Cancer constellation. Optical image of Cancer, the Crab, a zodiacal constellation that lies on the ecliptic between Gemini and Leo. In Greek mythology, the goddess Hera sent the crab to distract the hero Hercules while he was fighting the Hydra, a beast with many serpent-like heads during the 12 labours of Hercules. In different versions of the story, Hercules either kicks the crab into the night sky, or he kills it and Hera places it in the sky. Cancer is the faintest zodiacal constellation. None of the stars in Cancer are brighter than magnitude 3.5.