Daniel and Jorge Explain the UniverseDaniel and Kelly talk about how the gravitational dance of the Earth and Moon, and other objects in and out of our Solar System.
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Hey Kelly, I have a controversial question for you.
Oh boy, coming from you that makes me particularly nervous. Is this another episode I'm gonna have to make sure my kids don't listen.
To no, but it might cause some other kind of family drama.
Okay, now you have my attention.
Well, you know, since you're married to a cartoonist, I'm wondering if I'm allowed to ask you whether you're a fan of a different cartoon.
Well, that depends if it's a rival cartoon or not.
So I'm wondering if you have a favorite cartoon from the Far Side.
Oh, I mean he hasn't updated for a while, right, I mean, I suppose he's still competition. But anyway, all right, let's see, let's see. Uh, so hard to pick so many favorites. So there's one where there's a slide, you know, like a kid's playground, and some spiders have built a net or a web along the bottom, and one spider is saying something like, uh, if we can pull this off, we'll never be hungry again, nor something like that. And I, as an invert person, just thought that was that was amazing. And of course I liked the like School for the Gifted, the one that was on T shirts when I was a kid.
What about you, Well, I have to ask you, do you see that spider's eating a baby cartoon? Differently now that you're a parent?
No, still hilarious.
US. I think your children should be terrified if they listen to this episode.
Probably, Oh my gosh, poor kid won. The other day I told her to go get a bowl in my office, and she thought it was going to have something like candy in it. But she accidentally picked up the bowl of dead insects that I found in the barn that I was thinking of pinning at some point, and they were all over my office. So she must have seen it and thrown it up in the air and they went all over. My kids are going to have very high therapy bills at some point, but I love them very much.
I think that living in your house must be even weirder than the Far Side Cartoon, almost certain. Hi. I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I don't think I've yet traumatized my children with insects.
Hi, I'm Kelly Wiener Smith, and I study creepy crawley things at Race University, and I'm traumatizing kids all the time.
Education through trauma is set your strategy.
That's a good way to make people remember.
Stuff where you live out on the farm, but there's lots of access to creepy crawley weird things, and so I hope your kids are become ver acclimatized to.
That I'm making them because.
It takes a brave soul to explore the nature of the universe, or even just your backyard, depending on what's slithering out there. So whether or not you turn your eyes down to the ground or up to the skies, there's a whole lot of stuff to wonder about, and so welcome to the podcast. Daniel and Jorge explain the universe in which we do all of that wondering. We think about everything that's between your toes and everything that your eyes can gather as you look up into the night sky. We think about what's on the moon, what's on the backside of the moon, and what's in the deepest, darkest depths of the universe. We chew all of it up and try to digest it and explain it to you.
And it's not always pretty.
Sometimes the truth is just gross. My typical co host and friend, Jorge Champ, can't be here today, so I'm very glad to have one of our regular guest hosts, Kelly. Kelly, thanks very much for joining us again and terrifying your children.
Yeah, well, I hope I could terrify some people today too. Thanks for having me back.
But today we're not talking about the weird, creepy crawley bits that are inside Kelly's pantry. Strange as that might be, we are turning our eyes to the sky tonight. We are thinking about what's out there in the universe and what we can see and what we can't see, and how the gravitational forces that have built the structure of our universe also limit what we can see out there. If you were a kid like me, then you spent a lot of time looking up at the night sky, not just absorbing the stars, but also gazing on the closest object that's out there, the biggest, fattest thing in the night sky, and that's, of course our moon. It has so many fascinating features to it, but one of the most interesting features is that you always see the same features. Kelly, do you remember understanding this as a kid or wondering about why you're always seeing the same bits on the moon?
You know, I really wish you hadn't asked me that, because the answer is, when I was a kid, I didn't realize I was always seeing the same side of the moon. That hadn't really hit me until I think, baby, I was in college, and then I was like, wait a minute, and I guess I just wasn't I was looking down at the salamanders and not up at the sky. But so no, that is not a question I had when I was a kid, but it should be when did you learn this?
I remember learning about it, I think when I was seven or eight. But you know, my dad was pretty into astronomy and into science, and so we talked about this kind of stuff. One thing I'm still curious about, though, is when humanity understood that, or when humanity understood it? Was an interesting question, like did ancient Chinese astronomers, who spend a lot of time looking up at the Moon and predicting its eclipses, did they figure out that the moon was a sphere and that it was rotating and that it was weird that we always saw the same side of it? Or is this just something they overlooked?
Do you know the answer to that question.
