The constellations lacked official boundaries until 1930, when the International Astronomical Union defined precise borders to designate the area of the sky belonging to each. At the same time, the IAU broke the enormous constellation Argo Navis (the ship of Jason and the Argonauts) into four smaller, more manageable constellations, making Hydra, the Water Snake, the largest of the 88 constellations by default.
Hydra is nonetheless huge, covering more area than 6600 full moons, or the twelve smallest constellations combined. It snakes roughly a third of the way around the sky. From the time its head begins to clear the east-northeast horizon, nine hours elapse before the last of its tail has risen. Not long after that, its head starts vanishing below the west-southwest horizon. Currently, it’s completely visible only between 9:15 and 11:15 p.m.
It would be futile to describe Hydra in sufficient detail for someone unfamiliar with it to be able to find it, since it is so large and faint. Its brightest star is easily found, however, because there are no comparable stars near it — its name, Alphard, means “solitary one”. Just look low in the southwest around 10:45 p.m.
In addition to its role as the multi-headed serpent slain by Hercules, the Greeks tell a second myth involving Hydra, in which Apollo sent a crow to fetch him water. Spotting a fig tree, the crow dallied until the figs were ripe before completing its task. To explain its tardiness, the crow lied that a water snake was guarding the spring, so the angry Apollo, recognizing the deception, condemned the crow to be forever deprived of water by an actual water snake. (Both the crow and his cup, Corvus and Crater, are constellations sitting side-by-side just north of Hydra.)
Next column: The myth of magnification.