The Supermassive Podcast - 40: BONUS - Are Lunar Poles Arbitrary?

Episode Date: May 13, 2023

What's the most efficient way of space travel? How does longitude change your night sky view? Why does M87* look different? This month, Izzie Clarke, Dr Becky Smethurst and Dr Robert Massey take on your questions in The Supermassive Mailbox. Want to support The Supermassive Podcast? Why not buy our book The Year In Space - https://geni.us/jNcrw The Supermassive Podcast from the Royal Astronomical Society is a Boffin Media Production. The producers are Izzie Clarke and Richard Hollingham. 

Hello and welcome to another bonus episode from the Supermassive podcast from the Royal Astronomical Society with me, science journalist Izzy Clark, astrophysicist Dr Becky Smithers and deputy director of the Royal Astronomical Society, Dr Robert Massey. Thank you so much to everyone who has sent lovely messages that they're enjoying the extra episodes from us these are the little bonus ones you know you guys send in so many questions we couldn't possibly fit them in our normal episodes and you give izzy more work also i just really love these are the slightly more out there questions that i really really enjoy so please keep them coming because people

ask questions that i wouldn't necessarily think of and yeah i just love it it's brilliant i get very excited to ask them to you guys to actually see if we can get to the bottom of them but i think we're starting on an easier one this week and there's this really nice question from timsy on twitter and i thought we could all answer it and they say looking at saturn through a school telescope when i was a kid made me absolutely fall in love with astronomy what made you fall in love with astronomy and of course Saturn Robert do you want to go first I can because I know Becky's gonna rave about Saturn however however yeah I'll ask that I think it was probably my late dad taking me out with a really rubbish telescope actually because

you know there's a lot a lot of them were not very good and still aren't and uh just going out into a field and looking at some stars and trying to work out how to focus it and just realizing that you could see much more through a telescope than you could with your eyes so that for me was it i mean this would have been in in light polluted north bristol in in the well in the 1970s you know looking at things then so yeah that really did it for me and then actually slowly building up my own knowledge over time as well and you know getting slightly better telescopes and pairs of binoculars and starting to understand the night sky and then wanting to study it as well becky what about you so i vividly remember the 1999 almost total eclipse that we had in the uk i think it was total down in cornwall, wasn't it? But it was around about 97% or something in Lancashire where I grew up. And I was nine years

old. And I remember thinking, wow, like this is incredible. My school did a big thing for it. We had the little eclipse glasses that I was, you know, watching at home. And just, I think that was the moment I was probably hooked. I think from that point on, I was like, right, space books, please, mom. This is what I want for my birthday birthday and I think that was when I fell in love with Saturn just from the images in those in those books as well but I didn't see Saturn through a telescope for a long long time until after that I like I think the first time I actually saw it was during my undergrad and I'd already picked to study astronomy and astrophysics by that point but it was the first time that I'd actually had access to one of these not so good

telescopes that's what i was saying but they're not so good but they're also so incredible that you can see as much as you can with them and that you can get your hands on it was just like the student astronomy society's telescopes that they'd you know cobbled together and i just remember being absolutely blown away i think what's nice about saturn as well is if you've got you know even a 60 millimeter refractor is just about enough to see the rings. I guess it shouldn't surprise us because, you know, Galileo almost saw the rings, right? They were discovered in the 17th century. So you sort of think you've got to have something really amazing to see them.

But no, you can glimpse them. And my experience is even a tiny telescope showing people that they're blown away by just that, you know, they know they're real. Yeah. blown away by just that you know they know they're real yeah i think for me it was at high school where i was doing physics as a gcse and then we started doing a couple of questions and modules on astronomy and i just always had so many questions i was just like but but the thing and why and what why is it like that and how can we see that? And I think I just continued studying physics and the further I went along in education, the more questions I just had about astronomy

and somehow here we are. I call it a gateway science, Izzy, so it definitely worked for you. Okay, so Becky, we've had this question from Andrew in Australia. He says, Dr. Becky and Izzy are awesome and fun to listen to. It's so refreshing to hear women talking nerdy. We've been doing it for a long time.

It's just there's now platforms for you all to hear us talking nerdy. We now have microphones in front of us. Okay, to my question. Preamble. I read the Earth's moon doesn't have a dipolar magnetic field like Earth. So that's like a bar magnet if you have a magnetic North and South pole and the magnetic field travels North to South.

