Precession (Babylonian-Stratum)

Definition

Precession of the equinoxes is the very slow westward slide of the equinox points along the ecliptic — the Sun's yearly path. It moves about one degree every 72 years, so it takes roughly 25,772 years to come full circle. The cause is the gravitational tug of the Sun and Moon on Earth's slightly bulging equator. The Greeks credit the discovery to Hipparchus in the 2nd century BCE, and Ptolemy gave a slow figure of 1° per century in Almagest VII.

In Tradition

Read at the Babylonian stratum, Babylonian mathematical astronomy did not recognise precession at all. Rochberg shows that its two computational schemes fixed the equinox and solstice points against the stars — at 10° Aries in System A, 8° Aries in System B — and that those anchors stayed put throughout the systems' use, so the slow drift went unnoticed. The Babylonian zodiac is therefore sidereally fixed, counted from a star-anchored Aries 0° — unlike the Greek tropical zodiac, tied to the moving spring equinox.

In Practice

For a historian tracing how the idea travelled, the Babylonians' non-recognition of precession is a sharp contrast with later Greek astronomy. The Babylonian zodiac worked as a sidereal mathematical reference grid — twelve 30° segments counted from a star-anchored Aries 0° — with no need to track equinox drift, because its anchor points were fixed to constellations, not to the equinox. This is the root of the Babylonian-versus-Greek zodiac split that still drives the modern tropical-versus-sidereal debate: tropical astrology inherits the equinox-anchored zero point of Hipparchus and Ptolemy and so tracks precession, while sidereal systems — Indian astrology, and the modern Western sidereal revival — inherit the Babylonian convention. The rate of 1° per 72 years comes from observation, worked out from the roughly 25,772-year full cycle; Ptolemy's slower 1° per century, preserved in Almagest VII, is an undercount.

Historical Origin

The Greek discovery of precession is credited to Hipparchus (2nd century BCE) and preserved in Ptolemy's Almagest VII. The Babylonian non-recognition of precession is documented by Rochberg in Heavenly Writing (2004), Ch. 3-4 — System A norming the equinox at 10° Aries, System B at 8° Aries, both anchor points sidereally fixed throughout their use. This entry takes the Babylonian-stratum view; the Western primary lens lives at philosophy.ts:292.

Definition

In Tradition

In Practice

Historical Origin

Further Reading