Paul sent in a link to this story but see https://phys.org/news/2022-06-earth-feet-core-oscillates.html … the earth’s inner core oscillates. This contradicts previously accepted models that claimed it rotated at a faster rate than Earth does at the surface. The new study, in Science Advances, shows the inner core changed direction during a 6 year period, 1969-1974. Why that long ago? Well, back then the Soviet Union was conducting nuclear tests, and seismic data from them have proved useful. This may explain variations in the length of day, it is thought. The earth’s ssurfae shits as compared to its inner core, as various people have claimed. The latest findings also show the inner core span faster than the surface between 1969 and 1971, but switched speed between 1971 and 1974. The length of day grew and shrank in the process.
Our understanding of the inner core over the last 30 years has expanded. The seismic waves used were generated from underground nuclear bomb tests, from 1971-1974, and from 1969 at another location. The inner core is not fixed. It is moving back and forth a couple of km every 6 years. See https://www.science.org/doi/10.1126/sciadv.abm9916 …