Transits of Venus and the Astronomical Unit

Abstract

Our story starts with the German astronomer Johannes Kepler in the early part of the seventeenth century. Though Kepler never witnessed a transit himself, his significance in the story is enormous for two reasons. First, according to Kepler's Third Law, as it is now known, the ratio of the square of a planet's orbital period to the cube of its mean distance from the Sun is the same for all planets. From this law, the relative scale of the solar system can be determined simply by observing the orbital periods of the planets. In fact, Kepler's own estimates of the relative distances of the known planets from the Sun do not differ significantly from