Great Year

The term Great Year has multiple meanings. In scientific astronomy, it refers to the time required for the equinoxes to complete one full cycle around the ecliptic, a period of approximately 25,800 years. According to Ptolemy, his teacher Hipparchus discovered this phenomenon by comparing the position of the vernal equinox against the fixed stars, noting that it shifts westward by about one degree every 72 years. This means that a full cycle through all the zodiac constellations takes roughly 25,920 years. In the heliocentric model, this precession can be visualized as the Earth’s rotational axis slowly tracing a circular path around the normal to the plane of the ecliptic. Currently, Earth's axis points close to Polaris, the North Star, but due to precession, this alignment is temporary and will shift over time, returning only after one complete Great Year has passed.

By extension, the term "Great Year" can be used for any concept of eternal return in the world's mythologies or philosophies. Historian Otto Neugebauer writes:

The difficulty with the term "great year" lies in its ambiguity. Almost any period can be found sometime or somewhere honored with this name.