There is a permanently dark side of the Moon
The Moon has a far side that permanently faces away from Earth due to tidal locking, but this side receives just as much sunlight over a lunar month as the near side. The popular phrase 'dark side' historically referred to the far side being unknown or unmapped, not unilluminated.
What we know
The Moon is tidally locked to Earth, meaning its rotational period exactly matches its orbital period around Earth, both taking about 27.3 days. This synchrony is the result of gravitational interactions over billions of years that gradually slowed the Moon's rotation until it stabilized in this locked configuration, and it is the reason the same lunar hemisphere, the near side, always faces Earth, while the opposite hemisphere, the far side, always faces away, never visible from Earth's surface under any circumstance without a spacecraft.
This permanent orientation is the root of the popular but inaccurate belief that the far side is perpetually dark. In reality, the Moon rotates relative to the Sun over the course of its orbit in essentially the same way Earth does relative to its own day-night cycle, just on a longer timescale of roughly 29.5 days per full lunar day-night cycle (the synodic period, slightly longer than the 27.3-day sidereal rotation because of Earth's own motion around the Sun). Because of this, the far side receives sunlight and experiences day and night in almost exactly the same overall proportion as the near side, averaged over a full lunar cycle. When the near side is fully illuminated as a "full moon" visible from Earth, the far side is in its nighttime phase, and vice versa when the near side shows a "new moon," the far side is fully lit by the Sun, simply not visible from Earth either way.
The phrase "dark side of the Moon" most likely originated as a figurative reference to the far side being unknown, unmapped, and unobserved, using "dark" in the sense of "mysterious" or "hidden from knowledge," a use of the word paralleled in other English phrases such as being "in the dark" about a topic, meaning uninformed rather than in physical darkness. This is a completely reasonable historical description, since for the vast majority of human history, and indeed until the Soviet Luna 3 spacecraft captured the first photographs of the far side in 1959, no human had any direct observational information about what that hemisphere looked like.
The far side does differ from the near side in genuinely interesting ways that have nothing to do with sunlight. It has a much thicker crust, far fewer of the dark volcanic plains called maria that are prominent on the near side, and a heavily cratered appearance overall, differences that planetary scientists attribute to variations in crustal thickness and historical volcanic activity between the two hemispheres, along with the influence of Earth's gravity and heat on the near side's early geological history. NASA's Lunar Reconnaissance Orbiter and China's Chang'e missions, including Chang'e 4, which achieved the first-ever soft landing on the lunar far side in January 2019, have provided detailed data confirming and expanding on these geological differences.
Popular culture, most famously Pink Floyd's 1973 album "The Dark Side of the Moon," has reinforced the illumination misconception by using the word "dark" evocatively without necessarily intending scientific literalism, and this kind of cultural reinforcement likely explains why the phrase persists so strongly in public understanding despite being straightforwardly correctable with basic orbital mechanics. Astronomy educators and institutions such as NASA consistently point out that the more accurate and now more commonly used technical term among scientists is simply the "far side" of the Moon, which avoids the illumination confusion altogether.
Common claims
- The far side of the Moon is permanently dark.Not supported
- The far side of the Moon receives the same amount of sunlight as the near side.Accurate
- The far side of the Moon has never been photographed.Outdated, photographed since 1959
- The far side has geological differences from the near side.Accurate