Moon slowly receding from Earth at 1.5 inches per year, lengthening days over deep time

The Moon is moving away from Earth at about 1.5 inches (3.8 centimetres) per year, and that gradual outward drift is slowing Earth's rotation and will, over very long timescales, make days marginally longer, a physicist at Michigan State University said.

Dr. Stephen DiKerby described the effect in an essay for The Conversation, saying the recession of roughly 3.8 cm per year is driven by tidal interactions: the Moon raises tidal bulges in Earth's oceans that lead the satellite slightly because Earth is rotating. The forward pull of those bulges transfers angular momentum to the Moon, increasing its orbital energy and carrying it into a slowly widening orbit while Earth's rotation correspondingly decelerates.

The rate DiKerby cited is small relative to the average Earth–Moon separation of about 239,000 miles (384,000 kilometres). "1.5 inches per year compared to a distance of 239,000 miles is just 0.00000001 per cent per year," he wrote, adding that eclipses, tides and 24‑hour days will persist for millions of years. Geologic and fossil records are consistent with a shorter day in the past: around 70 million years ago, near the end of the age of dinosaurs, a day is estimated to have been roughly 23.5 hours long.

Scientists measure the Moon's recession directly using lunar laser ranging. Reflectors left on the lunar surface during the Apollo missions and by later spacecraft allow researchers to time laser pulses to the Moon and back, producing precise distance measurements over decades that reveal the steady outward drift and its variations.

The transfer of angular momentum that drives the effect has long been understood in celestial mechanics. As Earth's rotation slows, the length of the solar day increases imperceptibly on human timescales; the current annual change in day length is very small and detectable only with precise clocks and long-term records. DiKerby and other researchers stress that the change will not produce meaningful differences in daily life for millions to billions of years.

In the very long term, if tidal friction continued undisturbed for tens of billions of years the system could evolve toward tidal locking, in which Earth and the Moon would present the same faces to each other and the Moon would stop receding. That outcome is unlikely to be reached because stellar evolution alters the conditions first: models indicate the Sun's increasing luminosity over the next billion years will eventually boil away Earth's oceans, removing the primary mechanism—ocean tides—that currently drives the angular‑momentum transfer. A few billion years later, the Sun's expansion into a red giant is expected to radically change or destroy the inner planets' current configurations.

The Moon's gradual recession underscores the interplay of gravity, rotation and tides in shaping planetary systems. While the numbers are measurable and important for understanding Earth's past and future dynamics, scientists emphasize the changes are so slow that they pose no immediate concern for human society.