It hinges on observations made during the transit of Venus across the Sun: http://en.wikipedia.org/wiki/Transit_of_Venus http://en.wikipedia.org/wiki/Transit_of

It hinges on observations made during the transit of Venus across the Sun: http://en.wikipedia.org/wiki/Transit_of_Venus http://en.wikipedia.org/wiki/Transit_of_Venus Eli Maor wrote a great book "Venus in Transit" that accounts this multi-century quest to accurately measure this event, which occurs very rarely and requires accurate timekeeping across large distances to compute the triangulation. It's a fascinating story.