I'll continue. And that out of all of the moons in our solar system, Jupiter's Ganymede is the only moon that does. My question is, if that is true and given our moon is tidally locked, how can we say it has north and south poles? Doesn't that just make the idea of lunar poles arbitrary? How did they, whoever they were, decide it has poles and where to locate them? Yeah, so we talk about all the time being like, oh, we've, you know, found traces of water on the moon's south pole, or

there's these big craters on the moon's south pole and stuff like that so it doesn't have magnetic north and south poles so there's no liquid core in the moon to produce you know the sort of convection this moving sort of electrical charged particles in sort of like a liquid metal core that gives you the magnetic field the magnetic field of the moon it does have one but it's from magnetic materials in the core and on the surface. So just, you know, magnetic metals and things like this that are traced. And you can see maps of sort of like how much of a magnetic field does the surface of the moon have, which is literally just from the alignment of all of those materials on the surface and the concentrations of them. But that doesn't mean that the moon doesn't have poles because it still spins on an axis

that takes 28 days to spin on its axis, but it's the same amount of time that it takes to orbit the earth as well. So just like how the earth's magnetic north and south poles don't align with the geographic north and south poles that the earth spins on, it has like two poles, if you will. The moon just has the one geographic north pole

but then you'd say okay well how do we decide which way is north if which way is south if we don't have like a magnetic field sort of as you say traveling from north to south the guiders of which one is north and which one's south and essentially that's because um the way we decide which one's north and south is arbitrary. And it's based essentially because scientists all agreed on a same definition. And that was essentially the sun has a north pole and the earth has a north pole. And they both point in the same direction. So we'll take roughly that as north. And they take sort of an average of the solar system essentially.

What that means is that for any direction that a planet rotates in, you can work out which way is north based on if it sort of appears to be going sort of clockwise or anticlockwise so if you take your right hand and you make a thumbs up if you curl your fingers in the direction that an object is rotating in then the direction that your thumb then points whether up or down is your north and that's essentially how we determine north or south poles yeah i'm back in that science gcse lesson oh yeah okay robert hillary reached out on instagram and says hi i absolutely love your podcast i binged nearly two years of episodes when i discovered it and get a bit giddy when i see a notification for each one released since i laugh so much at every episode with you not at you i promise my question is about longitude i know latitude affects what you can see in the night sky and where but to what extent does longitude

affect what is visible for context i'm in seattle 47.6 latitude and i know latitudinally i'll see something similar to london but want to know if anything is significantly different from what Robert describes each month because of longitude. Thanks. Well, thanks, Hilary, as well. We're obviously not offended by laughter. Also, she would have been laughing at us if she could have just heard Izzy trying to say that question. Honestly, longitudinally. I can't even say it now. Let's, Robert, just carry on.

Yeah, I mean, look, the answer is that it mostly doesn't affect what you're seeing. You're quite right, Hilary, that latitude is much more important. It's just really the time that you see it. So obviously thinking, you know, how we're looking out at the sky during the day, someone on the other side of the earth is looking out at night. And that's really the difference is that you're just looking at things at different times. So sunset for you in Seattle is about eight hours later than it is for us. But there are some exceptions, and these are time-dependent events.

So, for example, if you've got, say, an occultation like the moon moving in front of a planet, then it might be that that's a fairly short-lived event. The moon is its own diameter about every hour or so. So if we're seeing it in the middle of the night, it might not be visible where you are just because the moon isn't above the horizon at that time. And you also, if it's very close, like an object, when you're looking at something like the moon, then you get a parallax effect. You get a slightly different view of it. It'll appear to be in a slightly different position depending on where you are on Earth. So in the main, the answer is not very much but there are some exceptions another one as well of course is

solar eclipses where you have to be on a particular ground track so you can be at the same latitude as the ground track but if you're east or west of it then you won't see say a total solar eclipse for that reason thanks robert and becky hannah on instagram messaged to say, Hi, can you please talk about the new image of M87? It's so different from the first image. The black hole shadow is much bigger and the accretion disk is much narrower. Can you please explain why and what it means? Yeah, sure, Hannah. I'd always talk about black holes. You know me. So the telescope that got the data for this image, the Event Horizon Telescope,

is lots of telescopes across the entire planet that are combined to make one earth-sized giant telescope the thing is obviously you don't have actually telescope dishes covering the entirety of the earth you've just got one sort of in lubama and one in hawaii and one in chile and one in antarctica all being combined at once um and so you've got gaps so it's almost like you've imagined the earth is like a disco ball, and you've taken away pieces of the disco ball. So you've just been left with little tiny sections that are reflecting the light, and you're actually getting some data from those, but for the rest of them, you've got gaps. Now, in the original image that was released back in 2019, to fill those gaps, they used a computer algorithm. But they didn't tell the

