Moon Phases and Visibility
As the Moon orbits Earth, it presents different shapes called phases. These phases include full Moon, gibbous Moon, half Moon, crescent Moon, and new Moon. Regardless of the phase, we only ever see one side of the Moon from Earth, never the far side.
The Far Side, or "Dark Side"
The unseen half of the Moon is known as the far side, back side, or dark side (a misnomer as it does receive sunlight). While 18% of the far side is sometimes visible due to the Moon's libration (wobble), humans have not yet observed the remaining 82%.
Tidal Locking
The Moon's lack of visible rotation is due to tidal locking with Earth. The period it takes to rotate on its axis (about a month) matches its orbital period around Earth.
Tidal Forces
Tidal forces between Earth and the Moon have slowed Earth's rotation and caused the Moon to become tidally locked. These forces deform both bodies, creating a gravitational attraction that aligns their shapes like footballs.
Consequent Visibility
The result of tidal locking is that the Moon always presents the same face to Earth, explaining why we only ever see that side.
Spacecraft Explorations
Despite direct human observation being impossible, spacecraft have captured images of the far side. In 1959, Luna 3 sent the initial photos, and the Apollo 8 astronauts were the first to physically observe it in 1968. More recently, China's Chang'e-4 probe landed on the far side in 2019 and transmitted high-resolution images.
Ongoing Exploration
In 2023, China's Chang'e-6 mission collected samples from the far side, providing valuable insights into the Moon's previously unexplored regions.
Summary
Tidal locking prevents the Moon from rotating fully, resulting in the constant visibility of only one side from Earth. Spacecraft explorations, including Chang'e-4 and Chang'e-6, have shed light on the far side's surface characteristics and composition, while also demonstrating the ongoing human endeavor to explore the Moon's hidden regions.