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The Moon is receding from Earth by about 3.8 centimetres per year, largely because of tidal interactions.
Earth's Moon seems like a permanent fixture in the sky. But its orbit is changing very gradually. Measurements show that our natural satellite is drifting away, even though the movement is far too small for people to notice from year to year.
Measuring an almost invisible movement
Astronauts on several Apollo missions left reflectors on the lunar surface. Scientists can bounce laser pulses off them and measure the round-trip travel time to estimate the Moon's distance with remarkable precision. NASA describes the present outward movement as around 1.5 inches, or roughly 3.8 centimetres, annually.
The role of ocean tides
The Moon's gravity raises tides on Earth, but the story involves more than waves on a beach. Earth's rotation shifts tidal bulges slightly ahead of the line between the two bodies. This offset produces gravitational forces that transfer energy and angular momentum, gradually altering the Moon's orbit while slowing Earth's rotation over very long timescales.
What it means for the future
The Moon is not disappearing any time soon. The measured rate describes a gradual physical process, and the interactions involved change over geological time. The discovery is valuable because it reveals that planetary orbits and rotation are dynamic rather than perfectly fixed.
Sources and further reading
Editorial note: This article explains documented research and links to the primary institutions used to verify its central claims.