Orion and Eridanus

as depicted on the Atlas Coelestis of John Flamsteed

Eridanus streams away from the left foot of Orion, as described by the ancient Greek astronomer Ptolemy, in this chart from the Atlas Coelestis of John Flamsteed (1729). Flamsteed was scrupulous about following the descriptions given by Ptolemy in his Almagest. In his list of stars in Orion, Ptolemy said that the ‘bright star in the left foot’ of Orion was common with Eridanus, the river. We know this bright star as Rigel or Beta Orionis. However, Ptolemy was inconsistent, for he did not repeat this description in his star list for Eridanus. Instead, under Eridanus Ptolemy said that the river began with ‘the star after the one in the foot of Orion’, which we now know as Lambda Eridani. Lambda Eridani can be seen just below right of Rigel on Flamsteed’s chart, above. The star marked ‘h’ on this chart, next to the left shin of Orion, is the one we know as Beta Eridani.

Image: Linda Hall Library.

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