The following image is taken from Wikipedia's article on the leap second.

Deviation of day length from SI based day with shorter days resulting from faster planetary rotation

Why is the slowest point each year in the middle of the year around July? Does being further from the Sun cause the Earth's rotation to slow down? What's the mechanism in play here?

  • $\begingroup$ This graph covers several centuries and the contribution of any annual cycle is small. How does it show the phase of the annual cycle as you suggest? $\endgroup$
    – CWPP
    Commented Aug 3, 2022 at 5:03
  • $\begingroup$ @CWPP There is a clear minimum in the middle of each year division. $\endgroup$ Commented Aug 3, 2022 at 5:26
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    $\begingroup$ As far as I know the Earth's angular velocity does not change throughout the year. The variation shown in the graph must depend on how the day is defined, but unfortunately the graph does not explain exactly what is being plotted on the y axis. $\endgroup$ Commented Aug 3, 2022 at 5:28
  • $\begingroup$ @JohnRennie The image on wikipedia points to to this document hpiers.obspm.fr/eoppc/eop/eopc04/eopc04.62-now, which tabulates measurements of UT1 - UTC. That's probably what's being graphed and there is another graph on the same page of UT1 - UTC that also shows an annual oscillation. UTC is TAI ("real" atomic clock time) plus some leap seconds. UT1 is the number of turns/angle of the Earth w.r.t. the celestial sphere plus scaling and shifting to make it look like a date. $\endgroup$
    – HTNW
    Commented Aug 3, 2022 at 9:30
  • $\begingroup$ @CWPP No, that graph covers 6 decades. Here's the SVG version: upload.wikimedia.org/wikipedia/commons/5/5b/… $\endgroup$
    – PM 2Ring
    Commented Aug 3, 2022 at 9:38

1 Answer 1


I don't have access to the paper but the abstract from Carter(1984) suggests that the major cause of rotational variations is due to "the exchange of angular momentum between the atmosphere and the mantle."

This is echoed by Earth Rotation Variations from Hours to Centuries:

Variations with periods of five years or less are driven primarily by exchanges of angular momentum with the atmosphere

One could hypothesize that the middle of the year corresponds with some weather pattern that maximizes atmospheric momentum.


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