From Astronomy to Ages

Nothing in Hipparchus's own work assigns cultural or religious meaning to the twelve roughly 2,150 year segments that the equinox point traces through the zodiac constellations as it precesses; that step belongs to a much later astrological literature, gathering force especially from the nineteenth century onward, that reads each segment as a distinct "age" governed by the character of its ruling sign. Nicholas Campion's History of Western Astrology traces how the idea of the Great Year as a framework for world history, already present in classical and Hellenistic sources in a more general sense, was progressively reinterpreted through the zodiacal ages scheme familiar today, culminating in the widespread twentieth century association of the coming Age of Aquarius with cultural transformation.

Because the twelve zodiac constellations are not evenly sized in the sky, dividing the roughly 25,772 year cycle into twelve equal 2,150 year segments is itself a simplifying convention rather than an observational fact, and different writers who instead try to match segments to the constellations' true boundaries arrive at different age lengths and different transition dates. This is the root of the disagreement over when each age begins and ends, discussed on the individual age entries below, and it means that no version of the ages scheme currently in circulation can claim to be the single astronomically correct one.