Quezi

Moon in Dec 2005 (photo by joka2000 - CC-BY)

We often hear the references to “the dark side of the moon”. But what does it mean? Does the moon really have a dark side?

The phrase conjures up an image of part of the moon being forever shrouded in darkness, never seeing the sun’s rays. But that simply does not happen.

The moon rotates approximately every twenty-eight days, so every part of the moon’s surface experiences sunlight for about two weeks followed by darkness for about two weeks. Because the moon’s “day” is so long, this causes extreme variations of temperature because the ground has so much time to warm up and then to cool down.

So there’s no such thing as a permanently “dark side of the moon”.

How did this phrase arise? It almost certainly arose from confusion with the phrase “far side of the moon”. Because the moon rotates at the same speed as it revolves around the earth, we always see the same side of it. When we look into the night sky, we always see the same patterns – the craters, mountains and lava flows that are sometimes colloquially described as the “man in the moon”.

The opposite side of the moon – the far side – remains hidden to our eyes because it is always pointing away from earth. We know what’s there because we have received pictures back from the spacecraft that we have sent into orbit around the moon, and a small number of humans have seen it during manned lunar missions.

Both the “near side” and the “far side of the moon” experience light and dark. That’s why we see the “phases” of the moon as it progresses from new moon to full moon and back again. There’s no permanently dark side of the moon.