#6: PRAESEPE: THE CELESTIAL WEATHER FORECASTER

About halfway up in the eastern sky is the faintest of all the zodiacal constellations: Cancer, the Crab.

For most sky watchers, this region of the sky seems nothing more than a dark and empty part of the sky. Sadly, the plague of light pollution is robbing us of our celestial heritage, and poor Cancer is one of the victims. Because it contains no star brighter than 4th magnitude, the crab is difficult, if not impossible to see under a light-polluted sky.

Cancer does have one significant attraction: appearing to the eye as merely a misty patch of light, binoculars will quickly reveal it as a beautiful open star cluster, containing hundreds of tiny stars. It is known to some as "Praesepe, the Manger." A manger is defined as "a trough in which feed for donkeys is placed." The cluster was apparently first called Praesepe 20 centuries ago. Indeed, two nearby stars, Gamma and Delta Cancri are also known as Asellus Borealis and Asellus Australis – the northern and southern ass colts – feeding from a manger.

The cluster also goes by the name "Beehive," a moniker that apparently evolved almost four centuries ago, when some anonymous person, upon seeing so many stars revealed in one of the first crude telescopes exclaimed: "It looks just like a swarm of bees!" Hence, the reason some astronomy books call the cluster "Beehive," while others still call it "Praesepe."

Interestingly, Praesepe was also used in medieval times as a weather forecaster. It was one of the very few clusters that were mentioned in antiquity. Aratus (around 260 B.C.) and Hipparchus (about 130 B.C.) called it the "Little Mist" or "Little Cloud." But Aratus also noted that on those occasions when the sky was seemingly clear, but Praesepe was invisible, that this meant that a storm was approaching.

Of course, we know today that prior to the arrival of any unsettled weather maker, high, thin cirrus clouds (composed of ice crystals) begin to appear in the sky. Such clouds are thin enough to only slightly dim the Sun, Moon and brighter stars, but apparently just opaque enough to hide a dim patch of light like Praesepe.

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