The Lower Back and Kidneys Under Libra

Libra takes the lower back, the loins and the kidneys, and this is one of the places where the zodiacal and planetary schemes agree partially, since Libra is ruled by Venus and the planetary scheme also gives the kidneys to Venus. The reasoning offered in the tradition is the paired and balanced character of the organs, which filter and regulate the fluids of the body and hold it in equilibrium, matching the scales of the sign and the temperate nature of its ruler. Saturn's exaltation in Libra was read as adding the sign's association with the bones of the lower back. The correspondence is a good example of how the two schemes were harmonised where they could be and simply used in parallel where they could not.

Libra is the one sign of the twelve with no animal or human form of its own, its symbol a pair of scales rather than a living thing, and some astrological writers took this as fitting the region it governs: the kidneys perform a mechanical, regulating function, filtering and balancing the body's fluids, which was read as more akin to the impersonal operation of a scale than to the animate qualities the animal signs were thought to lend their regions. The reading is an interpretive gloss rather than a claim found stated outright in Manilius or Ptolemy, and it is recorded here as part of how later astrologers rationalised the scheme rather than as part of its classical foundation.