Cancer
The Crab
Between the brighter zodiac constellations of Gemini and Leo hides the celestial Crab, Cancer. There are only six stars bright enough to be discerned without optical aid, with the brightest, Acubens, only of about 4.5 magnitude. So this most inconspicuous of the zodiac figures is hiding, indeed. However, Cancer is important in other ways, and it has a number of objects that are worth looking out for.
In early Greek times, the Sun was seen to move northward, day by day, throughout the spring. Then it slowed and, like a crab, appeared to sidle along turning to head south for the winter. The point in the sky where the Sun made this turnabout gave the crab’s name, Cancer, to that portion of the heavens. The circle of latitude on Earth’s surface where the Sun was directly overhead was thus called the Tropic of Cancer. Since those early times, the ecliptic point of summer solstice has moved westward out of Cancer and nearly all the way through Gemini, almost to the border of Taurus.
The Beehive cluster, also called the Manger, or M44 in the Charles Messier catalogue, is the most noteworthy of Cancer’s objects. This star group is so large it was known in antiquity, when it was thought to be a non-stellar object such as a nebula. In ancient times, the Japanese had regarded the cluster as a lump of souls, and they had been terrified by the sight of it. The Chinese title Tseih She Ke, means “the last exhalation of piled-up corpses.” To the unaided eye, the Beehive glows softly at magnitude 3.4 and spans more than two full moon diameters across the sky—see José Olivarez’s column on page 3. Galileo was the first to resolve the cluster into individual stars, with his new invention of the “optick tube” in 1610.
Interestingly, the visibility of the Beehive was used to help forecast the weather in ancient times. In 300BC, Aratus and Theophrastas noted that the approach of a storm was signaled when the cluster was invisible in an otherwise clear sky. Actually, any high cirrus clouds (clouds made of ice crystals) will effectively hide the cluster, and we know now that the presence of high cirrus clouds is often the first sign of an approaching warm front. Does the disappearance of the Beehive in an otherwise clear sky predict rain? Check your observations against the weather forecast.
Another Messier cluster, M67, is also found in Cancer. More compact than the Beehive, M67 needs a medium to large telescope to view its 200 stars which vary from 10th to 16th magnitude. Unique to this cluster is the age of its stars, considerably older than stars contained in other clusters and variously reported as from 3 to almost 10 billion years of age. A medium telescope also shows some of the stars’ colors, subtle blue-white, yellow and orange—a fine sight on a clear dark night.
Another object, for those with a telescope of 4-inches or greater, is the triple star system Zeta Cancri. The two brighter members, Zeta A and B (magnitude 5.6 and 5.9), orbit each other once every 60 years and are about 19 astronomical units (AU) apart, approximately the distance between the Sun and Uranus. The third member of the group is Zeta C, which lies some 175 AU from Zeta A and B.


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