Cancer the Crab in 18th century star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology
Grab your long-handled net, and let’s go crabbing!
How did a word that means crab come to refer to a group of cellular diseases? Early physicians noticed that tumors and the distended blood vessels around them formed a pattern that resembled a crab. Around the 17th century, they began applying the word “cancer” to the diseases that caused these tumors.
A dark sky--well away from city and suburban lights--is required to commune with the Crab. The faint constellation would be inconspicuous if it weren't for the renowned naked-eye star cluster at its heart: the Beehive. Even the brightest stars in Cancer are somewhat dim. If you see anything bright in Cancer, rest assured it’s a planet just passing through.
1) You’ll need to face south, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.
2) Wait at least one hour after sunset to begin observing, so that twilight’s finished and your sky’s good and dark. This observing week, you’ll have plenty of gazing time before the Moon floods the sky with light. It rises well after midnight on Friday the 25th and even later each night after that.
3) As you face south, tilt your head back slowly and move your gaze from the southern horizon up to the zenith, the spot directly above your head. Look for a fuzzy patch. It'll be almost as high as the zenith if you live in the southern latitudes of the continental U.S. If you live in the northern latitudes, it'll be slightly lower, up to a quarter of the way from the zenith down toward the southern horizon. It'll also be slightly west of due south. Find it? Excellent! This is the famous Beehive cluster.
If you’re having trouble finding it, first make sure you’re dark adapted. Avoid all white light for a minimum of 20 minutes. This will improve your night vision and help you see faint objects. Second, try to locate the Sickle, a prominent asterism (recognizable star pattern) in Leo. It looks like a big, backwards question mark. Right now the bright golden planet Saturn is hanging out at the base of the Sickle. The open end of the Sickle’s curving top points toward the Beehive. Also, if you can locate reddish planet Mars near the Gemini twin stars (Castor and Pollux) and draw a line between Saturn and Mars, it will cross the Beehive.
M44, the Beehive cluster
The Beehive is an open cluster, a loose grouping of stars that formed around the same time from the same gas cloud. It contains around 350 stars. It is alternately known as M44 and as Praesepe (pronounced pree SEE pee). The former is shorthand for Messier 44, the object’s designation in the catalog of famed 18th century French astronomer Charles Messier (pronounced MESS ee yay). The latter, Praesepe, is the oldest of these names and is Latin for manger. The origin of the whimsical Beehive name is unknown.
4) Once you’ve found the Beehive, look for two faint stars that bracket it on the left (east). The top star is Asellus Borealis and the bottom one, Asellus Australis. These are Latin for Northern Ass and Southern Ass. These two stars form the asterism known as the Donkeys or the Asses. In ancient mythology, the asses participated in a battle between the Greek gods and some giants. After helping the gods prevail by spooking the giants with their obnoxious braying, they were rewarded with a place in the sky next to a well-stocked feed trough, also known as a manger. Aha! Now we see why the Beehive originally had the name Praesepe.
5) The star Acubens, Arabic for the claw, marks the southernmost crab claw. Just south of Acubens (pronounced ak oo BENZ) is a circlet of stars--the asterism known as the Hydra’s Head, in the constellation Hydra. The water snake and the crab huddle together in the sky, sworn enemies of Hercules.
6) Al Tarf, the brightest star in Cancer, is an orange giant. Its name is from the Arabic for the end, and it marks one of the crab’s southern legs.
7) The only other star in Cancer with a traditional name is Tegmine (pronounced TEG-min-uh). It's Arabic for in the covering, referring to the crab’s shell. Tegmine is west of the Beehive.
8) The Beehive is a great binocular object. In fact, binoculars--rather than a telescope--usually give the most satisfying magnified view of a large object like this. If you have access to a pair of binoculars of any size, use them to peer at the fuzzy patch. The magnified patch will resolve, separating into distinct points of light.
Wander around in the cluster and notice how brilliant and sharp the stars are. I like to contrast this object with the Pleiades or "Seven Sisters" open cluster in Taurus. The Pleiades (pronounced PLEE uh deez) cluster is currently crossing paths with a nebula, a cloud of gas and dust. This makes its stars appear to be wrapped in cotton candy. In contrast, the sparkling Beehive is a visual 'palate cleanser,' not unlike a refreshing sorbet served between courses at a fancy banquet.