In 1761 and 1769, hundreds of astronomers from more than a dozen nations stationed at more than 130 locations around the world turned their telescopes simultaneously to the sky to observe the transit of Venus. They did so — at great peril and against heavy odds in many cases — because they believed that the transit held the key to one of the most pressing quests of the age: the distance between Earth and the sun and, by extension, the size of the solar system.