The Moon’s Non-Rotation & Dark Side

Two of the enduring misperceptions of astronomy are that the Moon doesn’t rotate since we see the same side of it all the time, and that the back side of the Moon is its dark side. But neither one is true! A simple demonstration in the one case and a bit of logic in the other will quickly dispel both myths.

If our lovely satellite didn’t rotate on its axis as many believe, we would alternately see both the front of it and the back of it during the course of its monthly orbit of the Earth. And here’s a fun demonstration to prove it. Sit or stand in the middle of a room. Then have someone walk around you in a circle with them always facing in the same direction of the room as they do–in other words, not rotating their body as they “orbit” you. As you turn to follow their movement (just as the Earth continually turns) you will alternately see the back of their head and half a revolution later their face! In between, the side of them would be visible. In fact, the Moon slowly rotates on its axis in the same number of days that it takes to orbit the Earth. This is the same as having that person continually face you while circling you—meaning that they would have to rotate themselves to do so (and would see different parts of the room as they did).

The mistaken belief that the back of the Moon is always dark can be easily dispelled by the following logical statements of fact. When our satellite is in its Full Moon position, the entire front side is illuminated–and, the back side is indeed dark! But half a lunar orbit (or month) later when at New Moon, the side facing us is dark and the back side if fully illuminated by the Sun! At both First Quarter and Last Quarter when the Moon looks half-illuminated, the other side is also half illuminated. Technically stated, the lunar phase presented to us is always the “compliment” of that on its back side. Incidentally, many wonder (and rightly so!) why the half-full Moon is called the First Quarter seen in the evening sky or Last Quarter in the morning one. It’s simply because the Moon at those phases is one quarter or three quarters, respectively, around its orbit at those times. When Apollo 8 circled the Moon for the first time in human history, the flight was planned so that at least part of the back half was illuminated so the astronauts could see and photograph its surface features as they passed over it.

Despite giving the above demonstration and explanation during my public lectures in answer to questions about these misperceptions, there are always those in the audience who don’t buy it and continue to believe that the Moon doesn’t rotate and that its far side is dark. They are also many of the same ones who are convinced that we never landed on the Moon!

-James Mullaney

Former assistant editor at Sky & Telescope magazine and author of eight books on stargazing.