For the week including January 28, 2011

M44 M44 Location Larger Illustration

THE CRAB

After sunset, the bright stars, Castor and Pollux, of Gemini are well above the eastern horizon. Just nearby, there is another constellation that has been plotted on star maps for thousands of years. Because it doesn’t contain bright stars, it’s often overlooked, but the constellation of Cancer, the Crab is definitely worth examining next time you’re out.

Although the ancient Egyptians and Babylonians called these stars, the Tortoise, the crab origin of this constellation comes from the Greek legends of the Twelve Labors of Hercules. One of his labors was to slay the Hydra living in the swamps of Lerna. This terrible beast devoured sheep, cattle, and the occasional person that crossed its path. If one of its many heads were cut off, another would grow to replace it, which made it very tough to kill. It was while the gods of Olympus were watching Hercules battle the Hydra that Hera, wife to the powerful Zeus, hatched a plot to help the Hydra win. Hera never liked Hercules much since he was the offspring of an affair between Zeus, and a mortal woman. So she decided to summon another monster to join the Hydra in defeating him. To her dismay, lots of other Greek heroes were busily pursuing their own legends and all the remaining monsters were booked solid. Fearing that Hercules might dispatch the Hydra before she could intervene, Hera cast about for the first creature she might send after him. This came in the form of a crab near the water’s edge. Hera ordered the crab to attack Hercules and so, pincers waving, it went after his toes. Unfortunately for the crab, Hercules accidentally stepped on it almost immediately, so its part of the story didn’t last long. The Hydra itself succumbed not much later. Her fellow gods were amazed that Hera would send such a puny creature after the great Hercules, but she puffed herself up and told them that this was actually the dreaded Nasty Little Crab of Lerna that had been a pest to beach-goers for years. Of course, none of the other gods had heard any stories about this crab before, so they just rolled their eyes and laughed. Although she placed her crab among the stars with much honor and ceremony, Hera eventually dimmed the constellation so the other gods would stop teasing her about it.

Still, there is a more probable history behind Cancer’s designation as a side-scuttling crab, or slow moving tortoise which has to do with our Sun’s apparent annual motion through our sky. During the course of a year, each day’s noon Sun rises to a higher place in the sky. This higher motion slows to a stop at the time of the summer solstice at which time the noon Sun slips lower in the sky until it reaches its lowest position at the winter solstice when the cycle begins again. Thousands of years ago, during the summer solstice, the Sun would slow, stop, and begin its slide back down the sky in the constellation of Cancer, perhaps inspiring thoughts of these animals. A slow wobble in the Earth’s rotation called precession has transferred this point to its current location at the foot of the constellation of Gemini, the Twins.

On a clear night, you can see a faint, luminous patch near the center of Cancer, the Crab. The light is coming from a cluster of hundreds of stars called the Beehive and if you look at it with a pair of binoculars you will see something like the photograph by Sven Kohle and Till Credner at upper right.

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