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How could a great year be explained in laymen's terms?

According to the definition in NASA's Aerospace Dictionary it is a complete cycle of the equinoxes around the ecliptic that completes in 26000 years:

GREAT YEAR. The period of one complete cycle of the equinoxes around the ecliptic, about 25,800 years. Also called platonic year. See precession of the equinoxes.

NASA SP-7, 1965

Does this mean that every X years the March 21st and September 22nd will be in a different part of the zodiac and it will take 26000 years to make a full circle?

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  • $\begingroup$ There's no better explanation than in the definition. Did you look up the link to "precession of the equinoxes"? For more detail, you should also look up "precession of the equinoxes" in Wikipedia. In summary: yes, every year the vernal equinox points to a slightly "earlier" (retrograde) position on the ecliptic, averaging about 0.014º change per year: if you live to 100, it will have moved a mere 1.4º. It's currently in Pisces. $\endgroup$
    – Chappo Hasn't Forgotten Monica
    Dec 18, 2018 at 0:20

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Only since February 2018 there is a Wikipedia Page about the Great Year which is indeed a good read, but I think the German wiki version has a more concise summary, here an (approximate) translation with my highlighting:

A great year or platonic year is the time during which the vernal equinox moves once through the zodiac. The designation of this cycle of precession as the Platonic Year derives from a somewhat unclear passage in Plato's dialogue Timaeus, in which Plato seemed to be referring to a movement of the fixed star sphere.

The time needed for such a rotation is $25\,772\, {\rm yr}$.

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