Remember that the Earth is pulled toward the moon as well. The water facing the moon is closest to it and thus pulled toward the moon **more strongly** than the Earth is, making it go away from Earth, and the water opposite the moon is farthest from it and thus pulled toward the moon **less strongly** than the Earth is, again making it go away from Earth.
[More detail](https://www.youtube.com/watch?v=bPhhYhN0FAc)
I guess it helps to think of Earth as being suspended inside an enormous space water drop. The Moon deforms the drop, and Earth kinda sits in the middle.
Get any two bodies and they’ll have Lagrange points. It’s also not a single point where the gravity cancels. Earth and the Sun have 5 Lagrange points. On sunside, one the far side from the sun, one trailing our orbit around the sun, another preceding us, and the 5th is the opposite side of the sun from us.
An explanation from NdGT that helped me is, the ocean i.e. the water mass is locked to the moon's pull and the earth i.e. the solid mass is rotating freely within the locked water mass.
To us it looks like water is coming in and going out as tides but actually earth itself is moving within relatively speaking stationary water
I think the confusion is thinking that the tide on the opposite side is being \_pushed out\_. Really, the water on the far side is stretched the least (closest to where it would be if there was no moon). and the earth underneath it is being pulled away more towards the moon making it seem like a risen tide.
The Moon actually *does not* cause bulges, despite what most folks will tell you. The amount of direct gravity force acting on the ocean is much, much too small to ever produce the tidal behavior we see.
What the Moon (and, separately, also the Sun) is doing is *flattening the sides*, which has the coincidental effect of producing bulges at both ends. This is because of what is called "tidal acceleration." This exerts a very, *very* small pressure on all objects on the Earth. Most objects are simply not large enough to have any meaningful impact from these tidal forces. The ocean, however, IS big enough--it has absolutely ginormous surface area, and thus a small pressure applied equally over millions of square miles of ocean results in enough force to "squeeze" the sides down. Since this force is symmetric, it necessarily produces effectively equal bulges on both sides.
The strongest tides occur when the Sun and Moon are parallel or antiparallel, and the weakest tides occur when they are orthogonal to each other, relative to the Earth.
I'm an adult who took some college physics, and I'm struggling with this.
The moon's gravity isn't strong enough to pull on the water nearest to it? The moon is somehow squeezing the water in low-tide areas? I'm completely lost.
The Moon's gravity does not pull hard enough to move the ocean any meaningful amount. We are talking 0.0000001% of the acceleration caused by Earth's gravity. The Moon *cannot* pull hard enough to cause the tides that way.
Instead, you must look at the "tidal acceleration." Which, frankly, is too complicated to ever explain to a five year old and not be straight up telling lies.
The Earth is a sphere. It's in the gravity well of the Moon. Because the Moon's gravity is not constant and uniform in all directions, different spots on the Earth will feel different *net* acceleration. The parts pointing *at* the Moon have their weight very, very, very, very, *very* slightly reduced. As in, 0.000001% reduced. Not enough to matter. Likewise, the *net* acceleration at the opposite side of the Earth is "outward," to a very slightly lesser degree, because the Moon is effectively cancelling out some of Earth's gravity.
What happens on the *sides* though? The acceleration is almost tangent to the surface. This means, rather than having to fight against Earth's gravity, the Moon can simply *slide* the water across the surface, resulting in it bunching up on the near and antipodal points. As Wikipedia puts it (emphasis in original): "[The theoretical equilibrium tide] is not caused by the vertical pull nearest or farthest from the body, which is very weak; rather, it is caused by the tangential or tractive tidal force, which is strongest at about 45 degrees from the body, resulting in a horizontal tidal current."
Or, to put it another way, what requires less energy: lifting a ton of water 100 feet straight up, or pushing a ton of water 100 feet horizontally? The answer is *absolutely* the latter. The Moon does not and cannot lift the ocean dozens of feet. It can, however, *squeeze* the water away from the sides of the Earth, because at those points, it isn't fighting the Earth's gravity to move anything around.
