Why Do We See Only One Side of the Moon?
Here at the Irene W. Pennington Planetarium we have several shows that include information about the moon. One of the things I’m asked about is why do we only see the one side. Some people seem to think that moon doesn’t rotate at all; some people think the “dark side” of the moon is always dark. So let’s clear some things up about the moon and illuminate why that “man in the moon” always faces us.
First of all, there is no “dark side of the moon.” The entire moon is illuminated at some point during its month long lunar-day. The problem is that we can’t see the other side in its high noon phase because it’s on the back side.
It’s believed that billions of years ago the Moon was much closer to us and rotated much faster. So much so that the entire lunar surface could be seen over time. However, over time the gravitational influence of the Earth on the moon slowed it down. Just like how the moon’s very small gravitational influence on the Earth affects the tides.
Over time this tidal friction caused the side of the moon closest to the Earth to slightly bulge, acting as a type of brake to slow it down. Eventually this got the moon to the point where it’s rotational speed matched it’s orbit and became what’s called “locked” or “synchronous” orbit. It’s something found throughout the solar system. Since the moon’s rotation is tidally locked to the Earth it means that the “man in the moon” side is always facing us. As it orbits around the Earth it completes one full rotation on its axis.
Here’s a nice little video demonstrating the point: