A tiny widening gap between Earth and the Moon is silently stretching the length of a day. Each year, the lunar orbit expands by about three point eight centimeters, a shift almost too small for human senses yet clear in precision measurements using laser ranging reflectors on the lunar surface.
The mechanism rests on tidal friction and conservation of angular momentum. Ocean tides, raised by the Moon’s gravity, drag against the rotating Earth and dissipate energy as heat. That drag slows Earth’s rotation, while angular momentum is transferred to the Moon’s orbit, pushing the Moon slightly outward and increasing its orbital radius.
Over geological timescales, the compounding effect is significant. Fossil coral growth bands and sedimentary tidal rhythmites indicate that past days were noticeably shorter and that Earth once completed more rotations during a single lunar orbit. The same physics of orbital resonance and rotational deceleration implies that the length of the day will keep increasing as the Moon continues its slow departure.
