The day is getting longer
Tidal friction slows Earth’s spin. The length of the day increases by a few milliseconds per century, measured with ancient eclipses and with lasers.
Established2 min read
Babylonian and Chinese eclipse records, interpreted by Stephenson and Morrison, show that the day has lengthened since antiquity at roughly 1.7 to 2 milliseconds per century. Lunar laser ranging measures the present torque directly: the Moon recedes, the spin drops, angular momentum is conserved.
On top of the slow brake there is weather. Winds and the core move milliseconds around from year to year, which is why leap seconds were a nuisance. The geological trend is cleaner than the annual noise. Devonian corals and the laser are the same story told from both ends.
Caveats
- The rate is not perfectly constant. Ice ages move water and change the moment of inertia.
- ‘A few milliseconds per century’ will not make you late for anything you can name.
Sources
- 1Stephenson F. R., Morrison L. V.. Long-Term Fluctuations in the Earth’s Rotation: 700 BC to AD 1990. Philosophical Transactions of the Royal Society A 351, 1995.