Karka is the fourth rashi, the Vedic sign of the crab, corresponding to Cancer. In jyotisha it is ruled by the Moon (Chandra), the only sign the Moon governs, and it is the sign of Jupiter's exaltation. Its quality is movable (chara), the cardinal mode, and its element is water (jala tattva). Jyotisha reads Karka through the Moon's nurturing, protective, emotional nature and the crab's shell: home, mother, feeling, and the instinct to shelter. As a sidereal sign it is fixed to the stars and runs about 24 degrees behind tropical Cancer. The Moon's rulership and Jupiter's exaltation here are dignities shared with Western astrology. In a Vedic chart the strength of the Moon in Karka is weighed heavily, since the Moon is the chief significator of the mind.
Karka, crab, is a direct translation of the Greek karkinos, carried into Sanskrit through the Yavanajataka, a versification of a Greek astrological text datable to roughly the second through fourth centuries CE. The twelve equal thirty-degree signs this name belongs to were fixed centuries earlier in Babylonian celestial divination of roughly the fifth or fourth century BCE, a Mesopotamian scheme with no precedent anywhere in the older Vedic corpus, where no twelve-sign zodiac appears at all. The Moon's sole rulership of Karka and Jupiter's exaltation here, dignities jyotisha shares with the equivalent tropical Cancer, belong to a later classical layer, generally credited to compilations such as the Brihat Parashara Hora Shastra of roughly the seventh or eighth century CE, a text pseudepigraphically ascribed to the sage Parashara.
Sources Brihat Parashara Hora ShastraSage Parashara (attributed) with The Yavanajataka of SphujidhvajaDavid Pingree, Jyotihsastra: Astral and Mathematical LiteratureDavid Pingree