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CANCER
The Crab
Cancer - Celestial Atlas by Alexander Jamieson - 1822
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Sandwiched between the distinctive forms of Leo, the lion, and Gemini, the twins, lies the faint outline of the constellation Cancer (Latin for crab), and although this group of stars is relatively dim, it has been associated with the shape of a crab since the birth of Christ. The Tropic of Cancer (an imaginary line circling the globe at 23.5 ° north latitude) was given that name because that is where the Sun is directly overhead at the moment of the summer solstice, June 22, and 2,000 years ago when the line was named, the Sun was in the constellation Cancer on that date. Now, due to precession (the wobble of Earth's axis) the Sun is actually in Taurus on June 22, but no-one has suggested the line be renamed The Tropic of Taurus. Besides, in approximately 20,000 years, the Sun will be right back in Cancer again anyway.
Being an ancient constellation, Cancer is associated with not one but two myths. The first is the myth of Hercules. One of the labours of Hercules imposed by the gods was the killing of Hydra, the sea monster. As Hercules was battling the Hydra, and beginning to get the upper hand, the goddess Hera sent a crab to harass and distract Hercules. But Hercules quickly stepped on the crab, and crushed it. The gods put both the Hydra and the Crab beside each other in the sky to commemorate the occasion.
The other myth involves the two eyes of the crab, the stars Asellus Borealis, and Asellus Australis, Latin terms for the Northern Ass (donkey), and the Southern Ass. In between the two donkeys is a patch of blurry light called Praesepe, the manger, from which the donkeys appear to be feeding. The beasts were put in the sky to honour the donkeys ridden by the gods in their great battle with the Titans. It is said the loud braying of the donkeys confused the Titans (who had never heard such a noise), and helped win the battle for the gods.
Praesepe, or M44, is actually an open star cluster called the Beehive Cluster, pictured in the NOAO photo below. At a distance of 525 light years, it is visible to the naked eye, and an exceptional sight through binoculars. M67, a smaller, tighter star cluster, is 1,600 light years away.
The most exciting thing in the constellation Cancer cannot be seen, either by the naked eye, or the largest telescopes. It was detected after an exhaustive eighteen year study of minute fluctuations in the light emitted by a tiny little star in the middle of the crab's small claw, named 55 Cancri. The result of this study was the indirect discovery of no less than five planets in orbit around this star, with at least one of these planets occupying the habitable zone, where temperatures allow the existence of liquid water, essential for the formation of life as we know it. This discovery, made in November, 2007, has enormous implications regarding the potential for discovering life outside our solar system. Below is an artist's rendering of the newly discovered solar system in Cancer, courtesy of NASA.
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