Facing the Moon - MyMoon

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It has long been believed by scientists to be a pure coincidence that a single side of the Moon faces the Earth. However, recent research carried out by astronomers suggests that there is actually a reason for how things have turned out.

You have likely noticed that the Moon always appears the same to us on the Earth. We only get to see one side of the Moon from the Earth. This is because the rotation period of the Moon and the Lunar orbit take exactly the same amount of time. In other words, the Moon is tidally locked to the Earth.

It also turns out that the two sides of the Moon are not exactly similar. This was discovered just a few decades ago, when the first spacecrafts were sent to explore the Moon. The side that we see every time has a number of dark areas, the lunar maria, that most of us are familiar with as the Man on the Moon. The other side, though, consists of a large number of craters and higher mountains.

inline image The side of the Moon we see is shown on the left, while the side that is always opposite from us is on the right.

In order to understand why the lunar maria side always faces the Earth, it is necessary to go back about four billion years, when the Moon was just forming. At the time, it was still molten, and was still malleable to force of the Earth’s gravity. The constant gravitational pull from the Earth caused the Moon to develop into an oblong and elongated shape. Afterwards as the Moon cooled down, the oblong shape remained. However, the rotation of the Moon was still much faster than it is today. Later still, just as the Moon’s gravity helps cause the tides on Earth, the Earth’s gravity at the time pulling on the Moon caused tidal forces that created a bulge inside the Moon always pointing towards the Earth. This bulge added onto the already elongated Moon. As the Moon rotated, the bulge would position itself to move towards the Earth. In this process, as the bulge would migrate, the interior of the Moon would flex to accommodate the bulge, causing friction and dissipating energy. Eventually, the dissipation of this energy worked to slow down the rotation rate until it matched the time of the orbit, when the bulge would no longer be moving inside the Moon because it would continually be facing the Earth.

This help explains why there’s a single side facing the Earth, but why did it have to be the one we see every time we look up at the Moon? The researchers discovered that the lunar maria side and the opposite cratered site are both energy minima, meaning that these two sides are most likely to be the side that ends up as the side always facing the Earth. Furthermore, in their simulations, the researchers found that how fast the energy dissipated through the bulge affects the ending state as well. If the energy is dissipated very fast, then both outcomes are equally probable, but if the energy is dissipated more slowly, as is expected to have happened for our Moon, then the lunar maria side is twice as likely as the cratered side. Ultimately, it seems that a single side always facing the Earth is not just a random outcome for the Moon, but due to really interesting physical effects.

So the next time you take a look up at the Moon, you won’t just get to appreciate its face, but also understand why that face is what you always see.

(The research described in this article is published in the journal Icarus, and another summary of the research is available here.)


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