The term “dark side of the Moon” is a common misconception. The reality is that the Moon does not have a side that is forever hidden from the Sun’s light. The use of the word “dark” in this context actually refers to the side that is unseen from Earth, a geographical designation distinct from the Moon’s illumination. This distinction is important for understanding lunar cycles. To fully understand the Moon’s cycle, one must separate the concepts of lunar day and night from the Moon’s rotation relative to Earth.
The Simple Answer: Lunar Day and Night
The Moon, like Earth, rotates on its axis and experiences a cycle of day and night. Over the course of a lunar month, every part of the Moon’s surface is exposed to sunlight. The confusion arises because the Moon’s rotation period is extremely slow compared to Earth’s.
One full lunar day, the time it takes for the Sun to return to the same position in the Moon’s sky, is approximately 29.5 Earth days. This means that a single “day” on the Moon lasts for about two weeks, followed by a “night” that lasts for another two weeks. This long exposure to sunlight or darkness causes extreme temperature swings on the lunar surface, making the environment harsh. The “dark side” is therefore simply the hemisphere currently experiencing night, and it is constantly shifting as the Moon rotates.
The Moon is always half-illuminated by the Sun, just like Earth. When we see a New Moon from Earth, the side facing us is the night side, while the entire far side is bathed in sunlight. Conversely, during a Full Moon, the near side is in day, and the far side is experiencing its two-week-long night.
Why We Only Ever See One Side
While the far side is not permanently dark, it is permanently hidden from our view due to a phenomenon called synchronous rotation, or tidal locking. This means the Moon rotates on its axis at the exact same rate it orbits Earth. The time it takes for the Moon to complete one rotation is 27.3 Earth days, which is precisely the duration of one orbit around our planet.
Imagine a dancer circling a partner while always facing them; the dancer completes one full spin for every circle they trace. Similarly, the Moon is spinning, but its rotation is perfectly synchronized with its revolution around Earth. This gravitational lock is a natural end state for a satellite and is caused by the ongoing tidal forces exerted by Earth.
This fixed orientation makes the “Far Side” a geographical term for the hemisphere that faces away from Earth. A small sliver of the far side, about 9%, can be glimpsed over time due to slight wobbles in the Moon’s orbit, known as libration. However, the majority of the far side remains perpetually out of sight from any location on Earth. The far side was completely unseen until the Soviet Luna 3 probe photographed it in 1959.
What the Far Side Looks Like
The far side of the Moon is not only out of view but is also geologically distinct from the near side. It is far more heavily cratered than the side we see, lacking the broad, flat, dark plains known as maria (Latin for “seas”) that dominate the near side’s appearance. The near side is covered by maria over about 31% of its surface, but the far side has them on less than 1% of its surface.
This difference is thought to be due to the far side having a significantly thicker crust, which would have made it more difficult for magma to rise and form the volcanic plains. The far side features one of the largest known impact structures in the solar system, the South Pole–Aitken basin. This unique environment makes the far side a prime location for future scientific endeavors, such as radio astronomy, because it is shielded from the radio interference generated by Earth. China’s Chang’e 4 mission achieved the first-ever soft landing on the far side in 2019, paving the way for further exploration of this unseen hemisphere.