Why Do We Always See the Same Side of the Moon?
The Far Side of the Moon
You may have heard that the Moon has a “dark side” and a “light side”.
“Dark” can mean a few things in English other than “not bright”, such as “mysterious” or “gloomy.”
It turns out that the Moon doesn’t really have a dark side or a light side from the perspective of the Universe, it’s just that we as Earth-residing humans can’t see the “far side” of the Moon.
But why do we only see one side of the Moon?
To understand why we first have to define some terms.
Imagine sticking a shish kebab skewer straight through the center of the Moon and then spinning the Moon around this axis. The time it takes for the Moon to make one revolution on this skewer is called its rotational period.
The other important concept is the orbital period, or the time it takes the Moon to complete a full rotation around the Earth.
Many millions of years ago the Moon rotated much more quickly than it does today. Its rotational period used to be a lot shorter.
Over time, the Earth’s gravity slowed the Moon’s own spin so that today the Moon’s rotational period and orbital period are the same! Both are around 29.5 days.
It’s because of this that we see only one side of the Moon. A term used to describe this phenomenon is ‘tidal locking.’
In the image below, the situation on the left shows the Moon spinning on its own axis as it revolves around the Earth. This causes observes on Earth to only see one half of the Moon.
On the right, the Moon is shown without rotation, which would allow observes on Earth to view the entire surface of the Moon over time.
Tidal locking causes the same side of the Moon to always face Earth
Image Credit: Stigmatella aurantiaca
Another Interesting Fact
We on Earth can actually see a little more than half of the Moon’s surface.
The Moon doesn’t make a perfect circular orbit around Earth, but instead traces out an ellipse, which is kind of an oval shape.
Because the distance from the Earth to the Moon changes depending on where the Moon is at in its orbit, the speed with which the Moon travels also varies.
But its rotational period, how fast the Moon rotates on its own shish kebab skewer, doesn’t change.
When the Moon is farthest from Earth, the Moon’s rotational period is shorter than its orbital period, allowing Earth-based viewers to get a quick glimpse of a usually hidden area of the Moon.
This mismatch of the rotational and orbital periods causes longitudinal libration, which is a rocking motion of the Moon relative to the Earth due to the elliptical route of the Moon’s orbit. There are two other types of libration that together allow us Earthlings to see up to 59% of the Moon’s surface:
- Longitudinal libration – farther from the Earth, the moon lags behind in it’s orbit. As it moves closer to the Earth it speeds up.
- Latitudinal libration – the moon spins on an inclined angle relative to the Earth. Think of the shish kabob skewer through the Moon as tilted slightly away from the Earth.
- Diurnal libration – due to the Earth’s rotation since we observe the movement of the Moon from the surface of the Earth instead of from the Earth’s center.
Here’s a time-lapse of all three types of libration in effect:
Libration demonstrated by simulated views of the Moon over a single month.
Sources:
Moon Connection
Libration
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