Earth’s Day Lengthens 1.7 Milliseconds per Century as Moon Pulls Away 3.8 Centimeters a Year
Updated
Updated · Sky at Night Magazine · Aug 18
Earth’s Day Lengthens 1.7 Milliseconds per Century as Moon Pulls Away 3.8 Centimeters a Year
3 articles · Updated · Sky at Night Magazine · Aug 18
Summary
Atomic-clock measurements show Earth’s rotation is slowing enough to add about 1.7 milliseconds to the length of a day every century.
The main driver is the Moon: its gravity raises tidal bulges that Earth’s faster spin drags ahead, and the Moon’s pull on those bulges acts as a brake.
That braking transfers momentum to the Moon’s orbit, pushing it about 3.8 centimeters farther from Earth each year while Earth spins more slowly.
Internal mass shifts in the molten outer core, along with winds, ocean currents and glacier melt linked to climate change, also nudge rotation by tiny amounts.
The slowdown is measurable with VLBI and laser ranging but far too small to notice directly; full tidal locking would take tens of billions of years, beyond the Sun’s remaining lifetime.