[okamiueru]

The distance to the sun was first calculated in very much the same way. When Venus passes in front of the sun, it crosses over at different places on the sun disk. Using Kepler's laws for orbits, we knew the distance to Venus. Together with the distance between the observations and angle of crossing, the only unknown was the distance to the sun. They were off by 0.8% in 1769.

The story itself is pretty cool. The idea was presented in 1663, and it wasn't until 1761 and 1769 that the planets would align. And, this also doesn't happen all that often, although it comes in pairs. 1761 and 1769, 1874 and 1882, 2004 and 2012. And it won't happen again until 2117. So, this was considered so important, and the transit of Venus had to be observed from distances far away on earth. In spite of wars going on, these expeditions were given safe passage.