in a century, with an interval of about 8 years between transits. Venus, the planet, crosses the face of the sun, and if you are in the right place and have the right instruments you can make some very nifty observations. Earthbound astronomers, professional and back yard, are delighted. But the transit has historical importance as well. It’s hard for us to accept this but the fact is that most of the great sea voyages before the 1800s were carried out by people who had very little idea of where they were. Latitude, or distance north/south of the equator, could be figured out pretty accurately. But the trick was longitude: location in the other directions. So, Columbus, and Magellan, and the Armada, the Pilgrims and the great fleet battles of the Age of Fighting Sail were all under the same necessity of groping for some clue about longitude. Go west and you’ll bump into something. We think. The fact the Europeans got to America, or to India or Australia and then home again, meant that they were very good sailors, and also that the survivors were very lucky dudes.And a lot of sailors weren’t so lucky. This whole story has been admirably told by Dava Sobel in her wonderful, and prize-winning, book Longitude. What everybody wanted was a reliable and fairly simple way of calculating East-West distance for use on merchant ships and men o’ war to take the guess-work and finger-crossing out of operations. In the 18th century, that had to be found in the sky. The other method, using a clock set to a fixed time, say London time, and calculating the difference between that and time at observation was theoretically possible, but practically out of reach. Clock technology couldn’t come up with a device that would stand up to the pitching and rolling of a ship, the moisture, the heat, etc. So, look upwards, sailors. The transit provided and opportunity to measure celestial objects very accurately. Maybe there was a method using the facts of astronomy to help captains find their way around. A new book recounts the course of three scientific expeditions whose purpose it was to observe the transit from different points on the Earth’s surface. An expedition, in 1769, did not involve 14 hours on a jet liner and a few weeks away from home. Each of the three forces faced terrible hardships, and two of the destinations were, and still can be, rather grim places. The English, under Capt. James Cook (“The Great Sailor”) were headed to Tahiti…rough duty, right? But getting there required a trip around Cape Horn, and that was enough to make anybody think twice. The Franco-Spanish group was headed to what is now Baja California. And the Austrian Empire’s outfit was headed to the North Cape of Norway. Oh, goody! It was just as bad as it sounds. Lots of people didn’t make it back, and all three groups suffered very severely. It’s all told again in Mark Anderson’s book, The Day the World Discovered the Sun: An Extraordinary Story of Scientific Adventure and the Race to Track the Transit of Venus (Da Capo Press, June 2012)
And take a look at this web site, which has a precis of the book, plus some interesting other fare on the Transit itself.
So on June 5, hold a good thought for those guys.