Is it purely coincidental that the moon rotates on its axis in synch with its revolution around the Earth, keeping the same face always pointed toward us?

Asked by: Zaheer Merchant


The Earth's moon rotates (spins on its axis), every 27.32166 Earth days. It revolves around the Earth in the exact same period - every 27.32166 Earth days. Because of the synchronization of revolutionary and rotational periods, the same portion of the moon's surface is always directed toward the Earth.

The phenomena of which you speak is not coincidental, and 'universally' speaking, throughout the galaxy, may well be considered more typical of planet moon relationships than perhaps an anomaly.

It is fairly well understood how the gravitational interaction of the moon with our Earth is responsible for the tides on our planet. But far less recognized and understood is the gravitational effects of the Earth on the moon. The mass and speed of rotation of the Earth influence the moon in that some of its rotational energy is actually transferred to the moon. The result of this being that the moon rotation is slowed while also being placed continually into a higher orbit and thus slowing its revolution. The net effect of this gravitational relationship is that the moon's rotation has been slowed to match its orbital period. Ironically, since the Earth is giving up some of its rotational energy to the moon, the Earth and moon will, in the far distant future, reach a synchronization of rotational periods, as Pluto and its nearer to its mass moon Charon have already done.

Many of the moons in the solar system have also reached this point of equilibrium. In Jupiter, the moons Amalthea, Thebe, Io, Ganymede, Callista, and Europa, all have identical rotational and revolutionary periods.
Answered by: Stephen Portz, Technology Teacher, Space Coast Middle School, FL