We can observe different shapes of the Moon at various times throughout the month, but regardless of its phase, we can only see one side of the Moon and never its far side.
When viewed from Earth, the Moon appears in many different shapes as it moves in its orbit, known as phases, which include Full Moon, Waning Moon, Half Moon, Crescent Moon, and New Moon.
The far side of the Moon has many craters and fewer dark spots compared to the side facing Earth. (Source: NASA)
Regardless of its phase, we can only see one side of the Moon and can never view the far side of this celestial body.
The side that is not visible from Earth is known as the dark side, far side, or backside of the Moon (sometimes referred to as the Moon’s dark region).
Approximately 18% of this side of the Moon can sometimes be seen from Earth due to the Moon’s libration effect. The remaining 82% remains unobservable by humans.
“The dark side of the Moon” is actually a term that is not entirely accurate as this region still receives sunlight, but since it is never visible to people on Earth, it is always considered dark.
From Earth, it seems that the Moon does not rotate at all; however, it does rotate on its axis, similar to Earth. The Moon is tidally locked with our planet, causing its rotation period to coincide with its orbital period.
This means that the time it takes for the Moon to rotate on its axis is the same as the time it takes to orbit Earth – about one month.
Astronomer Madelyn Broome from the University of California, Santa Cruz, states that about 4.5 billion years ago, when the Moon first formed, Earth’s rotation speed was significantly faster than it is now, with a day lasting only about 5 hours.
At that time, the Moon was also much closer to Earth, and the two celestial bodies continuously affected each other.
The gravitational pull from Earth’s tidal bulge affects the Moon. Conversely, the oceans’ movements caused by the Moon’s gravitational force create friction on Earth’s surface, causing our planet to rotate more slowly. As a result, today, a day on Earth is 24 hours long, and the Moon is “tidally locked” with Earth.
Some spacecraft have visited the far side of the Moon. (Source: NASA).
Tidal locking occurs due to the gravitational forces between the two celestial bodies. As the Moon moves around Earth, its gravitational pull distorts Earth, elongating it toward the Earth-Moon line and compressing it in the perpendicular direction.
This effect causes the tides on Earth (in fact, the land also experiences this distortion, but at a very small amplitude, making it hard to notice).
Conversely, the Moon is also deformed due to Earth’s gravitational force. As Earth rotates and the Moon orbits around it, the peaks of these deformations are misaligned; however, because the gravitational force between them is still greater than that at other points on their surfaces, the deformations tend to pull toward each other.
Robert Tyler, a physical oceanographer at NASA’s Goddard Space Flight Center, states that the gravitational attraction between the Moon and Earth distorts both bodies and pulls them toward each other, giving them a shape similar to an American football.
The result of this is the synchronization of the Moon’s rotation period (the smaller celestial body) with its orbital period around Earth.
Because the rotation period and orbital period are the same, the Moon always keeps one face toward Earth, which is why we can only observe this side of the Moon.
Although we will never directly see the far side of the Moon from Earth, spacecraft have captured images of this area.
In 1959, the Soviet spacecraft Luna 3 first captured images of the Moon’s far side. Subsequently, the Soviet Academy of Sciences published the first map of this hemisphere in 1960.
In 1968, the astronauts of the Apollo 8 mission were the first to directly observe this region as they orbited the Moon.
In 2019, the Chinese probe Chang’e 4 successfully landed on the far side of the Moon, becoming the first probe in human history to land successfully in this region.
The first images from the far side of the Moon sent back by Chang’e 4 on January 3, 2019. (Photo: AP).
Chang’e 4 captured high-resolution images of the far side of the Moon, revealing perspectives we had never seen before.
Most recently, on the afternoon of May 3, China launched the Chang’e 6 probe to explore the far side of the Moon and carry out a mission to collect samples from this area to bring back to Earth.
This is the first time humans have undertaken such a mission in the history of lunar exploration, aiming to learn more about the environment and material composition of the far side of the Moon – a place where humans have never set foot and where there is very little understanding of this region.
