Just as we have the solar (tropical) year of 365.242 days, there is also another longer, less known cycle. It is known as the Great Year. The Great Year is a cycle of 25,772 years and it is based on a movement of the Earth called precession. This movement is similar to the movement of a spinning top. The axis of the Earth traces 2 cones, which touch each other at their apices. There are a couple of visible effects (although it takes a long time for these effects to be apparent).
**Author: Ereenegee
License: CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0/legalcode**
One effect is the position of the Sun in relation with the constellations of the ecliptic. As time passes, at the time of the Spring Equinox, the Sun changes its position. Currently, the Sun, at the time of the Spring Equinox is in the region of the constellation of Pisces. In a few hundred years, the Sun, at the time of the mentioned Equinox, will be in the region of the constellation of Aquarius. This is why it is said that we are currently in the era of Pisces and will be moving to the era of Aquarius. Due to precession, it will take the Sun 25,772 years to be at the exact same position in relation to the constellations as it is today.
Another effect is the change in the position of the North Star. Today, Polaris is the North Star. In 12,000 years, Vega will be the North Star.
Plato mentioned the concept of Great Year first, however, he referred to the cycle of the position of the known planets of his time. A Great Year, according to him, will have occurred when all the planets return to their same relative positions in the sky.