# How long does sunlight take to reach Earth?

And when compared to the size of the sun, just how small are the planets in our solar system?
22 August 2022

## Interview with

Matt Bothwell, University of Cambridge

## MERCURY IN CAMBRIDGE

Julia Ravey takes a walk through space... Well, not really. But she went down to Midsummer Common in Cambridge where there is a space walk! Julia met up with Matt Bowell who gave her a guided tour of our planets...

Matt - It's the scale of 590 million to one. So the sun we are looking at here in front of us is about a bit less than two and a half meters in diameter; 590 million times smaller than the real thing. But the nice thing about this whole sculpture trail is that the entire solar system is to scale. Even when you see pictures of the solar system, it's never to scale. It's impossible to get the real scale of the solar system into one picture. The only way to do it is like this, to lay the solar system out over about 10 kilometres so you can see how much space there really is.

Julia - Shall we spacewalk?

Matt - Yes, let's space walk. We can start off if you want, by walking at the speed of light.

Julia - Relative to this?

Matt - Yes, relative to this, I'm not that fast a runner. So we are exploring a 590 million to one solar system. If we scale the speed of light down by 590 million, then you can walk at the speed of light on this scale. It's about this fast...

Julia - A bit of a shuffle...

Matt - It's a little bit less than two kilometres an hour. I think it's about a toddlers walking pace.

Julia - So if I ran along here and I mean, I am not fast, I'm slow - I would be running much faster than the speed of light?

Matt - You would, you would be super luminal.

Julia - Well, I'm going to take that and I'm going to do my run along here at some point and tell everyone I was running faster than the speed of light.

Matt - You could probably do 10 times faster than the speed of light easily.

Julia - Matt don't flatter me! But here we are, I'll take it.

Matt - It would take us eight minutes to do this short walk to the earth if we were going at the speed of light. So cosmically, the speed of light is sort of a crawl.

Julia - A cosmic crawl. I like that. I feel like that's what I should call this walk.

Matt - We're approaching Mercury now. Do you want to describe what you're seeing?

Julia - It's a nice big purple sculpture with "Mercury" on the top. And it's pointing down to what looks like a bead, the size of a generous garden pea.

Matt - It's sort of reddy-orange; a suspect garden pea, maybe. I think it's about five or six millimeters across. It's really extraordinary when you think of how enormous the sun is. And then you see this tiny thing that could fit on your fingertip. This is how big Mercury actually is.

Julia - It's an actual chickpea.

Matt - It is a chickpea.

Matt - It has the highest temperature difference in the solar system. Because there's very little atmosphere, the temperature of the sun doesn't get conducted around the planet very well. And so even though the side of Mercury facing the sun gets cooked to like hundreds and hundreds of degrees, the dark side of mercury is freezing. It's like minus 200 C.

Julia - A tale of two planets.

Matt - Yeah, absolutely. Actually, can I give you two facts? So Mercury is very iron rich and dense. Mercury really resembles Earth's core quite a lot. So if you think about Earth, Earth has this iron rich core surrounded by the liquid rock mantle. And the crust of Mercury is really just a big ball of iron. So one theory is that mercury is the core of a planet that used to be much larger and maybe was involved in some sort of cosmic car crash that smashed off all the mantle and the crust and just left behind the planet's core. So it might be the leftover centre of a dead planet. One fact about this scale solar system, which I really enjoy, is how far it would take to reach Proxima Centauri, the nearest starter earth. Laid out on this same scale, you'd have to walk all the way around the Earth one and a half times, and then you'd be at the nearest star.

Julia - So if I was stood here in this scale, it wouldn't be like I could walk to London and find a star?

Matt - No, on this exact scale, eight kilometres up to Waterbeach, you would have to carry on walking until you go all the way around the earth and then carry on once you've returned back to the same point and then go all the way around the earth, down to Australia. And then at that point, that's the nearest star.

Julia - That is ridiculous.

Matt - It feels like a long walk Pluto, but then this is the dense parts of the universe where we live, right? This is our nice, hot, dense, bright cosmic backyard. Once you've left Pluto, you would have months and months and months of walking through nothingness to reach a star.

Julia - Wow. Space is big.

Matt - I've heard this feeling called cosmic vertigo. This idea of almost intense, overwhelming dizziness you get when comprehending the scale of the universe. I think astronomers like that feeling.

Julia - I think they like it - that's like adrenaline for them. For me, I feel a bit car sick.

Matt - Maybe we're cosmic rollercoaster junkies or something. We just like it.

Julia - Keep riding that rollercoaster!