Jean Richer
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Jean Richer (1630–1696) was a French astronomer and assistant at the French Academy of Sciences, under the direction of Giovanni Domenico Cassini.
Astronomical and gravimetric observations made on the island of Cayenne by Jean Richer, after an engraving by Sébastien Leclerc.
Astronomical unit
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The astronomical unit is a unit of length defined to be exactly equal to 149,597,870,700 m. Historically, the astronomical unit was conceived as the average Earth-Sun distance, before its modern redefinition in 2012.
Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here, the so-called "black drop effect") and the rarity of observations.