I am lucky to have a public observatory five minutes drive from my house, Chabot Space And Science Center, so for yesterday’s Transit of Venus I took my boys to see it through the various telescopes, both the observatory’s and scads of amateurs’. Have mercy!, the crowd was the largest I’ve seen there for what looked to me like a small black dot in front of a large white one, so I asked a science teacher friend about the transit and got this.
The 1761 transit of Venus was a watershed moment in the history of astronomy. It was the first time astronomers would have the opportunity to measure accurately the size of the solar system. The distance between the Earth and the Sun had been estimated, with variable degrees of success, since the Greeks, but this was different.
Thanks to a rare celestial alignment, Venus was to pass in front of the Sun, taking about six hours to cross the fiery disc. By recording the times of the start and end of the event from widely separated locations around the globe, trigonometry could be used to calculate the distance to Venus and the Sun. With that, Kepler’s laws of planetary motion could be used to calculate the orbits of all the planets out to Saturn, the outermost known planet