No Permanently Dark Side of the Moon
The notion that there is a permanently dark side of the Moon is wrong. But there is a far side of the Moon, a side we never see from Earth.
The Moon does not radiate its own light, but its surface reflects the Sun’s rays. Half of the Moon's surface is always illuminated, and during the course of a lunar month, most areas of the surface have about 14-15 days of sunlight. On the far side, the Moon phases are exactly opposite from the ones we see from Earth, on the near side. When we see the Moon fully illuminated at Full Moon, the far side is in darkness. When it is New Moon here, it is Full Moon on the far side.
A Mystery until 1959
Humans had no idea what the far side of the Moon looked like until October 1959, when a Soviet spacecraft, Luna 3, transmitted the first grainy images.
During the next five decades, the quality of lunar images has vastly improved, especially because of NASA’s Lunar Reconnaissance Orbiter (LRO), a spacecraft which has been orbiting the Moon collecting data since 2009. Today, we have detailed maps of the Moon’s topography, including of the once mysterious far side.
Why Do We Never See the Far Side?
The reason the same side of the Moon always faces Earth is that the Moon rotates around its axis at the same speed as it revolves around Earth. This effect is known as tidal locking, and it is caused by the tidal forces of our planet.
In the same way as the Moon exerts tidal force on our planet, causing tides on Earth, our planet exerts tidal force on the Moon. Earth is larger than the Moon, so its tidal force is much more powerful. Over time, this force has slowed down the Moon's rotation until it reached a speed matching the speed of its orbit around Earth.
However, there is a small variation to this rule. Since the Moon revolves around Earth on an elliptical path, the Moon's distance from Earth varies from day to day. The point of the orbit closest to Earth is called perigee, while the point farthest away is known as apogee.
Like all celestial objects with elliptical orbits, the Moon's speed varies a little on its path around Earth. It speeds up when it is closest and slows down when it is farthest away from us. At perigee, the Moon's orbital speed is a little faster than its rotational speed.
This variation in speed, along with other factors, causes an effect called lunar libration. From Earth, the Moon seems to rock slightly from north to south and wobble a little from east to west. Over time, it is possible to see up to 58% of the Moon’s surface, but only 50% at a time. In other words, over time, we can see up to 8% of the outskirts of the Moon's far side from Earth.
Far Side Conspiracy Theories
A quick search on the web shows plenty of references to the “Dark Side of the Moon.”
It is the title of a widely popular album from 1973 by English rock band Pink Floyd. There are also several films with the same name, including an American 1990 science fiction thriller and fantasy action “Transformers: Dark Side of the Moon” from 2011, and the German thriller “Die dunkle Seite des Mondes” from 2015 involving a lawyer who becomes a wanted man after a psychedelic mushroom trip.
You will also find a vast amount of more or less humorous conspiracy theories about secret Moon bases and UFOs, along with images from the far side of the Moon showing what some claim to be structures created by humans or aliens.