The phrase “dark side of the moon” has become a common expression in popular culture, often used to describe something mysterious or hidden. This notion suggests a perpetually unlit lunar hemisphere, a concept that has intrigued humanity for centuries. However, scientific understanding of the Moon reveals a different reality. This article explores the Moon’s actual illumination and the unique characteristics of its unseen hemisphere.
Dispelling the “Dark Side” Myth
There is no perpetually “dark side” of the Moon. The term is a misconception; astronomers refer to the “far side” of the Moon, contrasting it with the “near side” that always faces Earth. The “dark” in “dark side” likely refers to “unknown” or “unseen” rather than a lack of light. Both the near and far sides of the Moon experience sunlight and darkness over the course of a lunar cycle.
The Moon’s Synchronous Rotation
Only one side of the Moon is visible from Earth due to synchronous rotation, also known as tidal locking. This occurs when a celestial body’s rotation period matches its orbital period around another body. For the Moon, it takes approximately 27.3 Earth days to complete one rotation, nearly the same time it takes to orbit Earth.
This gravitational locking results from tidal forces between Earth and the Moon. Early in its history, the Moon rotated faster, but Earth’s gravity created bulges, slowing its spin over billions of years. Eventually, the Moon’s rotation rate decreased until one side became gravitationally “locked” facing Earth, reaching a stable state.
How Both Sides Get Sunlight
Even though the same side of the Moon constantly faces Earth, the far side is not always dark. The Moon continuously rotates on its axis and orbits the Sun along with Earth. As it orbits, sunlight falls on different parts of its surface.
Over a lunar month (about 29.5 days), every part of the Moon, including the far side, experiences approximately two weeks of daylight followed by two weeks of night. Lunar phases observed from Earth, such as the new moon or full moon, result from how much of the Moon’s sunlit portion is visible to us, not because one side is inherently dark. For example, during a new moon phase, the side facing Earth is largely unilluminated, while the far side is bathed in sunlight.
The Unique Features of the Far Side
Spacecraft images reveal the Moon’s far side is distinctly different from its near side. The near side features large, dark plains formed from ancient volcanic lava flows, known as maria. In contrast, the far side is heavily cratered with a rugged terrain and significantly fewer maria.
Scientists believe the far side’s crust is thicker, which may have prevented volcanic material from reaching the surface as frequently. The first images of the far side were obtained by the Soviet Luna 3 probe in October 1959, revealing a previously unseen landscape. Subsequent missions, including China’s Chang’e-4 (first soft landing on the far side in 2019), continue to provide valuable insights into this unique lunar hemisphere.