Relevant to current discussions on the talkshop concerning changes in Earth’s length of day (LOD) and the effect of planetary orbital resonances on the Moon’s orbital parameters and Earth climatic variation; this is a repost from Ian Wilson’s excellent Astro-Climate-Connection website. Ian very generously opens with a hat tip to this blog, (at which he is one of the ‘collaborators’ he mentions).
Connecting the Planetary Periodicities to Changes in the Earth’s LOD
Monday, October 14, 2013 : Ian Wilson PhD
[(*) Some of the findings in this blog post concerning the connection between the Earth’s rotation rate and the planetary configurations have also been independently discovered by Rog “Tallbloke” Tattersall and his collaborators]
A. The Connection Between Extreme Pergiean Spring Tides and Long-term Changes in the Earth’s Rotation Rate as Measured by the Rate-of-Change of its Length-of-Day (LOD). (*)
If you plot the rate of change of the Earth’s Length of Day (LOD) [with the short-term atmospheric component removed] against time [starting in 1962] you find that there is a ~ 6 year periodicity that is phase-locked with the 6 year period that it takes the lunar line-of-nodes to re-align with the lunar line-of-apse [see the first note directly below and reference  for a description of the method used to determine the time rate of change of LOD].
NB: The pro-grade precession of the lunar line-of-apse once around the Earth with respect to the stars takes 8.8504 Julian years (J2000) while the retrograde precession of the lunar
line-of-apse line-of-nodes once around the Earth with respect to the stars takes 18.6000 Julian years (J2000). Hence, the lunar line-of-apse and the ascending node of the lunar line-of-nodes will realign once every:
(18.6000 x 8.8504) / (18.6000 + 8.8504) = 5.9969 Julian years