The Celestial Water Snake


Last updated 4/6/2021 at 11:27am

Just after dark in late March and early April, we can find Hydra ascending over the southeastern horizon.

Springtime in the Southern California deserts is when the snakes begin to appear. They're beautiful creatures, of course, but considering how much time I spend prowling around after dark, I'm always concerned that I might unknowingly come too close to a sidewinder.

There is one snake, however, that I don't mind encountering at this time of year, and that's because it lives safely in the sky. We know it as Hydra, the water snake, and it forms the largest of all constellations.

Just after dark in late March and early April, we can find Hydra ascending over the southeastern horizon, and by 10 p.m. or so, its full length finally appears in the sky. With a total length of about 100 degrees, this constellation stretches a quarter of the way across the heavens and encompasses 1,303 square degrees of celestial real estate.

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. The beast was reportedly so hideous that people died of fear just from the mere sight of it. And, if its appearance didn't do it, its poisonous breath might!

Legend tells that Hydra found its way into the heavens when Apollo, son of the chief Greek god Zeus, sent a crow to fetch him a drink of cool water from a nearby cup. Considering this a waste of his valuable time, the crow brought back a water snake instead. In retaliation, Apollo tossed them all into the same region of the sky. Even today we can see Hydra, the water snake, guarding the cup of water (Crater) from the perpetually thirsty crow (Corvus).

Despite this story being ancient, the Hydra we see in the sky today isn't that old. And believe it or not, Hydra used to be even larger than it currently is. Over the ages, various stellar cartographers, including the famous 17th-century astronomers John Flamsteed and Johannes Hevelius, divided it into several pieces. Out of its faint stars, they created the constellations Crater, Corvus and an even more obscure grouping we know today as Sextans, the sextant. And, of course, the new Hydra "lite."

As immense as this constellation is, Hydra contains only one fairly bright star that marks the heart of the water snake. Alphard is an aging orange giant about 176 light-years away and appears only about as bright as those of the Big Dipper.

Now that moonlight is gone from the early evening sky, see if you can find Hydra. First, locate the bright star Spica in the southeast, and then look for Procyon in the southwestern sky. About a third of the way between Procyon and Spica lies Alphard, almost due south. If you have a fairly dark sky, you might be able to make out the tiny oval of faint stars to its right; these outline the snake's head. To the left of Alphard, try to trace the snake's long, sinuous body to a point just below Spica. Then search above the snake for the much smaller groupings of Corvus and Crater.

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