THE PERFECT day ought to have 86,400 seconds: 24 hours for Earth to spin round its axis, 60 minutes in every hour, and 60 seconds in every minute. However the obvious precision of those easy calculations ignores the messy actuality of planetary our bodies. Tidal forces, mixed with the roiling currents in Earth’s core and the redistribution of ice sheets at its floor, trigger the planet’s charge of spin to differ ever so barely from 12 months to 12 months.
