Why is the moon moving away from Earth?

We are on a space walk at the Our Place in Space trail in Cambridge this week. Julia Ravey was given a tour by Cambridge University's public astronomer Matt Bothwell... 

Julia - We're approaching Venus.

Matt - Yeah. So we've reached Venus. It's another trek away from Mercury. We've upgraded from chickpea to grape. This thing is maybe a centimetre across. So one interesting fact about these planets, because this entire solar system is to scale, when you stand at the distance of the planets and look back towards the sun, the sun is the same size, physical angular size, as it would actually look on that planet. When you look at the sun from Venus, that's how big the sun would be in the Venus sky. Wow. So it's about twice as big as it appears from earth.

Julia - I was going to say - it's really hot and sunny here today. So when we get to earth on our next little bit of the trek we'll have to have a look and give it a bit of a comparison.

Matt - Yeah, exactly. Well, without looking directly at the sun, of course.

Julia - Yeah. I have got my sunglasses on, but even I'm not that brave.

Julia - Well, let's head to Earth. Venus, it's been nice knowing you, we're on our way. We have arrived home. Well, nearly home. We've got a little pit stop first at the moon. The moon is pea sized as well.

Matt - Moon is pea sized. Yes. And earth is more cherry tomato size, maybe a bit more grape size. Earth and Venus are actually very similar sizes.

Julia - Yeah. Are they sister planets? Is that what they get called?

Matt - Yeah. You can think of the Earth and Venus as sort of sister planets, maybe Venus is Earth's evil twin. Because of its horrible climate and it's sulphuric acid rain - it's sort of like looking in a dark mirror or something.

Julia - So if we turn back now and look towards the sun.

Matt - Yeah. So if we look towards the sun, the sun is the same size that it appears in our sky. And there's actually a really nice trick you can do.

Julia - Oh, I love a trick.

Matt - So, because the entire thing is to scale, if you put your eye level with where the earth is, you can create an eclipse.

Julia - I can just see the spikes and it's a total eclipse.

Matt - It perfectly eclipses the disc of the sun.

Julia - Yeah. It's a spot on match. That is fab. So you've got your own little eclipse here as well.

Matt - Yeah, exactly. Eclipses won't be around forever, actually, because the moon is actually getting further away from the earth.

Julia - Why is that?

Matt - Because it's losing energy. So if you think about tidal energy, the moon raises and lowers the tides, we can get energy out of that to drive our renewable energy. But that energy has to come from somewhere. And so that energy in the tides, we are basically stealing it from the moon's orbits. And so the moon loses energy and moves away from the earth. It moves away at around the speed that your fingernails grow, about a couple of centimetres per year. But that means way back in the past, the moon would be much bigger in the sky and would've completely blocked the sun and you wouldn't get any night eclipses. 500 million years in the future, the moon would've moved so far away, it'll be smaller than the sun and you won't get eclipses anymore. So we are living at the right cosmic time to see these eclipses.

Julia - A very special time to be alive.

Matt - Yeah, exactly. We're very lucky. I find it interesting that Earth is the only planet that was named before we knew it was a planet. So all of the other planets in the sky, I guess we named them before we knew there were planets, but they were named after Gods. The ancient Greeks and the Romans looked in the sky and knew that they were special. And so they named them after their gods. Earth is named after just dirt. I sometimes feel we should name it something a bit more poetic than dirt. We should name it Gaia or something. That'd be quite nice.

Julia - Have you ever thought of that? Our planet is literally called mud.

Julia - So we've got Mars here. This looks like a, what are they called? Red currents - Christmas time. That's what I'm thinking of.

Matt - I feel like we're running out of small foods to name these planets.

Julia - In terms of science and research, what is something interesting that's going on about Mars at the minute?

Matt - There's a Mars rover called Perseverance, which is busy exploring the Jezero crater. It's the remains of a dried up lake on Mars and we're looking for any signs of ancient life. Because in terms of habitable planets, Mars, a few billion years ago was pretty spot on

Julia - Matt. I'm going to leave you on Mars or you might float back to the sun and I'm going to keep walk through space.

Matt - Amazing. Well, this has been really fun. I've enjoyed walking the solar system with you. It's been great.

Julia - Yeah. It's been brilliant. And now I must traverse the rest alone. Never know who I'll find on my way.

Matt - I believe in you.

Julia - Thank you.