Part of the Casswiki article series Cassiopaean Experiment
In the Cassiopaean material, the grand cycle refers to a period of approximately 309,000 years.
The Wave, or realm border, which is a cosmic phenomenon facilitating passage between densities (and which is usually accompanied by physical cataclysm is said to pass by Earth at such intervals.
The period of roughly 309,000 years (the more precise number 309,882 years was also given), is close to 12 full cycles of precession of the equinoxes. The equinox precesses about 1 degree every 72 years, amounting to roughly 2 millennia per zodiac sign. Whether the precession of the equinoxes has anything to do with the grand cycle is unclear.
According the the Cassiopaean material, the mythical fall of man took place at the previous passage of the Wave, roughly 309,000 years ago.