Cancer is the fourth sign, beginning at the summer solstice, the moment when the Sun reaches its greatest northern declination and the days stop lengthening. It is cardinal, watery and the only sign ruled by the Moon, with Jupiter taking its exaltation there in the classical scheme. The solstitial position is the key to the traditional reading: the sign stands at a turning point, the top of the year, where growth is complete and the movement reverses, and the tradition gives it the meanings of shelter, containment, nourishment and the beginning of return. The Tropic of Cancer takes its name from this position, marking the latitude at which the Sun stands overhead at the June solstice, though precession has since carried the solstitial point out of the constellation and into Taurus and Gemini.
Cancer's three decans run Venus, Mercury, Moon, continuing the same unbroken Chaldean sequence from Gemini's final Sun decan. As a water sign Cancer's triplicity rulership is one of the clearest points of disagreement between the classical authorities: Ptolemy's own table in the Tetrabiblos gives Venus by day and the Moon by night for all three water signs, with Mars added as a subordinate co-ruler, while the more widely used table transmitted by Dorotheus and followed by William Lilly drops Venus and the Moon entirely for water and assigns the whole triplicity to Mars alone, day and night. The two systems genuinely conflict, and a horary astrologer's judgment of a water-sign planet's dignity can differ depending on which table is in use.