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, which we know as Rigel or Beta Orionis, was common with Eridanus. However, he did not repeat this description in his star list for Eridanus (which he simply called potamos, or river). Instead, 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 next to the shin of Orion marked ‘h’ on this chart is the one we know as Beta Eridani.
Image © Linda Hall Library.