"The Dark Side of the Moon"

The Far Side of the Moon; Credit - Apollo 16 astronauts/Public domain: https://commons.wikimedia.org/wiki/File:Back_side_of_the_Moon_AS16-3021.jpg

“The dark side of the Moon.” This common phrase refers to the far side of the Moon which constantly points away from the Earth – we only ever see one side of the Moon. Above is a picture of this elusive side which you will never have seen with your own eyes from Earth. Despite what the phrase suggests, the far side of the Moon is not dark, receiving the same amount of sunlight as the Earth-facing side. ^[1] So what causes this phenomenon?

Tidal locking

The Moon completes an orbit of the Earth once every 27.3 days. It also spins about its axis once every 27.3 days. ^[2] Scientists call this synchronous rotation: ‘the rotation of an orbiting body on its axis in the same amount of time as it takes to complete a full orbit.’ ^[3] However, this has not always been the case. After its initial formation, presumably following a collision where ‘an object smashed into early Earth,’ ^[4] the Moon had a rapid orbital period of 10 hours around the Earth. ^[5] It was the gravitational force exerted by the Earth that slowed the Moon down.

As you may know, the tides on Earth are, in part, caused by the gravitational attraction of the moon on our oceans. ^[6] The Earth also pulls on the Moon, to an even greater extent. In fact, it ‘distorts to Moon into an oblong shape [where] the sides pointed towards and away from the Earth bulge outwards while the others are pulled inward.’ ^[6] The regions on the moon which bulged constantly changed as the Moon continued in its orbit. Eventually, the Earth’s gravitational pull slowed the Moon’s rotation down until it became synchronous.

In the very distant future (50 billion years), the Earth will also become tidally locked to the Moon. This will mean that only one side of the Earth will be visible from the surface of the Moon. ^[6] This has already happened to the dwarf planet Pluto and its largest natural satellite, Charon.

 

Sources:

1. Wired: https://www.wired.com/story/the-know-it-alls-what-is-the-dark-side-of-the-moon/

2. Astronomy: https://astronomy.com/magazine/ask-astro/2012/08/the-moons-nearside

3. Dictionary.com: https://www.dictionary.com/browse/synchronous-rotation

4. Space.com: https://www.space.com/19275-moon-formation.html

5. MinuteEarth (YouTube channel): https://www.youtube.com/watch?v=6jUpX7J7ySo

6. Fraser Cain: https://www.youtube.com/watch?v=wDNDz_VP3PE