Length of Day Variation

Length-of-day variation is the small fluctuation in Earth’s rotation period about the nominal 86,400 seconds, measured by very-long-baseline interferometry and satellite geodesy. On timescales from days to a few years it is dominated by exchange of axial angular momentum between the solid Earth and its fluid envelopes, chiefly the atmosphere and, at smaller amplitude, the ocean and continental water. Strong zonal winds slow the solid Earth, lengthening the day by milliseconds. Oceanic angular momentum from currents and bottom pressure adds a measurable contribution, and seasonal and ENSO-related signals appear in the record.