[peterlk]

> The scientists say their model of inner core movement also explains the variation in the length of day, which has been shown to oscillate persistently for the past several decades.

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?

[wongarsu]

Observing quasars and the like [1]. We measure it to figure out when we need leap seconds (to keep UTC within 0.9 seconds of UT1, the time given by earth's position and rotation in the solar system)

1: https://en.m.wikipedia.org/wiki/Very-long-baseline_interfero...

[shadowgovt]

Star readings. We can measure that accurately... In fact, a difference of 0.2 seconds is hugely significant with modern telescopes.

[dragonwriter]

> wobble (the motion that gives us seasons)

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.)

[peterlk]

Ah yes, of course. Thanks for the clarification!