Geometry of Transits
The utmost significance of planetary transits arises from the possibility they offer, of directly measuring the Earth-Sun distance - the Astronomical Unit.
Kepler’s laws gave relative distances between all the planets and the Sun, but, the absolute distances were not known. The 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first good value for the Sun's distance.
It may be useful, however, to first rationalize for our selves, the underlying geometry and methods involved in the determination of this quantity, using transit observations.
The operative quantity here, is the Solar Parallax – the angle subtended by a half diameter of the Earth, at the Sun.
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