The length of an Earth day is not fixed. Due to a gradual transfer of angular momentum, the planet’s rotation is slowing while the Moon drifts outward. Each century, the day stretches by roughly 1.8 milliseconds, a small increment that accumulates over geological timescales.
The mechanism is tidal friction. Gravitational pull from the Moon raises tidal bulges in Earth’s oceans and crust. Because Earth rotates faster than the Moon orbits, these bulges are carried slightly ahead of the Moon’s position. The misalignment creates a torque that acts as a brake on Earth’s rotation while adding energy to the Moon’s orbit.
Conservation of angular momentum links both bodies in a single dynamical system. As rotational kinetic energy is dissipated as heat in the oceans and seafloor, the Moon’s orbital radius increases. Laser ranging experiments, which bounce light off retroreflectors on the lunar surface, measure the outward drift and confirm the long term slowdown of Earth’s spin.
Paleontological records, including growth rings in ancient corals, indicate that past days were shorter and that a year contained more daily cycles. These independent lines of evidence align with tidal theory and orbital mechanics, turning a barely perceptible daily change into a key constraint on models of Earth Moon evolution.
