Keep leap seconds, and glitches in telecommunications, navigation by satellite, and legal marking of time could become more frequent and serious. Lose them, and astronomers will have trouble pointing their telescopes, and eventually the time of day will get out of sync with Earth’s rotation.
At the crux of a debate about whether to continue inserting leap seconds into Coordinated Universal Time (UTC), the standard in many countries, is the unpredictability of Earth’s rotation. Millennia-old eclipse data show that Earth has long been slowing down. Nowadays, Earth’s rotation is measured by observing quasars with very long baseline radio interferometry. The deceleration is chiefly due to the tidal pull of the Moon; fluctuations on shorter time scales come from, among other things, the oceans and atmosphere and core-mantle interactions.
A leap second every year or two has been the norm, though none has been added since 1999, and timekeepers predict that...
