Jimmy Westlake: Looking for Cancer
March 3, 2014
Of the 12 constellations of the zodiac, which mark the sun's annual path through our sky, Cancer the Crab is the faintest and most challenging to locate. By the first week of March, the Crab has climbed high up in our eastern sky, tucked in between the more prominent constellations of Gemini the Twins to the west and Leo the Lion to the east.
Greek mythology identifies this crab as the pesky crustacean that was nipping at the toes of Hercules during his battle with the giant sea serpent, Hydra. Hercules grew weary of the crab's distraction and squashed Cancer under his foot. Hera, the jealous queen of the Greek gods who had arranged for the battle, immortalized her martyred crab in the stars.
It was here, within the constellation of Cancer, that the sun used to reach its highest point in the sky during the summer solstice. Thus did the crab lend his name to our Tropic of Cancer.
Since that ancient time, the slow wobbling of the Earth on its axis has shifted the sun's highest point out of Cancer and into the neighboring constellation of Gemini.
To locate the stars of Cancer the Crab, choose a dark, moonless night and look about midway between Gemini's bright twin stars, Castor and Pollux, and Leo's bright star, Regulus. There, in that large blank region of sky, you can spot a fuzzy-looking smudge of light with a little star on either side of it.
The fuzzy patch marks the crab's carapace, or shell, and the two stars represent his protruding eyes. The numerous other faint stars that surround the fuzz mark the crab's claws and legs.
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To the ancient sky watchers who predated the crab's arrival in the sky, this fuzzy spot was known as the Praesepe, Latin for "The Manger," and the two little stars that flank the manger were considered to be two donkeys munching on hay. The stars' names still reflect this identification: Asellus Borealis and Asellus Australis, the Northern and Southern Donkeys.
For centuries before the advent of weather satellites and computers, the Praesepe was used to forecast the weather. If the Praesepe's misty glow is clearly visible, fair weather will prevail, however, if it in not visible, a change in the weather is due. This is because the least little bit of atmospheric haze, like the high cirrus clouds that signal an approaching cold front, can render the misty Manger invisible.
If you aim a small telescope or a pair of binoculars at the Praesepe, you'll learn why it is also known as the Beehive Star Cluster. Catalogued as the 44th object in Charles Messier's famous 18th century list of celestial fuzz balls, the Beehive Cluster contains several dozen stars. They are delightfully arranged into little pairs and triangles, all lying about 600 light years from Earth.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Jimmy's astrophotography website at http://www.jwestlake.com.
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