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.