Cancer: Hera's crab and the donkeys' manger

Among the twelve signs of the zodiac, Cancer is perhaps the hardest to find in the sky. Its stars are faint, wedged between the bright figures of Gemini and Leo, and in cities they almost vanish entirely. Yet this constellation gave its name to one of the fundamental lines of geography, the Tropic of Cancer, and it holds a small treasure that the ancients knew well: a cluster of stars called the Manger.

A crab against Heracles

The myth explaining the crab's presence among the stars is linked to the second labour of Heracles. The hero was fighting in the marsh of Lerna, in the Argolid, against the Hydra, the monstrous serpent with many heads that grew back as soon as they were cut off. Hera, who hated Heracles as the illegitimate son of Zeus, sent a giant crab, Karkinos, to help the monster, and it bit the hero on the foot.

Heracles crushed it with no difficulty and carried on the fight, defeating the Hydra with the help of his nephew Iolaus, who seared the severed necks to stop the heads from growing back. According to the tradition reported by mythographers, Hera wished to reward the little creature for its loyalty by placing it in the sky. The Hydra itself met a similar fate: it is the very long constellation stretching below Cancer and Leo.

The Manger and the Donkeys

At the centre of Cancer, on a dark night, a milky patch can be seen: it is the open cluster the Greeks called Phatne, “the manger”, known in English as the Beehive or Praesepe. On either side lie two stars called Asellus Borealis and Asellus Australis, the little donkeys. The picture is consistent: two animals feeding at the manger. Aratus already mentions it, and observed that when the cluster grows dim it is a sign of coming rain.

A legend reported by Eratosthenes and Hyginus explains the donkeys. During the war between the gods and the Giants, Dionysus, Hephaestus and the satyrs are said to have arrived riding donkeys; their braying, never heard before, put the frightened Giants to flight. Out of gratitude the donkeys were placed in the sky beside their manger. Galileo, in the Sidereus Nuncius of 1610, turned his telescope on the Praesepe and saw it resolve into dozens of stars.

The sign of the solstice

In antiquity the Sun reached the northernmost point of its yearly path, the summer solstice, while crossing this constellation. The terrestrial parallel over which the Sun passes at the zenith on that day was therefore called the Tropic of Cancer, from the Greek for “to turn”: there the Sun seems to reverse its course. Because of precession the solstice now falls in Gemini, but the name has remained.

The crab, which walks sideways and withdraws into its shell, lent itself to many symbolic readings. Astrological tradition linked the sign to water, to the Moon and to the home, to the idea of protection and return. The myth, more simply, tells the story of a small animal that challenges a hero in obedience to a goddess, and that precisely through this stubbornness earns an eternal place in the sky.