I don't know the answer to that question. I've been digging through the history of astronomy a little bit, and I know that Newton understood it because of his ideas about gravitation, but digging deeper into history, it's not really discussed very much, and I sort of wonder if people suspected that it was just a coincidence that the Moon rotates at this perfect rate, just so that we can only see one side of it, because you know, there are other fascinating coincidences about the moon. For example, the moon and the Sun happen to be the same size in our sky, right, the Sun is much much bigger, but much much further away, and those two numbers line up perfectly. So the Sun and the Moon appeared to be the same size, which is why we can get so many fascinating eclipse effects. And that, of course, is just a coincidence. There's no like deep reason for that. It just happens to be. These two numbers just kind of line up in our.
Universe, and humans are notoriously tripped up by coincidences, exactly.
And so I think ancient astronomers must have wondered about that size, and I wonder if they also wondered about the rotation of the moon, if they understood that. Now, I do know that ancient Chinese astronomers had a little bit of a disadvantage compared to, for example, ancient Greek astronomers because they didn't have geometry. Right, we think about the universe in terms of geometry. You imagine how clips has happened, and you think, probably in your head you have a mental picture of all these things happening and to explain it. But Chinese astronomers didn't develop geometry, so they didn't have the same sort of geometrical thinking that we're all very familiar with. Instead, they had like tables of numbers, and they had arithmetic, and they could manipulate these numbers to do their predictions. So I think they must have thought about these things very differently than we did.
Interesting, it's nice to have our modern tools.
Thank you to those folks who invented geometry. It turns out to be pretty.
Useful quote unquote modern.
Yeah.
Right.
And so we have been looking up in the night sky and looking at the moon and seeing the same side of it for thousands and thousands of years, and now of course we understand why. And so today on the podcast, we're going to be digging into the physics of the far side of the moon and many other objects in our solar system that exhibit the same behavior. On today's episode, well, I answer the question what is tidal locking?
And this is another thing that connects us to the dinosaurs right, because they also would have seen the same side of the moon.
If dinosaur geometers had figured that out, boy, they could have written a paper and got a lot of citations.
They really could have, and then maybe they could have started a you know, NASA and had an asteroid deflection program. Everything could have been different.
You sound like you're kind of hoping for that, but you know, in that scenario, we don't exist.
Oh, good point.
We're rooting for the asteroid on that one.
That's right, we sure are.
I'm sorry dinosaurs, which puts us on the anti Math Education for Dinosaurs committee as well.
It's an uncomfortable place to be. But no, it doesn't feel very good, but I'm firmly in this position. Yeah.
So this is a topic that affects our Moon and affects our planet, and affects a lot of things in the Solar System and in other Solar system. It's a fascinating little bit of physics. So I was curious what people knew about the topic of title locking and if they understood the physics of it. So I went out there into the Internet to ask folks if they understood this. Thanks very much to those of you who participate in this segment of the podcast to let us know what people think about the topic of the day. If you'd like to hear your voice for a future episode, please don't be shy. Write to me too questions at Danielandjorge dot com. So think about it for a minute. Do you understand the physical of tidal locking. Here's what some listeners had to say.
So, I believe that tidal locking is when, say, a planet has a moon that's locked in orbit at a certain distance, so the tides are always the same. So I guess you could say that we're in tidal locking because day in and day out, our tides go the same because.
The moon's in a fixed orbit.
Tidle locking is just like our moon. We see the same face of it at all times.
It has no spin, and the dark side of the moon is facing outward.
It is the deformation that a celestial body so first when it's orbiting or is orbited by another massive celestial body.
Tidal locking is when a lower mass body is orbiting a much higher mass body and the same side always faces it. So I know that we always see the same side of the moon. Means tidally locked to the Earth.
I was impressed that none of the listeners seemed totally stumped by this, because I feel like title locking is not necessarily something that you hear, like that, at least there's a phrase I don't hear very often, and so like, it seems like some people knew exactly what you were asking about, and none of them were like me when I was a kid, and we're just totally unaware that this is a thing that was happening.
Yeah, we got some pretty well educated listeners. The word title I think makes some people think of tides, which is connected, of course to tidal forces, you know, like the moon pulls on the Earth and squishes its oceans and makes us have higher tides in some places and lower tides and other places. But that's not what we mean by title locking. Title locking is actually a different phenomena we're going to dig into today. But thank you very much to all of our volunteers. We really cherish your thoughts.
Yeah, you have a smarter than average audience, I think, or smarter than Kelly average audience. I'm always impressed by the answers.
They're smart and they're good looking too.
That's right. I think that's called the halo effect when you know what good thing about them, and you assume you know all the good things about them.
