Here's a question: Which is the largest of all constellations? If you answered Hydra, you know more about the night sky than you admit.

At this time of year we find Hydra, the water snake, low in the southern sky shortly after dark, winding a quarter of the way across the heavens. According to Babylonian mythology, Hydra was known as Tiamat, the dragon of Chaos. To the ancient Greeks, Hydra represented the terrifying seven-headed monster killed by Heracles as the second of his 12 labors. Hydra found its way into the heavens like this: one day Apollo sent a crow to fetch him a drink of water. Having wasted his time, the crow brought back a water snake as an excuse for being late. Apollo tossed them all into the same region of the sky where today we can see Hydra guarding a cup of water from the crow.

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Despite the story being ancient, Hydra isn't that old. And it used to be even larger. The 17th-century astronomers John Flamsteed and Johannes Hevelius broke it into pieces. Out of its stars they created the constellations Crater (the cup), Corvus (the crow) and an obscure grouping known as Sextans (the sextant).

Later this week and next see if you can find Hydra. First locate Spica and Saturn in the southeast, then look for Alphard, low in the southwest. To its right, you should see the tiny circle of faint stars that form a snake's head. To the left of Alphard, trace the snake's body to a point just below Spica. Then look above the snake for the much smaller Corvus and Crater.