The Moon Has a Permanent Dark Side

The popular image of a lunar hemisphere perpetually shrouded in blackness has more to do with mystery than reality. This hidden face of the Moon is more accurately called the "far side," and its existence is the result of a gravitational dance called tidal locking. Over eons, Earth's gravity tugged on the Moon, slowing its rotation until the time it took to spin once on its axis matched the time it took to complete one orbit around our planet. This cosmic synchronization means the same side is always oriented toward us, leaving the other hemisphere completely unseen from Earth's surface.

For all of human history, that far side was a complete unknown. This changed dramatically in October 1959, at the height of the Cold War's Space Race, when the Soviet Luna 3 spacecraft successfully flew around the Moon. It captured the first-ever, grainy photographs of the hidden hemisphere, revealing a landscape that was surprisingly different from the familiar face. The far side is far more rugged and heavily cratered, and it almost entirely lacks the large, dark volcanic plains, or "maria," that create the "Man in the Moon" illusion on the near side.

This stark geological contrast (Review) remains a subject of intense scientific study. The leading theory suggests the far side's crust is significantly thicker, which prevented magma from erupting onto the surface to form the vast, smooth seas we see on the near side. This ancient, battered landscape holds clues to the early history of the solar system, and missions like China's Chang'e 4, the first to land on the far side, continue to unravel the secrets of this once-mysterious realm.