computer algorithm what they expected the final image to look like. They gave it no information, essentially. And it used the data that it already had to infer what is the most likely thing to fill those gaps, given the fact that this is an image of an object, right? But we didn't tell it what that object looked like. Now, this new image, they've not taken any new data. They've gotten the exact same data that was used to produce the 2019 image. But this time, they've not taken any new data they've gotten the exact same data that was used to produce the 2019 image but this time they've used an algorithm where they have essentially trained it on like as in like sort of like this is what we expect the image to actually look like on simulations of black holes so general relativistic using einstein's gravity simulations of what the material around a black hole should look like. And so that gives the computer a bit more information

and essentially makes its prediction of sort of filling in those gaps and inferring that missing data a lot more precise, which is why you end up with a much thinner ring. The reason they have decided to sort of do this is essentially because we've got two images of black holes now that look very similar to what our simulations suggested they should look like without us telling computer algorithms any information on what they should look like.

So that gives us a sort of basically more confidence in the fact that our simulations are probably right or probably giving us something close to what is reality. So using that simulation to train or using lots and lots of simulations from all different angles from you know different accretion rates around the black hole with different brightnesses all this kind of stuff you can then infer a much more precise ring around the black hole which is why it is much thinner and then also the properties of that ring that the thinness the thickness whatever you want to call it that can also help you infer a much more precise mass of the black hole as well it's a little bit circular because you've used general relativistic simulations to infer what this object should look like and then you're using it to derive a mass using general relativity again. So it is a

bit circular in that respect but we're more confident that that would be a reasonable thing to do now that we've got a few more images showing that it is looking like we expected. now that we've got a few more images showing that it is looking like we expected. Nice. Okay, and finally, Robert, PlanetQuest asks, what would be the most efficient way of space travel? Well, if only it was that easy, really. Yeah, I mean, we're still a long way from it being trivial,

despite what you hear about space tourism and so on. Putting people in orbit en masse is a really big deal, let alone travelling further afield. But it is fair to say that there's lots of work going on to try and improve it, I think simply because the commercial drive is to get satellites in space more cheaply, for example. So things that come to mind are, well, reusing components. So SpaceX has obviously pioneered that and looking at things like trying to bring the rocket stages back to Earth, looking at things which are not on stream yet, but might be in the next decade, like the Skylon design, trying to go from a jet engine to a rocket engine. So, you know, going from ground to orbit, flying up, literally, you know, very science

fiction stuff. And once in space, one thing that missions do do routinely now is they have ion drives, electric propulsion, and they consume their fuel much more slowly than a chemical rocket does and they can operate over a very very long time we've seen that pretty routinely used now for for many years another idea that's really just in its infancy is solar sails which is where you use the pressure of sunlight because light has a has a pressure and you only really feel it in the vacuum when you're out in space but you can use that to steer spacecraft, at least in theory. Well, I say it's not quite in theory. It's actually been done with the Icarus probe to Venus, but not quite routinely yet.

There is also a really exotic proposal to send a mission to another star that way, using a laser based near the Earth to power it to send it to Alpha Centauri. I mean, that's hugely ambitious. And the weight of the probe, from what I recall, has to be about a gram. So you've also got to design instruments that can work within that mass and then somehow send the signals back to Earth. And then closer to home, there are ideas like having a nuclear thermal engine. So basically using a nuclear reactor to heat gas and eject that. And that would be a bit quicker, for example example if you wanted to send astronauts to mars and then finally and this is also very far future i think then there's been conversations over the

years and ideas developed around space elevators which is where you essentially have something say near the equator the earth perhaps in the ocean tethered to the earth's surface and this big thing going right out into space and essentially using the rotation of the earth to throw things out into space but we're a bit away off from doing that because we haven't really got the materials that are strong enough just yet but maybe if you're recording this in 300 years time that'll be a routine way of getting to orbit or off to mars i just love the idea of like getting in an elevator and it's like what floor would you like to moon oh brilliant well thank you everyone keep the questions coming you can email podcast at ras.ac.uk tweets at royal astro sock and we're also on instagram at supermassive pod we'll be back

next time with an episode about ground-based observatories so think the extremely large telescope and the vera rubin observatory and the square kilometre array. Get excited now. But until then, everybody, happy stargazing.