Hydra is the largest of all the constellations, one of Ptolemy's forty-eight, a long water snake winding for more than a quarter of the way around the sky south of the ecliptic. Despite its great length it is faint, its brightest star Alphard, the solitary one, marking the serpent's heart. In Greek myth it is associated with the many-headed Lernaean Hydra slain by Heracles, or with the water snake of the tale of the crow and the cup, which places the small constellations Corvus and Crater upon its back. In astronomical tradition the constellation is a star pattern rather than an astrological sign, and its watery identity groups it among the water-associated figures. These are the features of Hydra as the classical and modern traditions describe it, distinct from any astrological reading of the wide band of sky it crosses.
Hydra's great length made it a natural quarry for later astronomers wanting to carve out new figures: Sextans, the sextant, was cut from its faint northern stretch by Hevelius in 1687, commemorating the astronomical instrument he used in his own observations after a fire destroyed his observatory, while Lacaille's eighteenth-century Antlia, the air pump, and Pyxis, the compass, were drawn from stars near the constellation's southern reach, close to the old figure of Argo.