Earth’s 24-hour day is steadily stretching by about 1.8 milliseconds per century as the Moon’s gravitational pull gently brakes our planet’s rotation. Decades of lunar laser ranging confirm the Moon is receding at 3.83 centimetres annually, transforming ancient epochs where dinosaurs experienced shorter days.
Decades of Laser Ranging Track the Moon’s Retreat
Scientists have tracked the gradual recession of Earth’s natural satellite through high-precision observations spanning five decades. Researchers measure the increasing distance by firing laser pulses at retroreflector arrays placed on the lunar surface by Apollo astronauts between 1969 and 1971.
Apollo 11 astronauts Neil Armstrong and Buzz Aldrin installed the first reflector array at the Sea of Tranquillity in July 1969. Additional arrays were deployed during the Apollo 14 and Apollo 15 missions in 1971, with the Apollo 15 array containing 300 corner-cube prisms that serve as the primary instrument for modern campaigns. Soviet Lunokhod 1 and Lunokhod 2 rovers also carried French-built retroreflector panels.
According to NASA’s Jet Propulsion Laboratory, four observatories currently conduct routine lunar laser ranging from New Mexico, France, Italy, and Germany. The Apache Point Observatory Lunar Laser-ranging Operation in New Mexico provides the highest precision, measuring the Earth-Moon distance to about one millimetre by timing laser pulses with picosecond accuracy.
Tidal Friction and the Transfer of Angular Momentum
The physical mechanism driving the Moon away is tidal friction, which operates through the combination of Earth’s rotation, the Moon’s gravity, and the geometry of Earth’s oceans and crust. The Moon’s gravity creates tidal bulges on both the near and far sides of Earth. Because Earth completes one rotation every 24 hours while the Moon takes about 27.3 days to orbit, Earth’s rotation carries these tidal bulges slightly ahead of the Moon’s position.
This offset produces a forward gravitational pull on the Moon, transferring energy into its orbit and causing it to move farther away each year at a measured rate of 3.83 centimetres per year, with a formal uncertainty of about 0.09 millimetres. Under the conservation of angular momentum, the Moon’s gain in orbital energy comes from Earth’s rotational energy.
“Over time, the moon has stolen Earth’s rotational energy to boost it into a higher orbit farther from Earth.”
Ross Mitchell, Geophysicist via Spacedaily
Clam Shells and Ancient Rocks Record Shorter Days
Nearly 3,000 years of eclipse and astronomical records show that Earth’s average length of day has increased by about 1.8 milliseconds per century. Rewinding the geological clock reveals a rapidly spinning early planet. In 2020, a team led by geochemist Niels de Winter sampled a 70-million-year-old clam shell with a laser, finding 372 daily growth rings per year. That indicates a day near the end of the dinosaurs’ time ran about 23.5 hours, roughly half an hour shorter than today.
Tidal sediments point to a day of about 21.9 hours around 650 million years ago, and ancient coral records suggest a 23-hour day about 350 million years ago. Near the Moon’s formation roughly 4.5 billion years ago, a day may have lasted less than 10 hours. While most models predict progressively shorter days deeper in the past, a 2023 study by Mitchell and geophysicist Uwe Kirscher gathered dozens of ancient day-length estimates and suggested Earth’s day may have stalled near 19 hours for roughly a billion years, though researchers view this mechanism as an ongoing debate rather than settled agreement.
Long-Term Consequences for Earth’s Climate and Tides
Over geological timescales, the continuous recession of the Moon carries profound implications for Earth’s rotational stability, ocean tides, and celestial events. Without the Moon, the stability of Earth’s rotational axis would falter. Martin Knapmeyer of the German Aerospace Centre notes that without the Moon’s gravitational pull, ocean tides would diminish massively.
Furthermore, because total solar eclipses rely on a near-perfect alignment where the Moon and Sun appear the same size in the sky, the Moon’s gradual retreat means its apparent size will decrease.
