How do we measure persistent changes to the length of a day on Earth? This seems like a maddeningly slippery problem.
- Geosynchronous sattelites? What if they're slightly off?
- Star readings? Can we measure that precisely enough from the surface to find tiny fractions of change?
- Any surface-based reading also has to deal with the non-spherical nahure of Earth, and its wobble (the motion that gives us seasons)
How do they do it?
1: https://en.m.wikipedia.org/wiki/Very-long-baseline_interfero...
Axial inclination plus revolving around the sun gives seasons, wobble is a separate effect (and internal forces—mantle convection is the one I’ve mostly seen cited but core movement would also be in this category—are a source of wobble.)
Anyway, it would be cool if this led us to understand where mantle plumes come from. I don't know how likey that is, but it seems that the dynamics between the core and mantle is the right place to look.
[0] https://www.science.org/doi/10.1126/sciadv.abm9916 <-- Just read the abstract of this instead
No, it's all like that.
Though I should note that the model of "the core is shifting position relative to the surface" is fully justified here. Here's the statement:
> “The inner core is not fixed — it’s moving under our feet, and it seems to going back and forth a couple of kilometers every six years,” Vidale said.
And here's the concept:
> Research published in 1996 was the first to propose the inner core rotates faster than the rest of the planet — also known as super-rotation — at roughly 1 degree per year.
But rotating at a speed that is so slightly different means exactly that the location on the surface of the core that is directly below a particular point on the surface shifts -- slightly -- over time. Positional change relative to the surface is what's happening.
And the perspective of positions on the core and surface moving relative to each other in some cylindrical coordinates may well be what the article meant, but they sure didn't make it clear, and it's ultimately ill-defined: the correct unit for that change is still an angle. A particular linear distance can only accurately describe it given a particular latitude, which is expressed in, guess what, an angle. This is of course an overly technical way if explaining how the "moving" terminology just obfuscates a pretty simple and accessible underlying concept. They went out of their way to make it less clear.