So let's dig into the physics of tide locking what it is, what's going on, and why I play such a big role in what we see in our night sky. And I think the first thing to understand is just like the geometry of what's going on. Obviously, the Moon is orbiting the Earth, and the Earth is spinning, and the Moon is spinning around the Earth, and the Moon is also spinning on its axis, so there's sort of a lot of spinning going on. But you got to get all that spinning in your head to understand, like why we only see one side of the moon.
Yes, so I am staring directly ahead trying to picture everything, So go ahead, what should I be imagining?
So first, just put yourself on the Earth and forget the fact that the Earth is spinning, and just move the Moon around the Earth in your mind and imagine the Moon is like a shoe or something. If the Moon is not spinning, it's just orbiting the Earth, then the Earth is going to see different sides of the shoe. It's going to see the back of the shoe. It's going to see the front of the shoe because as the shoe moves around the Earth, different sides of it are going to be closer to the Earth or closer to outer space.
So the shoe is not rotating either.
That's right. So first the shoe is not rotating, and so we see one side of it, and then we see the other side of it. So if the shoe or the moon or whatever is not spinning, then you would see different sides of it. Right now, the Moon, of course is spinning. It's spinning on its axis. But if it was spinning at some random rate, like really really fast or really really slow, you would still see both sides of it because eventually it would spin in a way that revealed every part of the Moon. The only way for us to not be able to see part of the Moon is for its spin to be perfectly syncd up with its orbit, so that as it goes around the Earth, it turns just the right amount every second, so the same side of it is always facing the Earth, like if the back side of the shoe is always facing the Earth. Because the shoe itself is spinning at just the right rate as it goes around the Earth.
And everyone on every part of the Earth is always seeing the same side of the moon. Right, It's not like you know, in the US you see one side, in Chinese the other side or right, you're always seeing the same side.
That's right. The same side of the Moon is always closer to the Earth, and the other side of the moon that we call the far side, which I assume is the origin of the name of the far Side and comic it's always facing outer space. And this is the sort of coincidence we were talking about, Like these two numbers, the rate at which the Moon goes around the Earth and the rate at which the Moon spins around its axis have to be perfectly synced up to get this effect.
And why are they perfectly syncd up. Is this one of the few instances in which we have something special going on and they just happen to sync up? Or are we just like every other moon Because this happens all the time.
So it's not that special and it's not a coincidence, but it's not something that's always happened. Like to really get into it, you have to understand the history of the formation of the moon, like how did it get made how is it spinning originally? And then how has that changed? You know, our moon is not just like some object that we captured as it was flying by. We think that the Moon actually comes from a huge collision something like one hundred million years after the Earth was formed. So we're talking like four point four billion years ago. You have some like very hot proto Earth. It got slammed into by some giant Mars sized planet they called Fea, And so you're talking about a huge collision like Earth versus Mars in space, and both objects are essentially vaporized.
But we still have thirst in the moon.
You're like, how that to current reality? You know, you're exactly right. It got vaporized, but then it coalesced. Right. Gravity is very very patient, and so even though all of its original work in the first one hundred million years was ruined by this collision, it got back to work and it pulled those blobs together and it made two new blobs, one that formed the Earth and then a big ring around the Earth, like a ring of debris that was spinning sort of too fast around the Earth to fall into the central plump, which formed this like huge spray of material.
All right, So and that's why the Moon's made out of the same stuff the Earth is made out of.
Exactly when we visited the Earth first and we sampled it, people were surprised to discover that it's basically the same mixture of stuff as the Earth. It's not like a different Solar System body that was formed in another place, and so it has a different mixture of like ices and rocks and different isotopes and stuff. It's basically made out of the same stuff as the Earth. And that's because it comes from the same mixture of those two planets. Like it was the same vaporized blob of stuff, and some of it fell in towards the Earth, and the stuff that was spinning around sort of too fast ended up in this big debris ring, but then gravity pulled that together into a moon.
Did we have this hypothesis before we went to the Moon and found what it was made of, or did going to the Moon and physically collecting samples give us this hypothesis when we were like, whoa, it's the same stuff.
This hypothesis has been around for a while, but it really became the favorite once we went to the Moon, and also once we understood something about the Moon's inner core. The Moon has sort of a small iron core, smaller than you would expect, and the hypothesis is that a lot of that iron was probably lost from when the impactor hit the proto Earth, and some of that was like melted and vaporized, and so it would have a bigger iron core otherwise. This became a much more popular hypothesis after we went to the Moon and visited and understood what it was made out of. And there's a lot of really interesting physics there that's relevant to it, Like why doesn't it all just collapse into a planet, right, Why doesn't it all just become one huge giant planet. How come part of it ended up as a ring? And that's just because you know, the whole thing is spinning and a lot of it has angular momentum, and so some stuff was just moving too fast to fall in the way like the moon now is moving too fast to fall into the Earth. It's all about that velocity. And then when that stuff gathered together into a moon, that stuff was spinning. So the moon had some original spin, right, that came from the spinning motion of all of that stuff. After the vaporization, we think that spin rate originally was much much higher than it is today, So the Moon used to be spinning faster than it is now.
