Cancer, the Crab

Everyone has heard of this constellation, because of all that astrology nonsense, so you might think it should be easy to see. But when the ancient astrologers divided up the Sun’s yearly path into twelve, the real sky didn’t necessarily provide nice neat star patterns to match. So they had to make do with a rather faint group of stars, in fact the faintest in the whole Zodiac.

Back then it didn’t matter too much that they were faint – everyone could see them. But today, with all our wonderful streetlighting and other attempts to turn night into day, the stars of Cancer are virtually invisible. If you have trouble finding it, work on the basis that it is roughly midway between Pollux in Gemini and Regulus in Leo, two constellations that are easy to find in the spring sky in the northern hemisphere.

Cancer is Latin for Crab. Tumours in the body can also resemble crabs, which is why the name is also used for the disease. As always with constellations, there’s a mythological background, and this luckless creature was trodden on by the hero Perseus as he was fighting the monster with many heads, Hydra. Can’t blame him really: crabs had better watch out when there is a fight with a multi-headed monster under way.

Map of Cancer

For hints on understanding the star map, click here.

Cluster M44, with the Donkeys to its left

Things to look for in Cancer

There are two nice star clusters in Cancer, which make up for the general faintness of the rest of the constellation. The brightest and easiest to see has the catalogue number M44, but it has three names – the Beehive, the Manger and Praesepe.

The Beehive is easy enough to figure out, because it really does resemble a swarm of bees; but the Manger bit comes from the names of the two stars on either side of it, Asellus Borealis and Asellus Australis. Asellus means donkey (ass – get it?) and Borealis and Australis are north and south. So now you’ve learned some Latin as well as some astronomy. And here comes another bit. The Latin for Manger is Praesepe (pronounced pry-see-pee), hence its third name.

You can see the cluster with the naked eye in a reasonably dark sky, and it’s even visible with with a certain amount of moonlight around. Through binoculars you can see a dozen or more stars, and it’s a really pretty sight, with the two donkey stars flanking it.

Cluster M67
Close-up of M67. The bright star to its left is Alpha Cancri

The other cluster in Cancer, M67, is a lot harder to see from the suburbs, but in a country sky it’s plain enough. Move your view down twice the distance between the Donkeys and you should spot it just to the right of the star Alpha Cancri. With higher magnification binoculars or small telescopes you can just see a faint speckle of stars.

The ecliptic runs through Cancer. This is the track of the Sun, and the Moon and planets can also be found fairly close to it, so from time to time there might be an extra bright planet in the constellation.

Text and photographs by Robin Scagell