Can you help me brainstorm some events that are supposed to happen on a predictable regular cycle, several years apart? In addition to:
* Leap year happens every 4 years. (US Presidential elections are in leap years.)
* You're supposed to get a tetanus booster shot every 10 years.
* Periodical cicadas emerge every 17 years.
* Halley's comet returns every 74 to 79 years.
This is for a project that I'm working on. What other cycles like this can you think of?
@frameacloud Chinese zodiac, cycles through itself every 12 years (and through all combinations of signs + associated elements every 60).
@frameacloud Ah, makes sense! Hmmm.
The sun, the moon and the Earth take roughly 18 years to return to (more or less) the same positions; this is used to predict eclipses.