Transit of Venus and Earth-Sun Distance
Date: Spring 2012
How do scientists determine the distance of Earth from Sun through the Transit of Venus?
This is a very good question. The way they calculated it was to use
(1) as precise longitudes they could find, and
(2) precise timings of first and last contacts of the transits. From these data they were able to arrive at a value for the distance between Earth and Sun. The problem was (besides weather) that the back drop effect prevented accurate timings.
A transit of Venus takes place Venus is directly between Earth and Sun. You can imagine two big triangles in the sky, one whose base is on the Earth and whose apex is at Venus The other has its base on the Sun and its apex on Venus. They can use observers at different places on Earth to produce slightly different triangles. Then they can use trigonometry to calculate the distance. Using this method can work only if observations are made from points widely separated on the Earth's surface. Knowing the distance between these points on Earth, one can calculate the distance between Earth and Sun.
This is a very good question. I hope I have been able to help.
It actually worked; by 1882 the distance had been calculated.
David H. Levy
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Update: June 2012