The crab is a minor character in one of the labours of Heracles (the Greek name for Hercules). While Heracles was fighting the multi-headed monster called the Hydra in the swamp near Lerna, the crab emerged from the swamp and added its own attack by biting Heracles on the foot. Heracles angrily stamped on the crab, crushing it. For this modest contribution to history, we are told that the goddess Hera, the enemy of Heracles, put the crab among the stars of the zodiac.
Fittingly enough for such a minor character, it is the faintest of the zodiacal constellations, with no star brighter than fourth magnitude. The star Alpha Cancri is named Acubens, from the Arabic meaning ‘claw’. As Ptolemy described it in the Almagest, this star lies on the southern claw of the crab; the northern claw is marked by Iota Cancri. Beta and Mu Cancri lie on the southern and northern rear legs, respectively.
Cancer, from the Uranographia of Johann Bode (1801). At its centre lies the star cluster Praesepe, flanked on the north and south by the stars Asellus Borealis and Asellus Australis (Gamma and Delta Cancri).
The asses and the Manger
Gamma and Delta Cancri are named Asellus Borealis and Asellus Australis, Latin names meaning the ‘northern ass’ and ‘southern ass’, and they have their own legend. According to Eratosthenes, during the battle between the gods and the Giants that followed the overthrow of the Titans, the gods Dionysus, Hephaestus and some companions came riding on donkeys to join the fray. The Giants had never heard the braying of donkeys before and took flight at the noise, thinking that some dreadful monster was about to be unleashed upon them. Dionysus put the asses in the sky, either side of the cluster of stars which the Greeks called Phatne (Φάτνη), the Manger, from which the asses seem to be feeding. Ptolemy described Phatne as ‘the nebulous mass in the chest’. Astronomers now know this star cluster by its Latin name Praesepe, but it is popularly termed the Beehive ( praesepe can mean both ‘manger’ and ‘hive’).
Tropic of Cancer
The tropic of Cancer is the latitude on Earth at which the Sun appears overhead at noon on the summer solstice, June 21. In the time of the ancient Greeks the Sun lay among the stars of Cancer on this date, but the wobble of the Earth on its axis called precession has since moved the summer solstice from Cancer through neighbouring Gemini and into Taurus.
In China the four stars surrounding the open cluster Praesepe (Delta, Gamma, Eta and Theta Cancri) were known as Gui, literally ‘Ghosts’, referring to the spirits of the deceased. The 23rd Chinese lunar mansion was named Gui after this asterism. Praesepe itself was called Jishi, a group of corpses. Praesepe and its four surrounding stars were sometimes seen as a ghost being carried in a sedan chair; hence this quadrilateral was also given the alternative name Yugui, Ghost Wagon.
To the north of Gui was another quadrilateral of stars forming a figure called Guan, a beacon fire on a watchtower; however, sources differ as to which four stars were involved and indeed the Chinese themselves probably used different stars at different times. The usual version places Guan in the area around Chi Cancri while another favours the region of Iota Cancri. A chain of four stars that crossed into Cancer from Canis Minor was Shuiwei, ‘water level’.
© Ian Ridpath. All rights reserved