Seeing you on the dark side of the Moon

I found it intriguing that reports about China’s Chang’e 4 lander, at work on the far side the moon, kept referring to it being on the ‘dark side’. In a way the epithet is appropriate; the unseen side has been unknown to us for much of human history. And that, reasonably, made it ‘dark’, at least in terms of what we knew about it. It wasn’t dark, though, in terms of the amount of light the far side received – which is exactly the same as the amount the visible side receives.

The far side of the Moon as seen in April 1972 by Apollo 16. Note that it is mostly illuminated – the day-night terminator, bottom left of the frame, indicates that the near side visible from Earth is mostly dark. NASA, public domain.

Let me explain. The Moon is tidally locked to Earth, meaning it completes one rotation in the same time as it completes one orbit around Earth. We see only one side of it, give or take a bit – there is a certain wobble, ‘libration’, which exposes a thin sliver of the unseen side as the Moon moves. Before the space age a fair number of astronomers, including the late TV presenter and science populariser, Patrick Moore, made careful observations of that edge as it peeked into view, trying to map out as much of the far side as was visible.

In practice, of course, most of the far side remained hidden. So to that extent we could call it the ‘dark side of the Moon’ in the sense that we had no real knowledge of it. I think that’s the meaning used in the Pink Floyd album. All that changed with the space age, of course; and now we have detailed maps of the far side. It’s not ‘informationally dark’ in the way it was. And the Chang’e 4 probe and its rover, Yutu 2, will give us further information about conditions on the ground.

Inevitably, though, the label ‘dark side’ has an entirely other popular meaning – the notion that it is somehow unlit. This just isn’t true; the moon does rotate as it orbits, however its orbital period and its rotation period are the same. This means that the far side gets exactly the same amount of light as the side we see. You can observe this by going outside and looking up. Any time other than full moon, part of the lunar side we see is always dark – and the corresponding opposite part on the unseen far side is illuminated. During a new moon or a solar eclipse, the whole of the far side is illuminated. So it’s kind of weird, to me, that the epithet ‘dark side’ carries that implication of perpetual darkness.

The more intriguing part is the fact that the far side is heavily cratered and lacks the ‘seas’ familiar to the side we see. Explaining why the Moon appears to have two distinct geologies – and why they’ve ended up in the orientation we see – has occupied a good deal of time and effort, and there’s no particular consensus just yet other than the possibility that the mass-distribution of the Moon as a whole might have something to do with it. One theory suggests that Earth originally had two moons, one larger than the other, which then came together in a kind of giant splat which left the new body slightly asymmetric. What is currently the far side gained a thicker crust. Over time, as the Moon became tidally locked, tidal effects coupled with the asymmetry of the lunar mass meant that heavier side ended up facing away. This same asymmetric composition also meant that the two sides responded differently under the Late Heavy Bombardment that is thought to have formed many of the lunar features we see, between 4.1 and 3.8 billion years ago.

The Moon during the Late Heavy Bombardment; and the Moon today. Tim Wetherell – Australian National University, via Wikipedia.

All of this has yet to be fully proven, but the theories are intriguing.

Doubtless missions such as Chang’e 4 and its successors – which must come, and not necessarily from China – will help shed light on the matter. We’ll see.

Copyright © Matthew Wright 2019