Rick Robinson

Sep 25th 2019

What Is the Dark Side of the Moon?


What is the dark side of the moon? Is it strictly the name of the classic 1973 Pink Floyd album, or is it a real phenomenon?

The moon does indeed have a dark side. So does every other moon and planet in the solar system, including Earth. In fact, if you happen to be reading this at night, you are currently on the dark side of Earth. This is where things get confusing.

When you look up at a crescent moon in the sky, you’re looking at part of the dark side, though you don’t usually notice it because it is, well, pretty dark. When people talk about the dark side of the moon they don’t usually mean the side experiencing lunar night. They invariably mean the far side of the moon, which is indeed invisible from Earth, but gets just as much sunlight as the near side of the moon, the side we see, Wired reports. So what is the dark side of the moon?

The Unchanging Man in the Moon

Still confused? Perhaps, says, some of the confusion can be blamed on Pink Floyd, and some of it is due to a cultural tradition of using dark to mean unknown. But the biggest culprit is likely the moon’s rotation period, which is the same as its orbital period — a month.

This is not mere coincidence, says Wired. The moon’s rotation is “tidally locked” due to Earth’s much greater mass, but the main effect is that the moon doesn’t seem to rotate at all. The same side always faces Earth, its shades-of-gray surface features forming the familiar man-in-the-moon pattern.

In fact, because the moon wobbles slightly as it orbits Earth — the technical term for this is libration we can see a little more than half the moon’s surface, though not all at the same time. But most of the lunar far side can never been seen from Earth, and remained totally unknown until the space age.

Seeing the Dark Side

The far side gets just as much sunlight as the near side does — in fact a tiny bit more. During a total lunar eclipse, the sunlit “bright” side of the moon passes through Earth’s shadow and for a couple of hours is temporarily darkened. On the lunar farside, where Earth is not visible in the sky, its bulk never blocks out the sun, so the farside cannot experience a lunar eclipse.

As for the actual dark side of the moon, the side facing away from the sun, it never gets completely pitch dark there thanks to starlight (and any planets visible in the lunar night sky). The side facing Earth, meanwhile, also gets reflected earthlight. If you look at a thin sliver moon, a day or two before or after new moon, you can often see the faintly lighted dark side, an effect that NASA’s Astronomy Picture of the Day describes as “the old moon in the new moon’s arms.”

Even during a total lunar eclipse, when the moon is facing Earth’s night side, our atmosphere bends a little sunlight into the shadow, sometimes producing the “blood moon” effect of a lunar eclipse.

So as it turns out, even the actual dark side of the moon is not always quite dark. And since we are still in the dawn of moon exploration, the far side of the moon remains pretty mysterious, even though most of it is brightly sunlit for two weeks out of every month.

Future Moon Exploration

The 50th anniversary of the Apollo 11 landing helped call attention to all things lunar. Apart from current and upcoming NASA missions, China and India are both carrying out robotic moon missions. Indeed, China’s Chang’e 4 mission made the first-ever landing on the far side of the moon early this year — an accomplishment that triggered a whole spate of headlines about the lunar “dark side.”