Why is the Moon slowly moving away from Earth?
Tides gradually transfer energy and angular momentum within the Earth–Moon system.
A measurable drift
The average distance between Earth and the Moon increases by about 3.8 centimetres per year at present. Scientists can measure the change with remarkable precision using laser pulses reflected from special mirrors left on the Moon by Apollo missions and other experiments.
The connection to ocean tides
Earth spins faster than the Moon travels around it. The tidal bulges raised on Earth by the Moon do not remain perfectly aligned with the line between the two bodies. Their gravitational interaction transfers angular momentum from Earth’s rotation into the Moon’s orbit. Earth’s rotation slows very gradually while the Moon moves outward.
A slow-motion cosmic partnership
The rate has not been constant throughout geological history, because the shape of oceans and continents changes tidal friction. The familiar monthly Moon is part of a dynamic system evolving over immense timescales. The measurement is a striking example of how a tiny annual change can reveal large-scale physics.
External references are provided for exploration; Curio Hour’s explanation is written separately.