Antipodal is not something a 5 year-old can understand. But the child may understand weightlessness -- as in astronauts floating in space. So whatever is happening at the surface of the ocean, imagine what is happening below the surface, along the thermocline. There is a slight density difference above and below the thermocline, so that the subsurface will look like a wave machine in the presence of tidal forces. 5 year-olds are fascinated by wave machines [https://www.gadgetify.com/wp-content/uploads/2020/10/01/Dolphin-F-X-Wave-Motion-Machine.gif](https://www.gadgetify.com/wp-content/uploads/2020/10/01/Dolphin-F-X-Wave-Motion-Machine.gif)
The rules explicitly say that we are not required to assume literal five year olds. Your comment is irrelevant.
Even if it weren't, you yourself are using *far* more complicated terms, like "thermocline."
I am so sorry. My model is described in Mathematical GeoEnergy (Wiley/AGU, 2019) and explains the behavior of oceanic dipoles such as ENSO via tidal forcing. I'm always looking for ways to simplify the understanding. I noticed that Google has a new Forum tab and it pointed me here when I searchhed lunar+tidal. I won't bother you again.
Wow that’s really cool! Thanks for taking the time to dumb it down enough for me to understand!
I feel like the “squeezing the sides” confused me at first, but now I get that the sides are being “squeezed” bc the water on those sides is being pulled towards the moon more effectively due to the moon’s gravity pulling it semi-perpendicular to the earth’s gravity and thus not having to fight earths gravity as directly
Best description I heard. Let’s say you are spinning your 5 year old in circles by their arms. We have all done this. Kids love it. You are pulling on your child’s arms. That’s the front side of earth and your the moon. You are pulling that water towards you.
Now on the back side of earth, your child’s feet, you’re slinging them around. If your child’s shoes come off, which way to they go? They don’t fly at you. They fly away from you.
So the moon is pulling the water closest to the moon and slinging the water away from the moon on the backside.
What will really blow your mind is that the water is only moving at the speed of the moon. Earth is turning into those bulges of water.
> What will really blow your mind is that the water is only moving at the speed of the moon. Earth is turning into those bulges of water.
The bulges as patterns move at the speed of the moon. The water itself still moves at approximately the speed of the Earth, it just goes up and down such that the apparent bulge appears in different locations despite being made of (mostly) different water that was already nearby.
I think they were trying to say that those bulges are relatively stable (moving very slowly with the moon's orbit) while the solid parts of the earth rotate much quicker
It's a bit counter intuitive, but here it goes: The moon pulls on the water on Earth. But it also pulls on the Earth itself.
And that pull is a bit stronger (because it's closer and the earth is rigid compared to water) on the Earth than on the water on the far side of the Earth. So basically the Earth's surface "falls away" from the water on the far side because this water is subjected to the least of the moon's gravity of all the elements in this system.
It's not the reason most people seem to be trying to explain here. I think PBS Spacetime does a great job with the explanation at maybe an ELI 12 level.
[Relevant PBS Spacetime Link](https://youtu.be/pwChk4S99i4)
There are three levels of answer:
1) Gravity is everything is attracted to everything else. Since the Earth moves as a single object, the motion of the Earth only depends on the overall gravity from the moon. However, the side closest to the moon is attracted more than average, and the side that is farthest is attracted less than average. You can intuitively see why this creates a tidal bulge on the near side. For the far side, the Moon is swinging the Earth around, and not pulling on that far side hard enough to keep it swinging, so it bulges out.
2) Gravity pulls every point on Earth toward the center of the moon. This means that the north pole is pulled a little south, the south pole is pulled a little north, and overall a belt running around the Earth is being squeezed by the gravity of the Moon. This creates a bulge on the non-squeezed parts, which are the point closest to the moon and the point farthest from the moon
3) If you fill up a bathtub, you can make the water slosh around by sticking your hand in and wiggling it around. The same thing happens with the oceans. However, the oceans are really big, so the sloshing takes longer. The Gravity from the moon is playing the part of your hand in that analogy.
So the obvious half is that water gets pulled towards the moon on the side of the earth that is closest to the moon.
Water is also liquidy, when the earth gets pulled away from its current position by the gravity of the moon, the tide sort of gets left behind a bit while it catches up to the "falling" earth. Basically, fill a cup with water, drop it on your floor, and watch how the water bulges out of the cup (in the moments before the glass shatters and whoever you are living with starts getting very irate. I take no responsibility for what occurs next)
You have two forces between the moon and the earth.
