© 2004 Pearson Education Inc., publishing as Addison-Wesley
is when the rotation period of a moon,
planet, or star equals its orbital period about
Tidal friction on the Moon (caused by Earth)
has slowed its rotation down to a period of
The Moon now rotates synchronously.
We always see the same side of the Moon.
Tidal friction on the Moon has ceased since
its tidal bulges are always aligned with Earth.