A period or series of events or numbers which recur everlastingly
in precisely the same order.
Cycle of the moon,
called “Meton's Cycle,” from Meton, who discovered it, is a period
of nineteen years, at the expiration of which time the phases of the
moon repeat themselves on the same days as they did nineteen years
previously. (See Callipic Period.)
Cycle of the sun.
A period of twenty-eight years, at the expiration of which time
the Sunday letters recur and proceed in the same order as they did
twenty-eight years previously. In other words, the days of the month
fall again on the same days of the week.
The Platonic cycle
or great year is that space of time which elapses before all
the stars and constellations return to any given state. Tycho Brahët
calculated this period at 25,816 years, and Riccioli at 25,920.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894