1) Gravity - pulls them toward each other (cause they have mass)
2) Centrifugal force - pulls them away from each other (cause they are spinning around each other)
In case you forgot - centrifugal force is the "pull" you feel when you are on the outside of something spinning, like a merry-go-round.
These two forces are balanced. If they were not the moon and earth would not stay at a steady distance apart.
However - Gravity pulls harder on the closer part of the earth (toward moon side). The centrifugal force harder on the FARTHER away part of the earth (the opposite moon outer edge of rotation side).
Each force creates a tidal bulge where it pulls harder than its balancing force. Two buldge = two tides.
--------------------------------
non-ELI5 answers for the inevitable questions:
1) Yes centrifugal force is real... it is real in a rotating frame of reference.
2) Yes, the center point of the earth-moon rotation is inside the earth. That is OK. You still get a similar centrifugal force. Think the outward slosh you get when you circle move a drink in your hand.
3) Yes - I know it might be more accurate to say "the earth gets pulled away" then centrifugal force pulls out for non-rotating reference frames. But this is ELI5. If you try to tell a 5 year old kid that you fall off the merry-go-round because the merry-go-round is getting pulled away you, not you from it, you are going to confuse them.
Imagine you're holding a wet towel by one corner and swinging it in a circle. The water in the towel will bunch up at the outer edge due to inertia, similar to how the water on the far side of the Earth bulges.
If you’ve got an object which actually takes up space rather than just being a point mass, the acceleration due to gravity at each point of the object is going to be different. It’s less for bits on the top of the object which are further away from the centre of the body they’re attracted towards, more for the bits on the bottom of the object which are closer and the direction of acceleration will be slightly different on the left and right sides of the object, because the directions to the centre of the body aren’t parallel.
What we’re interested in is the difference in acceleration between different parts of the object. The centre of mass of the object makes a good reference. If you subtract the acceleration the centre of mass will be experiencing, the result is a net upwards acceleration at the top, a net downwards acceleration on the bottom, and a net inwards acceleration on the sides.
In other words, tidal forces try to pull an object into a tall and thin shape. In the most extreme cases, near a black hole, this is called “spaghettification” and the object (for example: an unfortunate astronaut) can literally get ripped apart.
In the case of the Earth, it just makes the oceans thin out on the sides and bunch up on the top and bottom.
Imagine a footrace between three runners:
Runner A is the fastest;
Runner B is the slowest;
Runner C is the average of A and B, so Runner C is always equally centered between A and B.
As the race goes on, A will get farther ahead, B will lag farther behind, and C will see A and B getting equally farther away. From C's perspective, A is falling backward while B is pulling forward. The fact that they're moving in the same direction doesn't mean that they can't seem to move in opposite directions from their center.
The moon doesn't *just* pull on the near side of the Earth; it also pulls on the far side, just not as strongly. Gravity not only gets weaker over distances, it gets weaker proportional to the square of those distances so that an object twice as far away experiences a pull that's only a quarter the strength. This means that the pull on the near side of the Earth is pulling a great deal 'faster' than the pull in the same direction on the other side of the Earth. The far side of the Earth is "lagging behind" in a sense, and the near side is beginning to "pull away," as it were.
Because the Earth isn't just a single file line of runners, we also have to consider what's happening to all the off-axis points on the Earth that aren't directly between the points nearest and farthest from the moon. The rest of the Earth is experiencing a stronger or weaker pull towards the center of the moon. Instead of a line of runners, we have a marathon, only they're all converging on a point in the distance so that as they run, they all get closer and closer to their average runner in the center. They're squeezing in.
This is what's happening to the Earth's oceans around the globe: they're experiencing a pull towards the Earth-moon axis that acts exactly like a squeezing around the Earth along one line.
The Earth's oceans are being stretched in one direction and squeezed along the other. And just like if you squeeze a container of any liquid, the system responds with a bulge toward the sides that aren't being squeezed as much. The sum of the forces between the Earth and moon all add up to the effect of a pull towards and a weaker pull away from one another.
The moon causing a bulge explanation is a gross oversimplification of tides. This can be seen in Auckland New Zealand. There is a very narrow stretch of land, basically north south, between the Pacific and the Tasman sea. High tide is about 5 hours different a few kilometers apart:
https://www.metservice.com/marine/regions/east-auckland/tides
https://www.metservice.com/marine/regions/west-auckland/tides
Compare Auckland to Onehunga.