Was there this is maybe an unfair question. Was there anything alive on Earth when it was spinning fast enough that sometimes you'd see what is now the far side of the moon. So like dinosaurs probably never saw the far side of the Moon, but did like the first bacteria see the far side of the Moon. I mean, I'm sure they don't see anything that far away, but you know, could they have?
Yeah, it's possible. We don't really know exactly how old life is on Earth, but it stretches back billions of years, right, and so it's possible that there are critters that have lived on Earth that have seen other parts of the Moon. That's a very cool thought, because the Earth has been gradually slowing down the Moon's spin little by a little every year. And the opposite is also true. But you know, until nineteen fifty nine, no human had ever seen the backside of the Moon. It was this like dark spot in our vision that we just couldn't no matter how many thousands of years we've been staring up at the Moon, nobody had ever laid eyes on the backside until nineteen fifty nine, which feels really recent.
Right, And the Soviets did.
It right, Yeah, they certainly did.
Yeah, they beat us there with the lunar program. Their third one made it there. I think the first two maybe blew up. That was a thing that happened a lot for the Soviets. But anyway, so.
Sorry, no, there's a really fun set of stories there. Maybe you know about because of your book research, that the Soviets were there first and so they got to name a lot of stuff on the far side of the moon first, so they have all these Russian names and that like really annoyed the Americans at the time. And then you know, later this International Astronomical Union took over the naming, so it wasn't all just you know, named after kinds of vodka or whatever.
All come now, that's an unfair stereotype. Did the IAU change any of the names that the Soviet program had put in place, or they just like stopped the Soviets from doing any more naming.
I think to just stop them. And there's a lot of respect, I think for the people who discover something to name it. But then I think that the Civiets were so far ahead that they didn't want them to just like name the whole thing after their favorite Soviet flower or whatever.
Thank, yes, there we go. That's good. That's good. I think there is like a Gagaran and seas or something like them. All right, so we're gonna take a quick break and then we'll talk about what they found on the far side.
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The first time we got to see the first side of the moon was nineteen fifty nine. Not too long after that, the Apollo astronauts were flying around the Moon. We got to see the far side. What kinds of things were over on the far side To see Daniel, Well.
You know, in research, it's always exciting to see something nobody's ever seen before because you don't know what's going to be there. Right. Maybe it's like a crashed alien battleship or some weird new geological formation. Right, your imagination goes crazy when you're first landing on a spot and seeing something no human has ever seen before. But you know, in research, discoveries never guaranteed. Just because you land on new shores doesn't mean you find something fascinating. And in this case, basically we found that the far side of the Moon has more rocks and more dust, and it's not even very different from the front side of the Moon. I mean there are more craters, there are fewer of these like seas, these lunar lava flows, but all in all, it's not that exciting.
Yeah, that's the moon, good old, dependable.
Moon, And the first time anybody actually landed there was twenty nineteen, like just a few years ago the Chinese put a lander down and started to do a bunch of experiments. So it's only recently we started to in detail. So you know, there's still hope to find that crashed alien battleship.
Or the aliens might be living in a lavatude. We could find something even more exciting than what you've imagined.
Yeah, maybe so, or maybe we could build something that future aliens could land and discover for themselves, Like people are proposing building radio telescopes on the far side of the moon looking out into space, because then you'd have the whole moon to shield you from the noise of the Earth.
From my perspective, that's an interesting prospect geopolitically, because if you built one of those radio telescopes, it wouldn't work as well if people put other stuff on that side of the moon. So now there's like competition for that space. And can you tell someone they can't use the far side of the moon because your radio telescope is there. Anyway, these things are complicated, but yes, it would be a good place for a radio telescope.
Sounds like the kind of things a bunch of lawyers and a conference room are going to hash out one day. So the far side of the Moon is not that exciting on its own, but it is fascinating to understand why there is a far side of the moon. Before we dig into that, I just want to clarify something that's a common misunderstanding, which is the difference between the far side of the Moon and the dark side of the moon. Also, the Pink Floyd.
Album I don't think anyone needs clarification. We all know about that, and we've all probably seen the light show if we're around forty years old.
