The idea that there is a side of the Moon that remains perpetually dark is a common misconception. All parts of the Moon experience sunlight, just as Earth does. The Moon goes through regular cycles of day and night across its entire surface.
The Moon’s Illumination and Rotation
The Moon rotates on its axis while simultaneously orbiting Earth, ensuring sunlight eventually reaches all of its surfaces. The Moon completes one rotation on its axis in approximately the same time it takes to orbit Earth, which is about 27.3 days. This phenomenon is known as synchronous rotation, or tidal locking.
Due to synchronous rotation, the same side of the Moon, known as the near side, always faces Earth. However, this does not mean the other side is always dark. The Moon is consistently half-illuminated by the Sun, similar to Earth. Every location on the Moon experiences roughly two weeks of daylight followed by approximately two weeks of night. This means that over the course of a lunar month, both the near and far sides receive nearly equal amounts of sunlight.
The Misconception of the “Dark Side”
The term “dark side of the Moon” is misleading. It typically refers to the “far side” of the Moon, the hemisphere never visible from Earth. This invisibility is a direct result of the Moon’s synchronous rotation. This far side remained unseen by humans until space probes photographed it in 1959.
The far side of the Moon receives just as much sunlight as the near side, experiencing its own cycle of day and night. The origin of the term “dark side” may stem from its historical unknown and unexplored nature, rather than its illumination. Another reason for the term “dark” could relate to communication challenges, as the Moon’s mass blocks radio signals to spacecraft on the far side.
Understanding Lunar Phases from Earth
The lunar phases observed from Earth illustrate how the Moon’s illuminated portion changes from our perspective. These phases, such as new moon, crescent, quarter, and full moon, depend on the angle at which sunlight strikes the Moon and how much of that lit surface is visible from Earth as the Moon orbits our planet.
During a new moon, the side of the Moon facing Earth appears dark because it is not illuminated by the Sun. At this point, the Sun is illuminating the far side of the Moon, which is invisible to observers on Earth. Conversely, a full moon occurs when the entire near side is illuminated by the Sun and fully visible from Earth. This demonstrates that the perceived “darkness” is relative to Earth’s viewing angle.