Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees. Also one of the longest at over 100 degrees, its southern end abuts Libra and Centaurus and its northern end borders Cancer. It has a long history, having been included among the 48 constellations listed by the 2nd century astronomer Ptolemy. It is commonly represented as a water snake. It should not be confused with the similarly named constellation of Hydrus.