The Western astrological sign Cancer of the tropical zodiac (June 21 - July 22) differs from the astronomical constellation of Cancer and the Hindu astrological sign of the sidereal zodiac (July 21 - August 9).
In the tropical zodiac it represents the balmy expansiveness of mid summer. In the sidereal zodiac it is associated with the myth of the Lernaean Hydra, one of The Twelve Labours of Herakles.
In some cosmologies, Cancer is associated with the classical element Water, and thus called a Water Sign, forming Watery Trigon with Scorpio and Pisces. It is also one of the four cardinal signs (along with Aries, Libra, and Capricorn).
It is the domicile of the Moon or "House of the Moon", and this concept might originate from an ancient belief that Moon was located here at the creation of the world. Cancer is also the exaltation of Jupiter, and the traditional astrology claims it is the triplicity of Mars, one of the three lesser dignities which the modern astrology does not recognize.
Since Cancer is the opposite sign of Capricorn, which is the domicile of Saturn and the exaltation of Mars, Saturn naturally becomes detriment in Cancer, and Mars, although has the dignity of triplicity, becomes fall.
Many modern astrologers consider it to be significant that Pluto was discovered on the longitude of 20 degrees Cancer and the first atomic bomb was detonated at the moment when Saturn reached this longitude. Saturn is said to become particularly malefic in Cancer and Leo, for they are opposed to Saturn's own domiciles. Richard Saunders writes in Astrological Judgement and Practice of Physick (1677) that the Black Plague of 1593 was caused by Saturn's transit through Cancer, thus breeding "Flegm, thin Water, and Melancholy mixt".
Each astrological sign is assigned a part of the human body, viewed as the microcosmic counterpart of the heavenly signs. Cancer rules the breasts and belly. Caesius likened the sign to "the Breastplate of Righteousness" in Ephesians VI:14.
Elsbeth and Reinhold Ebertin extended this concept, claiming that every single one of the zodiac's 360 degrees corresponds to a specific part of human anatomy. In their Anatomische Entsprechungen der Tierkreisgrade (Anatomical Correspondences of the Zodiac Degrees, 1949), the section about Cancer starts by correlating the first degree of Cancer with the sixth rib and ends by correlating the 30th degree of the sign with the twelfth dorsal vertebra.
In the 1970s there was a proposal to rename the zodiac sign, as some astrologers felt that an imagined association with the disease Cancer was off-putting. Some people with the sun sign of Cancer refer to themselves as "Moonchild" instead, the sign being ruled by the Moon in astrology.