Cancer is traditionally the fourth constellation out of 12 in the Zodiac. It represents a crab most popularly, but has also been depicted as a lobster, crayfish, or water beetle in several Medieval astrological texts. In other cultures, it represented a Scarab (Egypt) or Turtle (Mesopotamia). It is a Water-element sign and a Cardinal sign (signs considered dynamic and able to initiate things). Cancer governs the 4th House (rules home, domestic affairs, heritage, roots) and it is ruled by the celestial body The Moon. Cancer is a dim constellation (the dimmest Zodiac constellation) bounded by Gemini, Leo, Hydra, Lynx, and Canis Minor. Its only named stars are (from Alpha to Zeta): Acubens, Altarf, Asellus Borealis, Asellus Australis, Praesepe, Tegmen. Traditionally, people born from June 21 (or June 22, depending on when the Summer Solstice falls) to July 22 are considered Cancer. Cancer is Latin for "Crab".
In Greek mythology, Karkinos (Greek "crab") or simply an unnamed crab, was sent by Hera to attack Hercules while he was fighting the Hydra. There are varying accounts of its size in this story, some depicting it as a regular sized crab, others as a giant crab. Regardless, it ends up being crushed by Hercules. Hera memorializes the creature's efforts by placing it in the sky as the constellation Cancer.