An Etymological Dictionary of Astronomy and Astrophysics

The physical process that slows the → Earth’s rotation rate due to → tidal friction. The → Earth rotates faster than the → Moon orbits the Earth (24 hours compared to 27 days). The → friction between the ocean and the solid Earth below drags the → tidal bulge ahead of the line joining the Earth and the Moon. The → gravitational attraction of the Moon on the bulge provides a braking action on the Earth and decelerates its
rotation. Tidal braking lengthens the day by 0.002 seconds every century. Because the total → angular momentum of the → Earth-Moon system in conserved, the loss in the angular momentum of the Earth is compensated by the orbital angular momentum of the Moon. Hence, the Moon moves away from Earth at a rate of about 3 cm per year. This process must continue until Earth’s → day and → month are equal, at which point the Moon will never seem to move in Earth’s sky and Earth is said to be tidally locked to the Moon (→ tidal locking).

See also: → tidal; → braking.