The question of whether we ever see the “dark side” of the Moon stems from a long-standing misconception about our nearest celestial neighbor. From Earth, we only ever see one hemisphere, correctly known as the Near Side. The hemisphere we cannot see is called the Far Side, which remained completely hidden from human eyes until the Space Age. While a portion of the Moon is always facing away from us, the entire spherical body experiences a cycle of day and night. This means the Far Side is illuminated by the Sun just as often as the Near Side.
Clarifying the Terminology: The Far Side is Not Always Dark
The enduring phrase “dark side of the Moon” is a misnomer because it implies a region that exists in perpetual shadow. The Moon rotates on its axis, and as it orbits the Earth, sunlight constantly bathes both the Near Side and the Far Side in alternating periods of light. Just like Earth, the Moon has a day that lasts approximately 29.5 Earth days. Each location experiences about two weeks of daylight followed by two weeks of night.
The Far Side is only truly dark when it is experiencing its night, which occurs during the lunar phase we observe as a Full Moon. Conversely, the Near Side is fully dark during the New Moon phase, when the Moon is positioned between the Earth and the Sun. Over the course of one full lunar cycle, both hemispheres receive a nearly equal amount of sunlight.
The Near Side receives an additional source of illumination that the Far Side entirely misses: the light reflected from the Earth, known as Earthshine. When the Moon is a crescent, the full Earth appears bright in the lunar sky, casting a noticeable glow on the Moon’s Near Side. The Near Side is also occasionally plunged into complete darkness during a lunar eclipse, when it passes through Earth’s shadow, an event the Far Side is shielded from.
The Mechanics of Synchronous Rotation
The reason we only ever see the Near Side is due to synchronous rotation, often called tidal locking. This is a stable orbital state where an orbiting body’s period of rotation on its axis is exactly equal to its period of revolution around its partner. For the Moon, this period is approximately 27.3 Earth days.
This synchronized motion is the result of billions of years of gravitational interaction between the Earth and the Moon. Earth’s gravity pulled on the Moon’s mass, which is not perfectly spherical, creating a slight elongation or tidal bulge. The gravitational forces exerted on this bulge acted like a brake, slowing the Moon’s initial, much faster rotation rate.
Eventually, the rotation slowed until one of the Moon’s elongated axes pointed permanently toward Earth, creating a 1:1 spin-orbit resonance. At this point, the gravitational torque ceased, and the rotation rate stabilized. If the Moon were not rotating, we would see all of its surfaces over the course of its orbit. However, because its rotation is precisely matched to its orbital speed, the same hemisphere is always facing Earth.
How Scientists Have Mapped the Unseen Lunar Surface
For millennia, the Far Side remained a complete mystery, but this changed with the advent of space exploration. The first images of the unseen lunar surface were captured by the Soviet Union’s Luna 3 probe in October 1959. This mission transmitted noisy, low-resolution pictures that revealed a heavily cratered landscape, distinct from the Near Side.
Subsequent missions, including the US Apollo program, provided clearer data. Apollo astronauts were the first humans to visually see the Far Side. Today, orbiting spacecraft like NASA’s Lunar Reconnaissance Orbiter (LRO) and China’s Chang’e probes have mapped the entire Far Side in high detail. The Chang’e 4 mission achieved a historic milestone in 2019 by performing the first soft landing on this remote hemisphere.
The detailed images confirm a stark geological difference: the Far Side is rugged, pockmarked with impact craters, and contains relatively few of the smooth, dark plains called maria. These basaltic plains cover about 31% of the Near Side, but only about 1% of the Far Side. Scientists determined that the Far Side’s crust is significantly thicker, estimated to be up to 20 kilometers deeper than the Near Side’s crust. This asymmetry is likely related to the early thermal evolution of the Moon, possibly involving a concentration of heat-producing elements like potassium and thorium on the Near Side, which fueled the volcanic activity that formed the maria.