Cancer, the Crab
This constellation represents the crab that attacked Hercules when he was fighting the Hydra. The poor crab was crushed underfoot by mighty Hercules, and subsequently elevated to the heavens.
In ancient times in the northern hemisphere the Sun reached its most northerly point in the sky each year while it was in the constellation Cancer. The Sun reaches this point about June 21st, and this day is known as the summer solstice. On the summer solstice, the Sun appears overhead at latitude 23.5° north on Earth. This latitude is called the Tropic of Cancer. It retains this name today even though because of a wobble of the Earth called precession, the Sun now lies in the neighbouring constellation of Gemini at the summer solstice.
To find Cancer look north-east in the late evening sky, and find bright Regulus with the upside-down sickle below it, marking Leo the "Lion". To the left are bright Pollux and Castor and above them Procyon in Canis Minor. From a dark sky the fuzzy patch of M 44 will be apparent, and is really the main distinguishing feature of this otherwise inconspicuous constellation.
Chart showing the constellation as seen to the north at about 11.00 pm NZDT in mid March.The chart is also valid for about 8pm NZST mid April and 7pm at the beginning of May.
Details of some of the objects shown in the chart.
α Cancris (Acubens, meaning claws) is a magnitude 4.3 white star 174 light years away. It has an 11th magnitude companion visible in moderate telescopes.
β Cnc is a magnitude 3.5 orange giant star 290 light years away and the brightest star in the constellation.
γ Cnc (Asellus Borealis, Northern Donkey) is a magnitude 4.7 white star 158 light years away.
δ Cnc (Asellus Australis, Southern Donkey) is a magnitude 3.9 yellow giant star 136 light years away.
ζ Cnc is an interesting triple star. The period of the close pair is 59.7 years. The separation between the stars is appearing to widen and will reach its maximum in 2020. The period of the third star is probably many hundreds of years and there is probably also a white dwarf around the third star. The system is 83 light years away.
ι Cnc is 298 light years away and a double star that can even be made out in steadily held 7 power binoculars, and is an easy object for small telescopes. This magnitude 4.0 yellow giant star has a blue-white magnitude 6.6 companion.
M 44 (NGC 2632) Praesepe (The Manger) or the Beehive Cluster is a large open star cluster visible as a misty patch to the unaided eye from a dark sky site. It is best seen in binoculars or very small telescopes at low magnification to show the many stars that make up this cluster. It lies about 520 light years away and appears to cover about three times the size of the Moon. It was known to the ancient Greek Hipparchus, who called it a "Little Cloud". (Note the two Donkeys γ and δ flanking the Manger).
M 67 (NGC 2682) is a denser and smaller cluster than the Beehive, visible as a misty patch in binoculars or small telescopes. This is one of the oldest of the open clusters and a recent age estimate is about four billion years. It lies about 2700 light years away.
Cancer lies to the north of the equator, consequently it is above the horizon as seen from mid New Zealand for only some 9.5 to 10 hours each day. By the end of April the constellation will be due north at about 7 pm, that is by the time the sky is completely dark following sunset. The constellation remains visible in the early evening until June when it will be low to the north-west once the sky is dark.