Sky Watch: Hydra, the Celestial Water Snake

Here's a trivia 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 can find Hydra, the water snake, low in the southern sky shortly after dark, winding a quarter of the way across the heavens. With a total length of about 100 degrees, this constellation encompasses 1,303 square degrees of celestial real estate.

In one sense, Hydra is an ancient constellation. 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 Twelve Labors.

For all its size, Hydra contains only one bright star that marks the heart of the water snake. It's an orange giant named Alphard, about 177 light years away and appearing only about as bright as the North Star.

Later this week and next, when the moon no longer dominates the evening sky, see if you can find Hydra.

First locate Spica and Saturn in the southeast, then look for Alphard, low to the southwest. To its right is the tiny circle of faint stars that forms a head of the snake. To the left of Alphard, try to trace the snake's sinuous body to a point just below Spica. Then look above the snake for the much smaller groupings of Corvus and Crater.