|Meaning:||The Crab||Print out the star map from Sky and Telescope
Things to look for:
Note: [x.x] indicates visual magnitude
|Associated Asterisms:||The Asses and the Manger|
|% of the sky:||1.23|
|Visible Stars:||23 stars brighter than magnitude 5.5|
The term ‘Tropic of Cancer’ is absed on this constellation but its original definition no longer applies due to the slow precession of the Summer solstice. A few thousand years ago the Sun was in Cancer when it was highest in the sky each year and overhead at a certain latitude on Earth; this became known as the Tropic of Cancer. Now the Sun visits a month later at the start of August.
The constellation appears to the Northern Hemisphere observer as an upside down Y although its a bit more like a stretched K and is seen as a Crab scuttling from West to East. Zubanah ι Cnc to the Northeast and Acubens, α Cnc to the Southeast mark the front claws of the crab. Asellus Boraelis, γ Cnc and Tarf, β Cnc are the central carapace or eyes.
Cancer is on the ecliptic and so often hosts planets within its borders so is a good constellation to know how to locate. Its stars aren’t very bright though so is often overlooked. One object, Praesepe or the Beehive Cluster is visible to the naked eye and is designated as Messier 44. It is an open cluster and was written about by Galileo in his work Sidereus Nuncius as he resolved the smudge into individual stars with his telescope.
Cancer hosts the δ Cancrids which peaks on 17th January each year although it only sports a maximum zenithal hourly rate of 4.
Cancer only contains two stars above 4th magnitude and so is the dimmest of the ecliptic constellations. It can be found sitting between Gemini and Leo. Alternatively, travel from the star Capella, between Castor and Pollux to find the constellation.
The constellation appears in Greek mythology as the crab Karkinos within one of the Twelve Labors of Hercules. Whilst Hercules battled the multi-headed Lernaean Hydra, Karkinos attempted to distract Hercules having been told to by Hera. Karkinos got booted into the heavens for his troubles.
Other than M44, the only other Messier object in Cancer is M67, another open cluster but dimmer than its sibling.
The diagrams below show each of the objects with a set of Telrad circles overlaid. The outer circle is 4°. The inner circle is 0.5°.