Mark Anderson, author of the book The Day The World Discovered the Sun on the Science Talk podcast last week talked about the transit of Venus that’s happening today. Also make sure you listen to part 2 of the podcast!
It’s an amazing story about the efforts in 1760 to measure the distance to Venus. They had to travel far north and also south to the tropics to measure the angle of view from two known locations. Astonishing that they did this so long ago, even using the moon as a GPS, a technique that had only just been figured out.
The global positioning satellite in this case was the moon. In the 1760s science was for the first time able to predict with uncanny accuracy the moon’s exact motion in the sky years ahead of time. So some of these Venus transit pioneers, especially England’s Astronomer Royal Nevil Maskelyne, had mastered a method by which a ship’s navigator anywhere on the Earth could observe the moon with a £8 sextant and a reference manual (costing a few shillings) — and in short order discover their exact location on the planet. The long-sought solution to the longitude problem was now easy, inexpensive and at hand. (source)