Until the late 1950s, the far side of the moon or the dark side of the moon was never seen by anyone on Earth. But that was changed by the Russian Space program that sent the Luna 3 probe to fly around the moon and beam back images of the unknown side.
Why can’t we see the far side of the moon?
The simple answer is that the moon rotates around its axis in the same time that it takes to revolve around the earth. This phenomenon is called Tidal Locking. Take a piece of stone, tie it to a string and put a colored sticker on one side. Preferably on the side of the string that you are holding. Spin it around yourself. The sticker will always face you. That is almost what is happening in the case of the moon. Except that it is also rotating around its own axis. Take a look at the GIF below to better understand this mechanism.
How is it different from the front side?
The two sides of the moon have some stark differences. For starters, the side facing Earth has darker spots. These are basaltic plains formed due to the rapid cooling of volcanoes. Yes, about 3-4 billion years ago, the moon used to witness volcanic eruptions. These darker areas are called Lunar Maria. Whereas, the far side of the moon has more craters, larger mountains, and almost no basaltic plains.
This difference between the two hemispheres of the moon has been explained by a number of theories. One of the theories attributed to this difference uses the findings of an experiment that showed that the side facing us has more heat producing elements. Another theory uses the Theia collision theory which states that the Moon was formed by the debris created from a planetary collision of Earth with another planet. Building upon that, the theory states that a part of the debris also collided with what would later form our moon, and resulted in some geological changes that then lead to the differences in the landscapes.
A fairly recent theory suggests that the front facing surface has fewer craters since it was warmer due to heat radiated from Earth. On the far side, which was cooler, the minerals that formed the lunar crust condensed quickly and acted as a shield against the meteorites. However, the ‘softer’ crust on the front side would spew lava from the meteorite hits, which on cooling down would cover the impact craters.
When did we first see it?
The first image of the far side of the moon was relayed to us by Russia’s Luna 3 probe. These images were released in 1960. Whereas, the first humans to see it were the astronauts on the Apollo 8 in 1968.
Is it really darker on the far side of the moon?
Slightly. Both the sides receive the same amount of sunlight. Nevertheless, the side facing the earth has a slightly brighter appearance because it gets some light from Earth.
Does it have any potential use?
Two potential use cases have been suggested.
- Radio waves from Earth get blocked by the moon. Hence, astronomers believe that it could be a good spot to set up radio telescopes that won’t face any issue of interference.
- The far side maria of the moon is also rich in Helium 3. A rare element on Earth that could be used as fuel in fusion reactors.
- Earthshine, the reason why the Earth-facing side of the moon is slightly brighter – Wikipedia
- Lunar Mare – Wikipedia
- Volcanism on the Moon – Oregon State University portal
- Theia collision theory – Wikipedia