ERIDANUS, in astronomy, a large constellation (q.v.) of the southern hemisphere, stretching from near Rigel at the heel of Orion (q.v.) to Achernar (a Eridani), not far from the South Polar Circle. a Eridani is the only star (q.v.) at present of the first magnitude in this constellation. The double star 0 Eridani is now of the 3rd magnitude, but is identified with the Achernar (the shining one) of Al-Sufi who described it as of the first magni tude ; it has therefore decreased considerably in brilliancy within historic times, and has thus surrendered its title of "the shining one" to a Eridani.
That Eridani is a ternary system was discovered by Herschel in 1783 ; it consists of a star of magnitude 4.5, associated with a close pair of about the loth magnitude and 82" distant from it.
Eridanus fluvius is the name under which the constellation ap pears in the catalogue of Tycho Brahe, Eridanus being a Greek river-god and also an ancient name for the river Po, in Italy. In the older catalogues it appeared simply as "the river" stars : Stellae Fluminis, according to Ulugh Beg, and the aorrEpea,u6s Iloraµov according to Ptolemy who catalogued 34 stars in it.