So some people imagine that the far side of the moon is also the dark side of the moon, that like, this part of the moon never sees sunlight, which is definitely not true. The dark side of the moon is the side of the moon facing away from the Sun. The far side of the moon is the side facing away from the Earth, and those two often don't agree. So when we have a full moon, the sun is illuminating the entire side of the moon that we are seeing because the Sun is sort of behind us in space. Then the back side of the moon, the far side that we don't see, is also dark. But the opposite scenario when the moon is totally dark when we see a new moon, and that's because the far side of the moon is totally lit up by the sun. Right half of the moon is always lit up, it's just sometimes not the side of the weird sie, So the far side and the dark side are different. It would be like a Gary Larsen Pink Floyd crossover event when they meet.
I'm pretty sure if you play dark Side of the moon backwards. It explains that.
That's how you understand the far side cartoons, right, you have to listen to the dark side of the moon backwards.
Yeah, there was a lot of cross talk between the arts back then. Okay, so how is this happening? The moon was going faster, you said, and it's been slowing down. Why has it slowed down to be exactly the same rotation rate and stuff? What's going on?
So it's not a coincidence, right. Physics seems to like this scenario, and it's called tidal locking because it's connected to the concept of tidal forces, which are in effect we see all the time in gravity. And it sounds complicated, but it's really pretty simple. The only thing you really have to understand is that gravity gets weaker as distances get longer. So if you're further away from the Sun, it's gravity is less powerful. As you get closer to the Sun, it's gravity is more powerful. That makes sense. But now imagine you're a pretty big object. So part of you is closer to the Sun than another part, then part of you is going to feel stronger gravity than the other part. And this happens all the time. Like if you are standing on the surface of the Earth, then the Earth is pulling on your legs harder than it's pulling on your head, right, and so that's a tidal force. Because there's a difference there. You can actually think about the earth as sort of like trying to pull your head off of your body.
Is this like many spaghetification.
Yes, exactly, when it's very powerful. When it's more powerful, then the force is holding an object together, that's spaghetification. And so when you get near a black hole, for example, gravity is super powerful, and the difference between the gravity at your feet and your head is much more dramatic and much more powerful than the force is holding your head on your body, and so it's effectively natural decapitation or spaghetification.
But lucky for us, instead of death, we get.
The tie exactly, so you don't have to worry about that because it difference of the force of gravity on your feet and on your head is very very gentle, and your neck is very powerful in comparison. But the moon is pretty big, and so the difference between the gravitational force on one side of the moon and on the other is much stronger. And so what happens is that the Earth basically turns the Moon a little bit into a football. It like makes it a little bit all blong, because it pulls on the closer bits more powerfully, and it pulls on the further bits more gently and has the effect of sort of like stretching it out, making it longer like a football instead of like a sphere.
This is another one of those things I'm surprised we ever figured out, because when I look at the Moon, I would not think football.
It's a subtle effect, right, It looks like a sphere, but is slightly distorted by the gravity of the Earth, and that has a big impact on how it spins, because now it has a gravitational preference to be aligned a certain way with the Earth. If it was a perfect sphere, then the gravity on the object wouldn't change as it spins. But if it has a bulge, then gravity prefers that bulge to be aligned with the object. Gravity has a handle now on the Moon, and it likes to pull that bulge, so one point is towards the Earth and one point is away from the Earth. That's like gravitationally less energetic a configuration than having the whole thing spin.
All right, Okay, so I'm trying to wrap my head around this, and picturing things in my head is not one of my stronger suits. But here we go. Okay, so we prefer to have a grip on the handle. Does the moon spin at a not constant speed where like we hold on to the handle for a little longer. I don't think that's what you're saying, And so what am I missing?
So what happened initially is that the Earth started to form these bulges on the Moon, and then the Moon was spinning, and so these bulges were like ripples through the surface of the Moon. There were like these waves that pass through the Moon. But Earth is also pulling on those bulges, right Like, if the Moon has a bulge, it's like a football, and then it starts to spin away from the Earth. The Earth is going to pull on that nearest point a little bit and try to pull it back, and that effectively moves the bulge through the surface of the Moon a little bit, and so that slows down the rotation of the Moon a little bit because the Earth is like tugging trying to pull that point back towards it, And so that slows the Moon down a little bit, and eventually the Moon settles into a pattern where these waves the bulge doesn't travel over its surface anymore, like settles into one location. And that's what the Earth prefers gravitationally.
Okay, I'm with you, And so.
That's how gravity makes the Moon a little bit pointy and also slows down its spin. It's like stealing some of its energy a little bit to slow down its spin.
But why does its spin get slowed down to exactly the right amount, Like it seems like that should just slow things down in general.
