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 there's also always orbital precession if you want a really long one.

@frameacloud for 20 yrs Neptune is within Pluto's orbital path. this occurs every 248yrs

@frameacloud *whoops, wrong way around. Pluto is within Neptune's orbital path during that time

@frameacloud Chinese zodiac, cycles through itself every 12 years (and through all combinations of signs + associated elements every 60).


Oh yeah, that's a good one! Astrology is great for cycles. For my project, I'm trying to not use anything astrological, but astronomical is good.

@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.

@frameacloud Is there a maximum cycle length to the data set you're looking for?


Yes, there is a maximum cycle length, thanks for asking. For my project, I'm looking for cycles that will happen at least once within the next 200 years.

@frameacloud Hebrew calendar has an extra month every two or three years as part of a 19 year cycle --

A lot of medical tests are supposed to be done every 3-5 years after a certain age (pap smear, colonoscopy, mammogram, etc)

@frameacloud there are lots of astronomical events like comment returns or eclipses that happened like this

@frameacloud Eclipse cycles:

It is possible to predict other eclipses using eclipse cycles. The saros is probably the best known and one of the most accurate. A saros lasts 6,585.3 days (a little over 18 years), which means that, after this period, a practically identical eclipse will occur. The most notable difference will be a westward shift of about 120° in longitude (due to the 0.3 days) and a little in latitude (north-south for odd-numbered cycles, the reverse for even-numbered ones).

(And a few other types of eclipse cycles: )

@frameacloud Tropical Storm Names.

A six-year cycle for both North Atlantic and East Pacific namesets. Central North Pacific and West Pacific both have their own lists which don't switch over with the year, but the Philippines have their own independent four-year cycle.

The North Indian Ocean has eight lists that don't switch to the next on a seasonal basis, just when the whole list is gone through, but there's a three-year cycle in the South-West Indian Ocean.

Indonesia, Papua New Guinea, Australia, the South Pacific, and the South Atlantic all rotate until exhausted (and then switch to the next list or start again from there.


Wow, cool! I don't think I'd heard of this one before. Thanks for sharing!


Cool! Eclipses are some of the coolest events that we can regularly predict about the future.

@frameacloud There are also 13-year periodical cicadas - I think I heard that cicada cycles tend to be prime numbers for reasons.


I was reading about how there are different groups of periodical cicadas, with different lengths of cycles. The largest brood of them is Brood X, which is on a 17 year cycle, but there are a lot of broods with shorter cycles. I hadn't heard about prime numbers being special to them, though, that's interesting!

@frameacloud I think it's so that there aren't shorter cycles that line up

like, 18-year cicadas would be caught by predators on a 2-year or 3-year cycle or a 6-year cycle, but 17-year cicadas will only line up with a 2- or 3- or 6-year cycle 1/2 or 1/3 or 1/6 of the time

@frameacloud mammogram every two years. Cholesterol levels every five years. Official publication of the US Code of Laws every six years.

A calendar for a leap year becomes reusable, that is to say, every date will fall on the correct day of the week, every 28 years. Other years repeat more often, in a pattern of 6, 11, 11. (Century-end leap year exceptions do disrupt these patterns.)

@frameacloud I think sunspots happen every eleven years. Need to check that with a solar scientist, though..

@frameacloud For similar reasons to cicadas (but with weaker selection bias towards longer intervals because trees live much longer), oak trees and many others have a strategy of biennial heavy seed production, keeping the population of their predators to what can be sustained by the off-year yield. I'd bet money that they synchronize within a species by chemical signaling, but don't know if we've figured out which one(s).

@frameacloud the 11 year solar activity cycle, and the 11.86 year orbital period of jupiter comes to mind

also in astronomy, precession of the equinoxes is on a 25772 year cycle

astronomy is a good source for long-period predictable cycles

in addition to 17 year cicadas, there are also 11 and 13 year cicadas, and probably some of other primes too

you get a new credit/debit card every five years, when the old one expires

@frameacloud someone liked this and i looked at it again and thought "wow that's a really mundane thing to end on after talking about astronomy"

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