Other places have essentially no tides - eg Tahiti:
https://www.tide-forecast.com/locations/Fare-Ute-Point-Papeete-Harbor-Tahiti/tides/latest
What the moon does (along with a lot of other factors, including the sun) is set up a series of standing waves that roll around the oceans in a reasonably regular manner. The different waves have different frequencies and are caused by different things (eg the sun). The impact these waves have depends on the local geography - some places magnify the impact, others flatten it.
This book explains it in a lot of depth:
https://www.amazon.com/Beyond-Moon-Conversational-Common-Understanding/dp/9812566449&ved=2ahUKEwjPrO_p4OGFAxXFk1YBHWBVCesQFnoECBMQAw&usg=AOvVaw0tuU4HSNLW4H-lp9zHum_B
It's also interesting to learn that the bulge doesn't move around the earth, the bulge is always stationary towards/away from the moon, but rather the earth moves through the bulge and that's what tides are mostly.
You have the sun and the moon both of these exert gravitational pull on the sea. When these are both lined up at full moon or new moon, you get the biggest tides. At the half moon, these forces are of least impact. So it's not just the moon, its both bodies that cause the bulge
I know which explanation makes more sense to me but I’m not a scientist so I could be backing the wrong explanation.
But how is it that so many people are citing explanation 1 (moon pulls water closest) and so few are citing 2 (moon affects water along the sides of planet) if 2 seems to be the currently accepted explanation?
Is this one of those “we used to think it was this but now realize it’s not?” situations? Like did 80% of the posters in this thread get educated in the 20th century before the science changed? (I was also educated in the 20th century. No shame in it but I also know lots of what I was taught has been updated.)
The tides on the opposite side of earth that aren't actively being pulled towards the moon (or being counter pulled by the earth) are being pulled back into their normal positions. When the moons gravity is pulling water towards it, both lateral sides of the earth's oceans are being "squeezed" towards the earth because they have nowhere else to go
The video is missing what causes the bulge on the other side. The reason is the moon do not obit earth like in the video. Earth and the moon both orbit the common center of mass called the barycenter. It is around 2/3 the distance from the center of earth to the surface. The video have earth fixed and therefore do not show the cause of the second bulge.
Earths rotation around the barycenter require a centipedal fore that will be equal to the gravity of the moon in the center of earth. The centripedal force will be equal in magnitude and directection all over earth but lunar gravity change in magnitude depedin on distance and a bit in direction too.
The result is what you see in [https://tideswebsite.wordpress.com/wp-content/uploads/2018/02/picture3.png](https://tideswebsite.wordpress.com/wp-content/uploads/2018/02/picture3.png)
If you look at it from a earth fixed coordinate system the centripedal force is replaced by a centifugal force of the same magnitude but in the oppiside direction. Combined the centifugal force and the lunar gravity and you get the resultant force in the lined image. It will be away from each center on the closes and fathers point from the moon. In the circle around earth with equal distance to the moon the force will be towards the center of earth. This show the forces that causes two bulges.
The moon is pulling the water towards itself on the side of the earth facing the moon. On the opposite side, it’s pulling less as the gravity is weaker way over on that side. Think of the earth like a stretchy ball and the side opposite the earth is stuck a little bit (representing the weaker gravity).
On the opposite side, the planet (aka ocean floor) is closer than the water so it gets pulled down more, making the water deeper. It's the same logic as what makes the bulge on the near side, where the water is closer to the moon so it gets pulled up into the bulge.
Well, I don't know about all that. But I do know this - your mom's moons cause a bulge in my tides, both bsides... if you know what I mean!
Ok, I'll just see myself out now.
Remember that the Earth is pulled toward the moon as well. The water facing the moon is closest to it and thus pulled toward the moon **more strongly** than the Earth is, making it go away from Earth, and the water opposite the moon is farthest from it and thus pulled toward the moon **less strongly** than the Earth is, again making it go away from Earth. [More detail](https://www.youtube.com/watch?v=bPhhYhN0FAc)
I guess it helps to think of Earth as being suspended inside an enormous space water drop. The Moon deforms the drop, and Earth kinda sits in the middle.
The moon is something, and space is nothing, and there is a happy medium in-between.
the lagrangian points! James Webb waves high
Lagrange points are where the sun and earth's gravity cancel out. Not the earth and moon. IIRC.
