Spot the Celestial Snake

SKY MAP: The sky as seen at 9 p.m. on April 27 from mid-northern latitudes. CREDIT:

Still shining sedately with a yellowish-white glow high in the southern sky after sunset is the beautiful ringed planet, Saturn. Not too far south of Saturn you can find a little group of five or six stars that marks the head of Hydra the Water Snake, one of those curving streams of stars which suggested a celestial serpent to the ancient imaginations.

Hydra is actually one of two snakes in the sky. Hydra represents a female snake; there is also a male snake named Hydrus that can be found curled in the far-southern sky.

If you extend an imaginary line from Denebola to Regulus in Leo and extended it as far again beyond, that will lead you to the head of Hydra. The bright star Procyon, in Canis Minor can also be used as a guide for finding Hydra's head, for it lies less than 10-degrees due east of Procyon (your clinched fist held at arm's length measures roughly 10-degrees).

Binoculars useful

The stars in Hydra's head make a rather attractive group in 7-power binoculars. One of them, the northernmost star in the circlet, Epsilon Hydrae, is one of the most outstanding examples of a multiple star system, astronomers having discovered no fewer than five stars here revolving about each other. Two-star and three-star systems are known to be common.

From there, if you have access to a clear and dark sky--and keeping in mind that Hydra's stars are for the most part rather dim--you can follow the scraggly stream of the Water Snake's body as it goes south and east below the Sickle of Leo, past the faint goblet-like star pattern of Crater the Cup and the moderately bright four-sided figure of Corvus the Crow, then south of the bluish star Spica before finally coming to end near Libra the Scales.

As you might have guessed by now, Hydra is the largest and most extensive of all the constellations. At mid-northern latitudes it takes well over seven hours for the entire figure of Hydra to rise. Its head has already passed the meridian as darkness falls this week; the tip of its tail, however, does not pass there until around 1:30 the following morning.

Find Polaris

Hydra's brightest star, not far below its head, is distinctly ruddy in coloration and shines about as bright as Polaris, the North Star. Its name is Alphard, which means "the Solitary One." It's also known as Cor Hydrae, or the Heart of the Hydra.

Once you locate Alphard, you'll immediately understand why it was so named; for it's the only conspicuous star in the large dull region of the sky that lies below Leo's Sickle. For this reason, Alphard appears brighter than it really is because it has no competition nearby.

Some star guides suggest that Hydra commemorates the fabled multi-headed Lernaean Hydra that gave the mighty Hercules so much trouble in the second of his twelve labors; when he cut off one of the heads, two others would sprout in its place! To make things even worse, one of those heads was immortal and could not be killed. Hercules, however, was able to dispose of the Hydra by enlisting the aid of his nephew, Iolaus, to stand by with a torch. Each time Hercules chopped off a head, Iolaus cauterized the stump to prevent any heads from growing back.

As for the immortal head, Hercules simply rolled a giant boulder over it, rendering it helpless.

That might be why, in the sky, our celestial Hydra only has one head. And that may also be why, when all but the tail of Hydra has set below the southwestern horizon that the constellation of Hercules stands triumphantly directly over our heads.

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.