Astronomers using Hevelius' double-arc bras octant, 1673. Plate taken from 'Machina Coelestis' (1673) by Johannes Hevelius (1611- 1687). Hevelius was an early German astronomer, who establshed the foremost observatory in Danzig in 1641, rebuilding it after a fire in 1679. He catalogued 1500 stars, discovered four comets and was one of the first to observe the transit of Mercury.
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