It does. It slows things down in general, but at some point it starts spinning at just the right speed so that the pointy bit is always pointing towards the Earth. And then the Earth is happy, and the gravity of the Earth no longer wants to slow down the spin of the Moon because the point of bit is always facing the Earth, and so gravity is happy because it's sort of like a ball rolling to the bottom of a valley. It's happy when it's settled in the bottom of the valley. So this is sort of like, you know, you let a marble go at the top of a hill and it's going to roll down. It's gonna awesantly pass the minimum, but eventually friction and everything, it's gonna settle down in the minimum, And the same thing happens. There's like friction inside the Moon, and the Moon gets heated up a little bit by this, and the Moon also speeds up in its orbit because there's like conservation of angular momentum. So you've slowed down the Moon's spin a little bit and you've sped up its orbit a little bit, which is why the Moon's orbital radius is increasing a little bit.
And so how rare is this? Like if Earth were a little bit smaller, would this not happen? If there was, like if Mars was a little closer, would this not happen?
Like?
How lucky are we that it worked out this way?
We're not that lucky. It's sort of the eventual fate of almost every pair of bodies that orbit each other for long enough. And in fact, we've done it to the Moon, and eventually the Moon will do it to the Earth. It's just a slower process.
Whoa like, how slow like are we gonna be swallowed up by the Sun before it happens? Or is my you know, kid gonna see it before they turn eighty.
Yeah, So the Earth's rotation has already been significantly slowed by this effect from the Moon. Over the four million years since the Earth and the Moon were formed, the length of an Earth day has lengthened from six hours to twenty four hours. So the Moon has made our days four times longer. It's slowed down the Earth spin by a big factor. And eventually, because the Moon keeps tugging on the Earth and making it a little bit longer than tugging on that handle as it passes, eventually it's gonna make the Earth take forty seven days to spin. And at that point, the Earth and the Moon will both be tidally locked to each other, and the Moon will only ever see the same face of the Earth.
I don't think that's very nice of the bit. I like having days, ah, so I'm glad that's not gonna happen in my lifetime.
That'd be fascinating, right, because it would mean that half of the Earth would see the Moon and the other half would never see it. If life of alt in that planet, then like half of life wouldn't even know there was a moon.
WHOA I bet we'd have all sorts of different like creation stories. If some of us had moons and some of us didn't, that would be that would be fascinating.
Or imagine being an explorer and like settling around the world and then discovering this huge thing floating in your sky. You're like, what, that would be crazy, that would be super amazing. But you're right, it's going to take a long time. And well before that happens, the Earth is going to be eaten by the Sun, which is going to end its life cycle in about five billion years, turn into a red giant and absorb the Earth. So the moon is not powerful enough to make that happen anytime soon.
Well, on the upbeat topic of the end of our planets, let's take a break, and when we come back, you can tell us if the other moons on the other planets are doing this as well. If I live on mars Am, I going to see all of Phobos or not.
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Okay, we're back. So, Daniel, the question that everybody wants to know when we move to Mars, are we going to see the far side of Phobos and Deimos or what?
So this is a process that happens all over the Solar System and all over the universe. Eventually, gravity will do its bit and dis door objects from spheres into like gentle little footballs. It'll grab on those handles and it will hidly lock stuff. And so as we look around the Solar System, we notice that all of the big moons, all of the twenty round moons that exist around planets, are all tidally locked to their planet.
Only the round moons. And what causes the moon to be round, It's that it needs to be a certain size, Is that right?
Yeah, if you're big enough, then gravity is going to be powerful enough to pull down any big features, which is why, for example, like neutron stars have no features on them that are higher than one millimeter, and the larger the gravity, the harder it is to keep a stack of stuff from falling down. And so if you're a pretty small object, you don't have to be very spherical because your gravity's pretty weak. If you're a big object like the Moon or the Earth, then you're going to end up as a sphere because stuff falls down basically, and stuff falls down more when you have more gravity. And so these round moons are technically not actually round, right, they're a little bit footballish and they're all tidally locked to their planet. Most of these also orbit pretty closely, and so their gravity is pretty powerful. And then example, you raised like Phobos and Demos on Mars. These are little moons and they're not very large, they're not very round, and they're actually spinning really really fast. And so while Mars is doing its job to try to tidly lock them, Phobos, for example, is not yet tidally.
Locked, but it will be.
It will be eventually. It's going to happen to everybody.
So we've been talking about moon planets. Has it happened to any sun and planets or planets yet?
