Same Old Face: Why We Always See the Same Side of the Moon


The familiar face of our Moon

Ah, the Moon. What would our sky be like without it? It waxes, it wanes, and it returns to do it all again. And every time it does we see the same face every time.

The Moon is tidally locked to face the Earth the same way as it circles around the planet. We never see the far side, as we should correctly call it. It’s not always dark – the far side gets to see just as much sunlight as the near side. To be technically correct we can see about 58% of the near side of the Moon due to a wobble in its axis, known as libration. We know what it looks like on the far side because we’ve sent spacecraft up there to orbit it and send photographs back. So we know it has lots of craters, but not as many maria, or “seas” of old lava flows across it. This is understandable in that this outward facing side would be more likely to be impacted upon over time from incoming asteroidal matter.

But some people aren’t aware of this. On the 8th August NASA released an animation of the Moon passing in front of the Earth as seen from its Deep Space Climate Observatory (DSCOVR) spacecraft, which lies about 1 million km from Earth towards the direction of the Sun. Among the denying cries of “It’s Photoshopped” and “Why isn’t there an eclipse?” there were the people who said “But the Moon doesn’t rotate!” Nope to all of that.

Tidal locking is not an oddity – all of the larger moons in the solar system are tidally locked with their parent planets, too. Iapetus, a moon of Saturn, is a good example of this. For a long time astronomers knew that Iapetus looked brighter when it was at one extreme of its orbit compared to the other. Once the Cassini spacecraft sent back close up images of this moon the answer became obvious; it was covered in a layer of dark dust that it collected on the leading side but was still a clean icy white colour on the trailing side. Examples of other moons that are tidally locked are Io, Europa, Ganymede, Callisto around Jupiter, Titan and Enceladus at Saturn, and Triton at Neptune.

An extreme example of tidal locking is Pluto and Charon. T they are close enough that the same side of Charon is forever locked to the same side of Pluto. They spin around a common centre of gravity, known as the barycentre, which actually sits outside of the surface of Pluto. This makes a convenient place to mark a line of zero longitude on Pluto for mapping purposes, directly under the gaze of Charon.

If you were on Pluto, Charon would never move from its spot in the sky. If you were on the Moon, you would see the Earth move just a little back and forth in your sky during the lunar cycle, due to the librational wobble of the Moon’s axis, but it still wouldn’t rise and set like we see the Sun and Moon do here on Earth. This simulation from the viewpoint of the crater Brisbane gives you an idea of what it would be like looking back at our blue planet. And take note that the Earth goes through phases as the Moon orbits around it as well.

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