# Definition:Time/Unit/Great Year of Plato

## Definition

The great year of Plato is a derived unit of time.

 $\ds$  $\ds 1$ great year of Plato $\ds$ $=$ $\ds 12 \, 960 \, 000$ days $\ds$ $\approx$ $\ds 36 \, 000$ years (of $360$ days)

## Source of Name

This entry was named for Plato.

## Historical Note

The great year of Plato was never actually mentioned by Plato himself.

The concept was raised in the context of the precession of the equinoxes, supposed to have been discovered by Hipparchus of Nicaea.

It was Claudius Ptolemy who (mistakenly or dishonestly) who determined that the time period was $12 \, 960 \, 000$ days of $360$ years.

The actual period is approximately $25 \, 800$ years, referred to as a Platonic year.