One Side of the Moon ALWAYS Faces Us. Why is that? | Tidal Locking

hey crazies you know how we always see the same side of the moon the side with the face on it well that can happen with planets too if there's enough time the same side of a planet could always be facing the star it's orbiting continuous sunlight on one side and continuous darkness on the other it would make the planet kinda look like an eyeball but will that ever happen to earth [Music] if you give a system enough time it'll settle into the most stable configuration it can find you could randomize the motion of particles in a room all you want but if they're bumping into each other a bunch of times they'll settle into something fairly predictable it's a stable configuration any minor deviation and the forces will bring them back in astronomy configurations like this are governed by gravity and they're called resonances we usually label them with some kind of ratio for example three of jupiter's moons have a one two four orbital resonance that means in the time it takes one moon to go around once the others go around two and four times respectively but today's video isn't about orbital resonance it's about spin orbit resonance rather than multiple objects having a ratio between them there's a ratio between the spin and the orbit of a single object kind of like how the planet mercury has a 3 2 spin orbit resonance with the sun that means for every three rotations or spins there are two full orbits for some places on mercury that means you could have two sunrises or sunsets in a single day which is kind of weird our own moon has a special kind of spin-orbit resonance called synchronous rotation that means the ratio between the spin and the orbit is one to one the time it takes to spin around once equals the time it takes to orbit once so the same side of the moon always faces the earth if you took a picture of the moon every day for a month the resulting gif would look like this and yes i pronounced it gif it's graphics interchange format graphics gif anyway synchronous rotation is actually a very common phenomenon even in our own solar system both of mars moons do it all four of jupiter's big moons do it some of saturn's moons even do it it happens a lot is what i'm saying all it takes is time but how much time depends on the circumstances in fact if the two objects were perfectly spherical none of this would ever happen it's the imperfections that make spin orbit resonance possible which if you ask me is the most interesting part of this whole process it's the imperfections that make it beautiful contrary to what we tend to think no object is ever perfectly rigid i've mentioned in a previous video that earth has an equatorial bulge because it spins well the moon spins too which means it also bulges at the equator those graphics are extremely exaggerated though the earth's oblateness is only about 0.3 percent the moons is about 0.1 percent we're exaggerating for clarity now spin is not the only thing that can cause ball there are other things that can cause bulging it had to be said let's try to get this line again now spin is not the only thing that can cause bulging gravity can do it too the earth and moon are pretty far apart but they still pull on each other that gravitational tug can and will cause bulging along the line between them that's not necessarily along the equator of either object so we can't call it an equatorial bulge we call it a tidal bulge tidal as in the tides this effect is most noticeable with the oceans the earth's gravity may hold the ocean down but the moon's gravity pulls a little to the side that combined with a massive amount of fluid pressure causes a bulge in the ocean this doesn't just happen with liquids though the earth may be solid rock but it can be deformed in exactly the same way remember nothing is perfectly rigid not only does the ocean bulge but all the rock underneath it does too just not as much how does this cause that sync lotron radiation though it's synchronous rotation also known as tidal locking and hi where have you been do you really want to know the answer to that actually no never mind anyway synchronous rotation occurs because of torque you know how forces can cause a change in speed well torque is like a rotational force it can cause a change in rotational speed for any spin-orbit resonance to occur there must be torque in our case that comes from the fact that tidal bulges can't change instantly if the moon is rotating quickly its bulge is going to lag behind of the planet moon line that means the forces on either side don't line up you get a torque opposite the rotation which slows it down a bit it's not a lot those bulges in the animation are exaggerated but if it happens over and over again for a hundred million year million million years worth of mill in here yeah the millionaires but if it happens over and over again for a hundred million years those little slowdowns are going to add up one snowflake may be pretty insignificant but a bunch of them is an avalanche planetary systems tend to be around for billions of years that's billions with a b the rotation of our own moon had enough time to slow down until the bulge was always along the planet moon line that's when its spin equals its orbit a similar thing would happen if the moon started out slower instead if it's rotating slowly its bulge is going to get ahead of the planet moon line that means the forces on either side still don't line up but this time you get a torque along the rotation which speeds it up a bit the end result is the same though synchronous rotation how long did it take to tidally lock like that getting a number is actually kind of tricky we do have an equation for it as long as we make a crap ton of approximations there are actually so many approximations that i don't feel entirely comfortable using it but it's the best we've got the general problem is so complex that it's completely unsolvable what this equation can do though is give us an idea of what factors are involved and how important each one is we've got some things you'd probably expect the rotation speed of the moon the radius of the moon the orbital radius of the moon the mass of the planet and the gravitational constant of the universe all things we either know or can find by taking a few measurements unfortunately these two we often don't know very well something called the tidal love number which measures the elasticity of the moon and the specific dissipation function which tells us how much energy is lost to heat we do know them for our moon which is how i knew it took 100 million years if you're trying to calculate this for any other object just just give up now we can see some patterns in this equation though if the elasticity is zero meaning the moon is perfectly rigid then time is infinite title locking never happens because no elasticity means no bulges the more energy that gets dissipated the longer it's going to take we can also see the radius and the orbital radius are to the 5th and 6th powers respectively that makes the time particularly sensitive to those values a moon orbiting much closer to its planet is going to tidal lock a lot sooner what about the planet's bulge oh that's a great question earth rotates about 27 times faster than the moon orbits so its own tidal bulge gets ahead of the planet moon line and the tidal forces slow the rotation given enough time the earth could slow down enough that it would tidally lock to the moon the same side of the earth would always face the moon how long would that take don't get your hopes up a generous lower bound says it'll take at least 50 billion years considering the sun will go red giant in four to five billion there isn't nearly enough time but this has happened elsewhere in our solar system pluto and its moon karen do this they're much closer to each other in distance size and mass than the earth moon system so it took a lot less time tidal locking with the sun will take even longer this scenario doesn't stand a chance of happening but we have seen it happen outside our own solar system in the trappist-1 system several of its planets are tidally locked to the star it's a small red dwarf so the planets are a lot closer to each other and to the star plus red dwarfs last for like a trillion years that's the kind of system where we'd expect to see an eyeball planet like this the lit side would be a desert hellscape the dark side a frozen wasteland life would only be possible in the thin boundary between the sides it would be a land of eternal dusk with dangers lurking in every shadow so would you like to live on a tidally locked planet please share in the comments thanks for liking and sharing this video a special thanks goes out to my patreon patrons and youtube members like online book club and christopher sheila for their generous support don't forget to subscribe if you'd like to keep up with us and until next time remember it's okay to be a little crazy what did i say i said don't at me but fine i'll clarify saying superconductors have zero resistance is sacrificing accuracy in favor of simplicity is there resistance below our ability to measure yes but that doesn't make it zero anyway thanks for watching

Channel: The Science Asylum

Views: 192,943

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Id: fivCstgXlDo

Length: 9min 52sec (592 seconds)

Published: Wed Jan 26 2022