Eridanus is a constellation. It is represented as a river; its name is the Ancient Greek name for the Po River, the main river of northern Italy. Its association with a river comes from the way its stars trace a tortuous path. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations defined by the International Astronomical Union. It is the sixth largest of the modern constellations.