The moon is leaving
Lunar laser ranging shows the Moon receding from Earth by about 3.8 centimetres a year.
Established2 min read
Apollo crews and the Soviet Lunokhod rovers planted retroreflectors. Observatories bounce laser pulses off them and time the return. The round trip is growing. The fit is 3.8 cm per year.
The cause is tides. Earth’s oceans bulge, the bulge is dragged ahead by rotation, and the bulge pulls the Moon forward. The Moon climbs to a higher orbit. Earth’s spin pays the bill: the day gets longer. The exchange is slow, measured, and one-way on human calendars.
Caveats
- The rate has not been constant over geologic time. Young Earth, closer Moon.
- 3.8 cm/year is the current average from decades of ranges, not a daily wobble.
Sources
- 1Dickey J. O. et al.. Lunar Laser Ranging: A Continuing Legacy of the Apollo Program. Science 265, 1994.