Myth: the dark side of the moon?

22 January 2019

Interview with

David Rothery, Open University

MOON

Moon

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When you look up at the Moon, you always see the same thing. To some it’s a face - the Man in the Moon - to others, it’s a giant rabbit or an elegant lady. But seeing the same face lit up has led to discussions on what must be on the other side, which some label as the “Dark Side of the Moon”. The phrase has been frequently repeated recently in the wake of the landing by the Chinese of their Chang’e 4 rover on that side of the Moon, and this has got David Rothery, Professor of Planetary Geosciences at the Open University, almost spinning off his axis, and he’s on a lunar mission to right this scientific wrong...

David - On the 3rd of January the Chinese landed on the dark side of the moon! Well did they?

I mean, I got fed up of hearing about it and reading about it put that way in the press. But the moon doesn't have a dark side, at least not a permanently dark side. I'm a Pink Floyd fan but, sorry, there is no dark side of the moon.

The Chinese landed on the far side of the moon. That's the side of a moon that permanently faces away from the earth. But it's no darker than the near side, it sees just as much sunlight as the near side of the moon does. In fact, a little bit more because on the far side of the moon you never see the Earth getting in the way of the sun.

So let's get rid of this phrase “the dark side of the moon”. Metaphorically it's dark but it's misleading. It is the far side of the moon.

Now, like almost every known moon in the solar system, the moon has this property of rotating once for every orbit it makes round its planet. So one side, the near side, always faces towards the planet. The opposite side, the far side, always faces away from the planet. And, if you're on the surface of the far side of any of these moons, you wouldn't be able to see the planet because it would be below the horizon.

Now why does this happen? Well, it's due to tides. Tidal forces have slowed down each moon's rotation, suddenly it rotates exactly once per orbit. And that then means that you don't have to be distorting the shape of the body by moving tidal bulges around the solid surface of the body because the total bulges can stay fixed in place facing towards underway from the planet.

So that's it. Our moon has a far side but not a dark side. So please if you hear somebody talk about the Chinese or anybody else landing on the dark side of the moon, gently put them right.

But what a great achievement to land there on the far side of the moon. There's no line of sight to the earth. You can't get radio signals to and fro. So the Chinese put the quasar relay satellite in a halo orbit around the Lagrange point on the far side of the moon from the earth so it could always see the far side of the moon and over the horizon to the earth. Brilliant technical job to land on the far side of the moon. And we're going to learn a lot about the far side of the moon when we put rovers down in more interesting places there...

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