The Moon is Receding, and the Tides are to Blame

The Moon gets further from the  
Earth by about 4 cm per 

year.

The graphic helps understand 

why:

1. The Moon raises tidal bulges in the Earth’s oceans.

2. As the Earth rotates, friction causes the tidal bulge to be 

pulled in the rotation direction, so that it is not directly under 

the Moon.

3. The water in the bulge exerts a small gravity force on the 

Moon. Because of the bulge’s offset, part of this force is to 

the left, and this causes the Moon to speed up slightly.

4. As it speeds up, the size of its orbit increases.

Distance measurements using lasers are so precise that the 

increase in orbit size has been verified. In a few hundred 

million years, the Moon will be far enough away that it can no 
longer cover the Sun, and there will be no more total solar 

eclipses.

The same phenomenon causes the Earth’s rotation rate to 

slow down. The total Earth-Moon angular momentum must 

remain constant, and as the Earth imparts angular 

momentum to the Moon, the Earth’s angular momentum (and 

rotation rate) must decrease.