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Using PyEphem, I calculated the time difference between consecutive summer solstices in the south hemisphere for the last 4000 years, and plotted them in a graph. It seems like random noise around the mean, but the amplitude seems to oscilate with a period of around ~161 years. What explains this behavior?

enter image description here

Code used:

import ephem

date = ephem.Date("2023/01/01")
solstice = ephem.previous_winter_solstice(date)
for i in range(4000):
    previous_solstice = ephem.previous_winter_solstice(solstice)
    print(solstice - previous_solstice)
    solstice = previous_solstice
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    $\begingroup$ I suspect that it's due to some cycle involving the Moon, similar to what happens with perihelion & aphelion, as I explained here. $\endgroup$
    – PM 2Ring
    Jun 4 at 15:44
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    $\begingroup$ OTOH, just playing with various mean month & year lengths I can't make a simple 161 year cycle. The best I can do is 239 years, just under 1½ of your cycles. Roughly, 2956 synodic months = 3168 anomalistic months = 239 tropical years = 87293 days. sagecell.sagemath.org/… $\endgroup$
    – PM 2Ring
    Jun 4 at 15:45
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    $\begingroup$ The paper Fedorov, V.M. "Interannual variations in the duration of the tropical year." Seems to be about this. The abstract says that it is due to "perturbations" but the article is behind a paywall. $\endgroup$
    – James K
    Jun 4 at 16:15
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    $\begingroup$ If I include the synodic period of Jupiter (398.88404 days), then I get 6011 synodic, 6442 anomalistic, 445 Jupiter, 486 Tropical ~= 177508 days, which is intriguing. ;) $\endgroup$
    – PM 2Ring
    Jun 4 at 16:17
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    $\begingroup$ @JamesK: I have read the article, but it mentions no 161-year period. I suspect, as PM 2Ring does, that it has to do with Moon cycles. Multiplying the apsidal precession period (8.61 years) by the nodal precession period (18.6 years) gives 164.61 years, which is close to 161 years… $\endgroup$
    – Pierre Paquette
    Jun 4 at 22:14

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