Absolutely it has, and absolutely it will. Right the Sun is the most powerful source of gravity in the Solar System, and so it's not going to be left out of this party. And it has mercury tidly locked in its grasp. But it's actually fascinating. It's not quite the same way as with the Earth and the Moon because mercury doesn't have as circular an orbit. It's much more elliptical. The mercury is tidly locked in this weird way. Instead of spinning once every time it goes around the sun's me three times for every two revolutions around the Sun. And this is not something we understood for a while because we can't always see mercury because it's so close to the Sun. There's a few spots in its orbit when it's like easy to see mercury, and so when people were observing mercury, they were always seeing the same side of it. So they thought, oh, it must have been a one to one tidle locking. And it's only later when we got more observations in mercury did we notice, oh, no, it's doing this even weirder thing. That's because the orbit is not circular and so it can do another little bit of spin as it gets further away and then it comes back around, but still it settles into this really cool pattern, this three two tidal locking.
All right, So let's see, my very elderly mother were on V I'm guessing Mercury is tidally locked in part because it's so close to the sun. So is Venus the next closest to being tidally locked or is it tidally locked?
Also, Venus is next on the list to getting tightly locked. It's not yet tidly locked. In fact, Venus has a really weird orbit ends. It takes five hundred and eighty three days to go around the Sun, but it takes two hundred and twenty four Earth days to rotate, So like a Venus year only has one point nine Venus days. Like you only see two sunrises and two sunsets in a Venus year.
Huh, But that wouldn't bother you, because Venus would have killed you in four or five different ways, long before you cared about whether the sunrise was coming.
That's right. But Venus is sot again there right, Like its spin is similar to its orbital period. We actually think that's because of a collision that Venus was like smacked in du By something which really changed its spin. We don't think there's been enough time for it to be like almost highly locked by the Sun. It takes a much longer time because Venus is so far away from the Sun and so small relative to the Sun.
All right, so we've done moon and planets. We've done comparisons between the Sun and the planets. Is there any other combination of tidally locked things for us to think about? Can asteroids get tidally locked? For example?
They could, but it's much harder because their gravity is so weak. Another really fascinating thing to look at are things that have similar masses, like dwarf planets and their moons. So, for example, Pluto no longer a planet, but it's still a dwarf planet. It's got a pretty big moon, Sharon, And these two things are mutually tightly locked already. So if you're like on Pluto and you look up, you always see the same side of the moon. And if you're on Pluto's moon and you look up, you always see the same side of Pluto.
Oh, interesting to how many moons does Pluto have?
Oh, Pluto's got a bunch of moons, but most of them rotate chaotically. They're too small to have been tidly locked so far. But it's sort of beautiful to see these things like orbiting their center of mass and facing each other. It's kind of like a dance.
Yeah, that's awesome. It's beautiful.
And that's not the only example of a dwarf planet. There's another one out there, a trans Neptunian object called Eris, which is also a dwarf planet, and it's got a pretty big moon called Dyspnomia, which happens to be the second largest dwarf planet moon after Pluto's moon, And these two are also tidly locked with each other.
That sounds like it's a pretty common thing for dwarf planets to have moons, that know, because how many dwarf planets are there.
There's a lot of dwarf planets out there. That's one reason why Pluto isn't really a planet anymore, because we discovered there's kind of a lot of them out there. And if we call Pluto a planet, we've got to call them all planets, and then we're going to have a lot of planet and so, you know, astronomers have big arguments about where to draw the line, But there's a lot of dwarf planets out.
There, right, and like, how are we going to have mnemonic if there's twenty of them or something like we just no way, no way, all right. So that's our solar system. This sounds like a fundamental physics thing. So I'm guessing that when we look at other solar systems, they've got the same thing going on.
Yes, exactly. We expect that the same thing will happen to planets around other stars and to moons around those planets. And because of sort of the way that we we can discover those exoplanets, we expect a lot of the ones we've seen so far have been tidly locked. Like the way that we discover these planets is by seeing their impact on their star. Either they pass in front of their stars, so they give like a little mini planetary eclipse which dims the star a little bit, or they wiggle the star because of the gravity of the planet pulling on the star. So in both cases they have to be pretty close to the star for us to see it, which means that of all the planets out there in the galaxy orbiting their stars, we're best at seeing the ones that are closest to their stars, which also means we're best at seeing the ones that are probably tidally locked. And so astronomers have done a bunch of calculations to understand the orbits of these things and the masses and predict their tidal locking, and they expect that a lot of these planets are tidly locked, maybe one to one, maybe like three to two the way Mercury is, or maybe like five to two. It depends a lot on the eccentricity of the orbit, which the sort of gravitationally preferred arrangement.
