Why Do We Only See One Side of the Moon?

We not only see the same side of the moon, but also the same side of Venus every time it comes close to the earth.

Amidst speculations to know what the dark side of the moon really looks like, the moon continues to wax and wane. What we know as the phasing of the moon is the moon revolving around the earth. We look up at the moon and all we get to see is the crater filled face of the moon. Ever wondered why the other side of the moon never really shows up?

It has been proven that the earth rotates around its own axis thus resulting in the phenomenon called day and night. The moon rotates around its axis in a similar fashion. The only difference is the time taken to complete one entire rotation. Seeing the face of the moon as constant and never changing is one reason most would think it is not really moving.

We all know that the earth exerts a gravitational pull on things around it. It is this gravitational pull that has caused the moon to be tidally locked with the earth. This means that the moon, although spinning around its axis, is forced to show the same side to the earth.

The moon completes one orbit around the earth in about 27.3 days, during which it completes one full rotation around its axis as well. Which means that the time taken by the moon to complete one rotation and revolution around the earth is the same. This kind of rotation is termed as synchronous rotation.

If the moon itself was static and never spun around its axis, we would have been able to see the other side of the moon. The gravitational tug exerted by the earth has slowed down the rotation of the moon to such an extent that it takes the same amount of time to complete one rotation as it does one revolution.

The gravitational pull of the earth has caused one side of the moon to lock itself to the earth in such a way that try as you might you are unable to see its other face. So each time you see the moon, it has turned just a little bit around in its axis that you feel it hasn't moved at all.

Another point worth considering is the fact that the orbit is elliptical which makes a little over half the moon visible from the earth. The elliptical path allows the moon to oscillate like a pendulum, allowing 59% of the moon to be visible from the earth. This rocking motion of the moon allows us to peek around the edges of the moon at different times.

It isn't uncommon to find many such satellites that have tidally locked themselves to their parent bodies. Just like our moon shows us only one side of itself, the four moons of Jupiter and the five inner moons of Saturn are tidally locked to the planet body.