Question

The period of the Moon's rotation is the same as the period of its revolution: 27.3 days (sidereal).

What is the angular momentum for each rotation and revolution? (Because the periods are equal, we see only one side of the Moon from Earth.)

Answer 1

The Moon (as well as all other moons of the solar system of comparable size) exhibit SYNCHRONOUS ROTATION. The physical reason for this is called tidal locking.

That means, that their rotation as measured relative to the starry background (about the truest basis there exists for establishing rotation) is of the same period as their orbit about their host planet.

That being said, from the perspective of the host planet, it will not *look like* the moon is rotating. But in truth the moon is rotating.



Earth's moon orbits Earth once every 27.3 days (sidereal month). Earth's moon rotates on its axis once every 27.3 days. CONTRAST THIS with the familiar phasing cycle (synodic month) that lasts 29.5 days.

Also, Earth doesn't "rotate once on its axis every 24 hours". That is an oversimplification. Earth has a sun viewing cycle lasting 24 hours (solar day).


Earth rotates a degree more than once on its axis every 24 hours...because it needs to make up the angle that the sun shifts against the starry backdrop every day.

Earth rotates on its axis once every 23hrs56min4sec. We call this the sidereal day.

FYI: it is essentially improbable for any celestial body to form and not be rotating on its axis. There is seldom ever such a thing as sidereal synchronicity. Distant stars are too far away to exert any tidal locking torque on bodies of other planetary systems. Plus, stars and planetary systems form out of initially rotating discs of gas and dust.