Would it be fair to say that it's too far away for us to have been able outside of our Solar system to see planets and their moons being tidally locked, and so the only thing we've been able to look at so far is tidal locking with suns and planets.
Yeah, that's exactly right. Exo moons is a brand new area of study. People looking for these moons around exo planets super exciting, and in the next few years, as we turn on more and more space telescopes, we're going to get more more information about these planets and their moons and we can start to measure their spin and to understand this and to figure out like whether these planets out there a lot more of them are tidly locked or a lot less. You know, a question we always have is is our solar system weird or is it typical? Like in most cases, is it just the first couple of planets closest to the Sun that are tidally locked, which would align with our understanding and tell us that our solar system's not that weird, or maybe would be surprised and we'll discover, oh my gosh, all those solar systems out there, they're all tidly locked. What's going on? And something is different from what we expect. You never know when you go out to the universe and ask these questions and look for the first time, if you're just going to see like more dust and rebbel or something really really exciting.
The universe is good at providing job security in the form of new questions.
And it's not just stars and planets we can study. We can also look at pairs of stars. We're used to thinking about stars as like individual and you have a solar system with one star at its core and a bunch of planets, but Actually, there's lots of binary star systems out there, and the reason is that stars when they're formed, it's a big cloud of gas and dust which collapses, and typically you get multiple stars from a big cloud of gas and dust. You have like little gravitational seeds that start this runaway gravitational effect. And you don't just have one seed, you have a bunch of them, so you get like a stellar nursery makes lots and lots of stars. So we have a whole podcast episode about binary star systems and even like trinary star systems. But the point is that lots of stars out there have brothers and sisters they were born with like twins, and so they're pretty close to another star, which means that you can have pairs of stars that are tidly locked to each other.
That's awesome.
Or if you have a really big planet, you could have a planet and a star that are tidly locked to each other, so the planet is always seeing the same side of the star. Right, that could be weird.
Yeah, but we haven't seen that yet, Right, we.
Have not seen that yet. There's a star out there Tau Buddhists, which we suspect is tidly locked to its planet. But we're not one hundred percent sure of that. But you know, this kind of stuff makes a difference because it really changes the experience of being on a planet. Like if Earth was tidly locked to the sun, we'd have half of the Earth at sunshine all day, and half of the Earth would be the dark side of the Earth. The far side of the Earth would be the dark side, you know where Pink Floyd plays concerts and Gary Larson draws his cartoons, And that would be the very very cold side of.
The Earth, right dead side, Yah, the.
Dead side exactly. And then you'd have this ring right in the middle of the Earth that was like right on the edge. It'd be like permanent sunrise, and that might be the only place on the planet you could live because one side would be too hot and one side would be too cold. And then you'd have this like Goldilocks ring around the planet that might be hospitable.
Location, location, location, exactly.
And so if that's the case on some of these exoplanets, that means it'd be a lot harder for life to form. Either they'd have to evolve to survive very hot conditions or very cold conditions, or there'd only be a thin strip of their planet that they could live on, and you know, then there wouldn't be seasons. In the same way, it would make for a very different kind of biology.
It would make it easy to study extremophiles, though, because you know exactly where you needed to go, just you know, the cold ones are over there and the hot ones are over there. But there probably wouldn't be a lot of funding for science that are world like that.
You never know, right, And we have all these expectations for what life would be like on these planets because we imagine what our life would be like on those planets. But of course if you evolve on those plants and if you think that's the normal way to live, and they would think it's super duper weird for like your planet to spin and get constantly bathed in sunlight, or for you to have no control over whether you're seeing the sun or not, because on tidly locked planets you don't want sun, you just move to the backside. You want sun, you move to the front side, totally up to you, whereas here we're like at the mercy of celestial objects. Spin rates to determine when we see sun and when we.
Don't, Yeah, I guess you get used to what you grew up with. It sure seems like it's nice to have a more moderate in between.
It certainly does. But in the end, the universe prefers tidal lock, and given enough time gravity, it's going to do its business make everything round and then make it a little bit footballish, and it's going to tug on those gravitational handles until everything is tidly locked, unless, of course, your sun explodes and eat you before it can even happen. But it's sort of the gravitational destiny of every other kind of object.
And there's the ending that'll keep my kids from listening.
And hopefully inspire a lot of crazy music and silly cartoons. Yes, all right, Thanks very much everybody for exploring the physics of title locking with us. It turns out to play a big role in what it's like to live on our planet, what it's like to look up at disguise and understand the universe or wonder at its mysteries, and it will continue to play a big role in life on Earth. Thanks for listening. Tune in next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeart Radio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases, Many farms use anaerobic digesters to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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This is Malcolm Gladwell from Revisionist History.
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