Cancer, the crab in the sky

¡SkyCaramba! Weekly astronomy blog for the week ending February 9, 2013

One of the dim constellations of winter is a crustacean crawling in the sky in the space between Gemini and Leo. The stars of Cancer have been recognized as a crab for at least 2,500 years. Despite having no bright stars to get our attention, Cancer is a well storied constellation.

About 2,500 years ago, the sun reached the northern solstice when it was in Cancer. That’s why the line on the globe showing how far north the sun is around June 21 is called the Tropic of Cancer. Some ancient writers described the sun as reaching that far north in the sky and then crawling backward like a crab. Because of a long slow wobble in the earth’s orbit, the sun is now in Gemini at northern solstice. It will be in Cancer again at northern solstice in about 20,000 years.

In one ancient legend, the goddess Hera sent the crab to bother Heracles while he was fighting the monster Hydra. Cancer bit the warrior who then either stepped on the animal or beat it with his club and killed it. Hera honored the crab by putting him in the sky. But she didn’t give him any bright stars because he failed in the duty she assigned him.

Two stars in Cancer, Asellus Borealis and Asellus Australia, are the northern and southern donkeys, respectively. The gods Dionysus and Silenus rode into battle against the Titans on them. The animals made noises that scared the Titans and the gods won the battle.

Between the two donkeys is a marvelous star cluster called Praesepe. It looks like a fuzzy cloud to the naked eye. Hundreds of years ago, astronomers thought it was a gas cloud. But the invention of the telescope revealed a sight that reminded astronomers of bees at a hive. Praesepe is so nicknamed the Beehive. The astronomer Messier labeled this cluster number 44 in his famous list of already known fuzzy objects that could confuse someone hunting new comets.

One of the stars in Cancer is named Acubens, an Arabic word for claws. Another, Al Tarf, means the end, referring to the end of one of the crab’s legs.

Many backyard astronomers like seeing Cancer when a planet is in or near Praesepe. That happens somewhat often, because the Beehive is barely north of the ecliptic and most planets travel near that line. Venus will pass right into the Beehive in early July this year. Mercury will be there in the middle of August. And then Mars visits in early September.