This is a table describing geophysical periods impacting the Earth's nutation (from [1] below)


The calculation appears to use the Delaunay arguments, which is essentially a sum of frequency components. My problem is the first entry. That has one component which appears to be close to the Draconic lunar month of 21.212 days. And I think that is what it should be according to other sources; yet it says that it is 27.20986 days.

Is this a typo or am I missing something fundamental in how the Earth responds to the lunar cycle? This frequency should be synched solidly to the observed lunar frequency, otherwise it will gradually go out of phase. The difference between the two gives a time of over 400 years before the two numbers phases cancel, so it is a slight difference but significant over long intervals.

So its not much of a gap, and in fact the 27.20986 number is closer to what I am seeing as an optimized aliased frequency component in the QBO. In other words, the optimal fit occurs for around this value.

According to the paper *F* is the difference between the mean longitude of the Moon and the mean longitude of the node of the Moon. That sounds like the same definition for the Draconic month, if the reference is the Earth.


This is an older paper, but still, these geophysicists work on the numbers to make them more precise, and being sloppy about it completely defeats that purpose. So I have to treat this with number with due respect.

Anybody have any ideas on the discrepancy?

[1] C. Bizouard, M. Folgueira, and J. Souchay, “Comparison of the short period rigid Earth nutation series,” presented at the IAU Colloq. 178: Polar Motion: Historical and Scientific Problems, 2000, vol. 208, p. 613. [Online](