How Does The Earth’s Rotational Gravitation Affect The Moon?

How Does The Earth's Rotational Gravitation Affect The Moon?
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Is it just a coincidence or a fluke that the Moon’s period of revolution and rotation are the same so that we always see the same face? And how did this situation come to be?

Our only satellite keeps the same face pointing towards the Earth. And this is because its rate of spin is tidally locked so that it is synchronized with its rate of revolution (time needed to complete one orbit). In different words, the Moon rotates exactly once every time it circles the Earth.

Furthermore, the same forces that generate tides in our oceans from the gravitational pull of the Moon and, and to a lesser extent, the sun, further act on the Moon’s body. 



Earth’s Gravitation Distorts The Moon

The Earth’s gravitational force on the Moon distorts it into a slightly prolate or football shape; also, the Moon’s natural form is somewhat egg-shaped. 

And if the tip of the football/egg does not point toward the Earth, then gravitational forces exert a torque that makes the tipping point back toward the Earth. So, in reality, the Moon oscillates a small amount around perfect alignment, a motion called the lunar libration.


The Earth photographed from the Apollo 11 mission.
Credit: NASA.
The Earth photographed from the Apollo 11 mission.
Credit: NASA.

The Moon Synchronized Spin Started Millions of Years Ago 

It is questionable that our Moon started out synchronized. That would certainly be a surprising coincidence. Our Moon’s synchronized spin state is believed to have started millions of years ago. 



This was when the Moon was much closer to the Earth, and so tidal forces were much stronger than at present. The Earth’s gravity maintained this spin state even as other gravitational interactions caused the Moon to move outward to its present orbital radius.


That’s it. Thanks for reading this short article. If you want to know more fascinating facts about our Moon, then head over to this article; 11 Things You Didn’t Know About the Moon.


Watch Phil “CrashCourse” for a tour of our Moon. From surface features, inside to the core, and back in time to theories about its formation.