Moon always faces Earth with one side, its spin period equal its orbital period - this is what we learn at school.

But I know Nature rarely operates in absolutes. I'd find it hard to believe Moon wouldn't spin relative to Earth at least very slowly. I searched but found no data other than "doesn't", so I'd like to ask - does the Moon stay turned the same side precisely - or do, over many years, new features show at one edge? If it does, how long would it take to make a full turn?

  • $\begingroup$ This is already addressed in the accepted answer in Why is only one side of the Moon visible from Earth?, under the section about libration. $\endgroup$
    – called2voyage
    Oct 18, 2013 at 12:37
  • $\begingroup$ @called2voyage: This is not what I meant!! Libration is a cyclic process strictly coupled with orbital period. I'm interested in a long-term process of full rotation, difference between period of rotation and period of revolution! $\endgroup$
    – SF.
    Oct 18, 2013 at 12:50
  • $\begingroup$ Libration is the only effect that will allow you to see different sides of the moon. Tidal locking will stay in effect until the moon flies out of Earth orbit in the distant future: space.com/3373-earth-moon-destined-disintegrate.html $\endgroup$
    – called2voyage
    Oct 18, 2013 at 12:56
  • $\begingroup$ Unless the Earth-Moon system is interrupted by some other process. $\endgroup$
    – called2voyage
    Oct 18, 2013 at 12:56
  • 1
    $\begingroup$ That's exactly what I'm trying to tell you. It is effectively unmovable. $\endgroup$
    – called2voyage
    Oct 18, 2013 at 13:10


Browse other questions tagged .