Finding the Largest Constellation

By Dennis Mammana

April 5, 2018 4 min read

Week of April 12-18, 2015

If someone were to ask you which is the largest of all constellations, would you know? 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 a couple of hours before midnight, 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. That's more than 3 percent of the entire heavens!

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. 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 have!

Hydra found its way into the heavens like this: One day, Apollo, son of the chief Greek god, Zeus, sent a crow to fetch him a drink of cool water from a nearby cup. 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, even today, we can see Hydra, the water snake, guarding the cup of water from the perpetually thirsty crow.

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

Even an obscure French astronomer got into the act, creating a constellation he named Felis, the little cat. Astronomers never adopted Felis as one of the 88 official constellations, however; it appears nowhere in the sky and remains simply a historical curiosity.

For all its size, Hydra contains only one bright star that marks the heart of the water snake. It's an orange giant about 177 light-years away that appears only about as bright as the North Star. The 16th-century Danish astronomer Tycho Brahe named this star Cor Hydrae, meaning "the Hydra's Heart," but the proper name we use today, Alphard, derives from the Arabic phrase "Al Fard al Shuja," which means "The Solitary One in the Serpent."

After dark this week, see if you can find Hydra. First locate the bright star Spica in the southeast. Then look for Alphard in the south-southwestern sky. To its right, you should be able to make out the tiny circle of faint stars that form the head of the snake. To the left of Alphard, try to trace the snake's long, sinuous body to a point just below Spica. Then look above the snake for the much smaller groupings of Corvus and Crater.

 Find the largest constellation after dark this week.
Find the largest constellation after dark this week.

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