The dark side of the moon, also known as the ‘far side of the moon’, refers to the hemisphere of the moon that faces away from Earth and therefore, can only be seen from space. But have you ever wondered why we are limited to seeing only around 59 percent of the Moon?
I mean, why do we only see one side of the planetoid? Since Earth rotates on its own axis, theoretically, the Moon should also do the same, allowing us to have a full picture of the Moon, right?
Well, because all loyal I’m A Useless Info Junkie readers should be the ones answering these kind of questions once they pop up in conversations, here’s the answer. (the article continues after the ad)
It all has to do with the speed at which the Moon rotates on its own axis. Millions of years ago, the Moon was actually spinning at a much faster pace than it does now. Earth’s gravitational influence though has affected the Moon’s rotation and gradually slowed it down. Just like Moon gravitational influence acts upon our planet to create tides, Earth’s influence slowed down the rotation period of the Moon in order to make it match exactly that of its orbit – approximately 27.3 days – and it is now locked to this period. Simply said, because the Moon’s rotation coincide with its orbit around the Earth, we only see the same surface of the Moon. But why 59 percent and not 50 percent?
Well, this is due to the fact that the Moon’s orbit is elliptical and not circular. Because of that, its orbital speed increases or decreases depending on how close the Moon is to our planet. However, its rotational speed is always the same. When it’s farthest from our planet, this difference between orbital and rotational speed gives observers a glimpse of the Moon’s dark side – the side that’s usually hidden. This is called libration and it allows us to see 59 percent of the Moon (over time) instead of just 50 percent.
Now you know!