We only ever see one hemisphere of the Moon from Earth. The hemisphere that perpetually faces away from us is known as the far side. This constant presentation of the same lunar face is the result of a precise cosmic synchronization between the Moon’s rotation and its orbit around our planet. While we cannot see the entire far side, small shifts in the Moon’s orbit allow us to glimpse slightly more than half over time.
Why We Only See One Face
The phenomenon that dictates the Moon’s visibility is called synchronous rotation, often referred to as tidal locking. This means the Moon takes the exact same amount of time to complete one rotation on its axis as it does to finish one full orbit around Earth, which is approximately \(27.3\) Earth days. The two movements are perfectly matched, ensuring the same face is always directed toward us.
Earth’s gravity slowed the Moon’s spin over billions of years until this synchronous state was achieved. This gravitational interaction created a slight, permanent bulge on the Moon’s near side that is aligned with Earth. Any deviation from the synchronous rotation would cause Earth’s gravity to tug on this bulge, acting like a brake to restore the balance. Small shifts in the Moon’s orbital speed and inclination, known as libration, allow us to glimpse an additional \(18\) percent of the far side over time.
Is the Far Side Always Dark
A persistent misconception is that the far side of the Moon is also the “dark side,” permanently shrouded in shadow. This is incorrect, as the far side experiences a full cycle of day and night just like the near side. The term “dark” arose simply because it was unseen and unknown to humanity until the space age.
The Moon’s illumination is determined by its position relative to the Sun, not the Earth. As the Moon orbits, sunlight continuously covers half of its sphere. When we observe a Full Moon, the near side is fully illuminated, and the far side is experiencing its two-week-long lunar night. Conversely, during the New Moon phase, the near side is dark because the Sun is shining entirely on the far side. Over the course of the Moon’s approximately \(29.5\)-day cycle, both the near side and the far side receive nearly equal amounts of sunlight.
What the Far Side Looks Like
The far side presents a dramatically different appearance from the familiar face we see nightly. Its surface is much more rugged and heavily pockmarked with impact craters. This hemisphere lacks the vast, dark, smooth plains known as maria (Latin for “seas”) that cover about \(31\) percent of the near side.
These maria formed from ancient volcanic eruptions that filled large impact basins with basaltic lava. On the far side, such formations are scarce, covering less than one percent of the surface. This difference is attributed to the fact that the Moon’s crust is substantially thicker on the far side, making it difficult for molten material to breach the surface and create the dark plains.
The first glimpse of this unseen terrain was achieved in October \(1959\), when the Soviet Luna \(3\) probe captured and transmitted low-resolution images. Subsequent orbiting spacecraft have provided detailed mapping, revealing a geological dichotomy. The far side features one of the solar system’s largest known impact structures, the South Pole-Aitken basin.