Casey Chin

Our in-house Know-It-Alls answer questions about your interactions with technology.

Q: What is the “dark side” of the moon?

A: The short answer? It’s a misnomer. A cool-sounding misnomer! But a misnomer. Assuming they aren’t talking about the Pink Floyd album or the French mockumentary, people who say “the dark side of the moon” are almost always referring to the moon’s far side—which, despite pointing permanently away from those of us planetside, actually sees as much sunlight as the side facing Earth.

Maybe you already knew that. But! Did you also know that slivers of the moon’s elusive far side are constantly slipping into view? Or that certain lunar regions are, in fact, shrouded in permanent darkness?

To understand why, you first need to understand why one side of the moon is forever pointed away from Earth. To those of us here on the ground, our planet’s natural satellite appears to never turn. But it’s actually turning all the time—it just spins on its axis and loops around our planet at the same rate: once every 27 days or so. When a cosmic body revolves around its parent and its axis at the same rate, astronomers say that it is “tidally locked.”

Our moon wasn’t born this way. Astronomers think that, like many natural satellites, it began its life spinning at a very different rate. (In the case of our moon, astronomers think it once whirled faster about its axis.) But over time, gravity from our planet exerted torque on bulges in the lunar surface, forcing its rotation into synchronization with its orbital period. This phenomenon is actually quite common: Many of Saturn and Jupiter’s moons are tidally locked with their parent planet.

Tidal locking is why we had no idea what the moon’s far side looked like until 1959, when the Soviet space probe Luna 3 snapped the first photos of that crater-laden landscape. We’ve gotten several good looks at it since: In 1968, astronauts aboard NASA’s Apollo 8 mission became the first people to gaze upon the moon’s far side with their own eyes. NASA’s Lunar Reconnaissance Orbiter has been mapping the entire lunar…