Axial precession, the slow conical wobble of Earth's rotation axis, was identified by the Greek astronomer Hipparchus of Rhodes around 127 BCE, who compared his own measurements of stellar positions against earlier observations, chiefly those of Timocharis a century and a half before him, and found that the equinox and solstice points had shifted against the fixed stars. He estimated the rate at roughly one degree per century; the modern figure, refined by centuries of subsequent observation, is closer to one degree every seventy two years, giving a full cycle of approximately 25,772 years. James Evans's history of ancient astronomy places this discovery among the genuine technical achievements of Hellenistic science, made without a heliocentric model and well before the physical cause, the gravitational pull of the sun and moon on Earth's equatorial bulge, was understood.
The term Great Year itself is older than Hipparchus's discovery: Plato's Timaeus speaks of a "perfect year" in which all the heavenly bodies return to their original relative positions, a concept rooted in observed planetary cycles rather than in precession specifically, since Plato wrote roughly two and a half centuries before Hipparchus measured the effect. Later astrological and esoteric writers have frequently folded Plato's perfect year and Hipparchus's precessional cycle into a single idea, and the two are commonly treated as equivalent in the ages literature discussed below, even though only the latter was ever an empirical astronomical measurement.