Transit of Venus and Distance Measure
Country: United Kingdom
Date: Summer 2012
How did 17th century astronomers calculate the sun-Earth distance using the transit of Venus and parallax?
Very good question.
The astronomers used precise timings on the ingress and egress of Venus to get a value. They used what we called the parallax method and Kepler's third law. By making precise observations of the durations of the transit as it was viewed by diverse points across the Earth. The distance between these points could be used as a baseline to calculate the distance to Venus and the sun using trigonometric triangulation. Because of the black drop effect, which made it impossible to get accurate timings of ingress and egress until recently, the transit method never gave the best value of the distance between Venus and the sun, and Earth and Sun, and hence the valued of their astronomical unit and the size of the solar system. Spacecraft have now refined that value successfully; we now know the size of the astronomical unit to about 30 meters.
Hope this helps a bit.
David H. Levy
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Update: December 2011