Graham asks :
> "So after 91 years everything repeats?"

Checking ... there are 3 cycles of 2.33 in 7 years and 2 cycles of 6.5 in 13 years, so the repeat is 7*13 = 91 -- gotcha. (Even though the Chandler wobble is not exactly 6.5)

I am applying a Hill/Mathieu equation so the repeat period is much longer in that situation. One has to figure out the repeat of this kind of waveform:


That's why sloshing is erratic, and is one of the reasons that this problem has not easily been cracked.

My latest model of combining the factors:

I have gained a weird insight into deciphering these waveforms after playing with them off and on over the last year. It probably helped that I stared at 2D and 3D diffraction patterns for hours on end during my grad school daze :)