Chasing Venus: Observing the Transits of Venus 1631-2004
For ages, astronomers looked for a way to measure the distance between Earth and the Sun. This distance, called the astronomical unit was a yardstick that could measure the entire solar system. Since the astronomer Johannes Kepler had determined the relative distances among the planets and the Sun in the early 1600s, the Earth-Venus distance could be used to calculate the Earth-Sun distance. Observing the transits of Venus across the surface of the Sun would help to find the value of this measurement.
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