​ERIDANUS, or Fluvius (“the river”), in astronomy, a constellation of the southern hemisphere, mentioned by Eudoxus (4th century B.C.) and Aratus (3rd century B.C.); Ptolemy catalogued 34 stars in it. θ Eridani, a fine double star of magnitudes 3.5 and 5.5, is now of the third magnitude. It is supposed to be identical with the Achernar of Al-Sufi, who described it as of the first magnitude; this star has therefore decreased in brilliancy in historic times. The star ο₂ Eridani (numbered 40 ​by Flamsteed) was discovered to be a ternary star group by Herschel in 1783; it consists of a close pair, of magnitudes 9.2 and 10.9, revolving in a period of 180 years, associated with a star of magnitude 4.5, which is distant from the pair by 82″; these stars have an exceptionally swift proper motion, about 4″ per annum. Eridanus was the ancient name of the river Po.