Posted by Astronomer_Guy on December 29, 2002 at 10:53:58 from 212.77.0.2 user Astronomer_Guy.
In Reply to: Re: Bach? posted by Harry Miller on December 28, 2002 at 19:00:59:
Taking up Harry Miller’s challenge...
Transits occur when Mercury or Venus cross the face of the Sun. They’re useful because if you time the same transit from two known, and widely-separated, points on the surface of the Earth you can use a bit of tricky trigonometry to calculate the actual diameter of the Sun; from that and the apparent size of the Sun you get the distance from the Sun to the Earth, and from that the distances to all the other planets. And that, in turn, forms the base line for measuring the distances to the other stars. So this measurement helps define the size of the Universe.
Edmund Halley, of comet fame, in 1716 worked out how to make these observations. He also realized, however, that Venus transits the Sun very rarely; the next events weren’t due until 1761 and 1769. It also turned out to be a tricker observation than anyone realized. That plus the fact that the 1761 event occurred during the 7 Years War (French and Indian War, in North America) meant that most of the 1761 events were a washout. So the Royal Astronomer hired a British Navy boat to go to the South Seas for the 1769 event, with instructions to try out their techniques on Mercury before the main event with Venus.
The Royal Navy insisted on having their own man on board to keep an eye on their ship, so they appointed a young lieutenant, James Cook, to captain the Endeavour. That began a long tradition, both real and fictional, of combining voyages of discovery with scientific expeditions. Notable literary descendants of James Cook include the Starship Enterprise’s captain James Kirk (the similarity in names is no coincidence) and of course Captain Flint of the Sea Bear.
(In 1761, the Astronomer Royal had sent out his assistant Charles Mason, along with a surveyor, Jeremiah Dixon, to observe the events; on their way home from South Africa, they stopped off in the North American colonies to determine the border between Maryland and Pennsylvania.)
You’ll be hearing more about transits in the upcoming year. The last transit of Venus was in 1882, but the next one is coming up soon, on June 8, 2004. It will be visible from Europe, but not North America. Meanwhile, the next transit of Mercury is on May 7, 2003.