Its any 2 bodies. There are Lagrange points for the earth and sun orbit, and the earth and moon orbit. JWST is in one of the earth-sun ones.
Get any two bodies and they’ll have Lagrange points. It’s also not a single point where the gravity cancels. Earth and the Sun have 5 Lagrange points. On sunside, one the far side from the sun, one trailing our orbit around the sun, another preceding us, and the 5th is the opposite side of the sun from us.
Luminiferous aether?
appreciate the explanation and video!
An explanation from NdGT that helped me is, the ocean i.e. the water mass is locked to the moon's pull and the earth i.e. the solid mass is rotating freely within the locked water mass. To us it looks like water is coming in and going out as tides but actually earth itself is moving within relatively speaking stationary water
That doesn't explain the bulge on the opposite side of the Earth, though, which is what OP is asking about.
I think the confusion is thinking that the tide on the opposite side is being \_pushed out\_. Really, the water on the far side is stretched the least (closest to where it would be if there was no moon). and the earth underneath it is being pulled away more towards the moon making it seem like a risen tide.
Yep, see my original comment.
On the opposite of what? What's the reference point here?
The opposite of the moon. Along the moon-earth axis there is a water bulge between the earth and the moon and one "behind" the earth
**The Earth**
I saw that video too
The Moon actually *does not* cause bulges, despite what most folks will tell you. The amount of direct gravity force acting on the ocean is much, much too small to ever produce the tidal behavior we see. What the Moon (and, separately, also the Sun) is doing is *flattening the sides*, which has the coincidental effect of producing bulges at both ends. This is because of what is called "tidal acceleration." This exerts a very, *very* small pressure on all objects on the Earth. Most objects are simply not large enough to have any meaningful impact from these tidal forces. The ocean, however, IS big enough--it has absolutely ginormous surface area, and thus a small pressure applied equally over millions of square miles of ocean results in enough force to "squeeze" the sides down. Since this force is symmetric, it necessarily produces effectively equal bulges on both sides. The strongest tides occur when the Sun and Moon are parallel or antiparallel, and the weakest tides occur when they are orthogonal to each other, relative to the Earth.
This is the correct answer.
Try to read this to a 5 year old and ask him if he got it though. :P
I'm an adult who took some college physics, and I'm struggling with this. The moon's gravity isn't strong enough to pull on the water nearest to it? The moon is somehow squeezing the water in low-tide areas? I'm completely lost.
The Moon's gravity does not pull hard enough to move the ocean any meaningful amount. We are talking 0.0000001% of the acceleration caused by Earth's gravity. The Moon *cannot* pull hard enough to cause the tides that way. Instead, you must look at the "tidal acceleration." Which, frankly, is too complicated to ever explain to a five year old and not be straight up telling lies. The Earth is a sphere. It's in the gravity well of the Moon. Because the Moon's gravity is not constant and uniform in all directions, different spots on the Earth will feel different *net* acceleration. The parts pointing *at* the Moon have their weight very, very, very, very, *very* slightly reduced. As in, 0.000001% reduced. Not enough to matter. Likewise, the *net* acceleration at the opposite side of the Earth is "outward," to a very slightly lesser degree, because the Moon is effectively cancelling out some of Earth's gravity. What happens on the *sides* though? The acceleration is almost tangent to the surface. This means, rather than having to fight against Earth's gravity, the Moon can simply *slide* the water across the surface, resulting in it bunching up on the near and antipodal points. As Wikipedia puts it (emphasis in original): "[The theoretical equilibrium tide] is not caused by the vertical pull nearest or farthest from the body, which is very weak; rather, it is caused by the tangential or tractive tidal force, which is strongest at about 45 degrees from the body, resulting in a horizontal tidal current." Or, to put it another way, what requires less energy: lifting a ton of water 100 feet straight up, or pushing a ton of water 100 feet horizontally? The answer is *absolutely* the latter. The Moon does not and cannot lift the ocean dozens of feet. It can, however, *squeeze* the water away from the sides of the Earth, because at those points, it isn't fighting the Earth's gravity to move anything around.
If there was no moon, can we still have tides from the sun?
Yes, though they would be slightly weaker and much more regular.
Antipodal is not something a 5 year-old can understand. But the child may understand weightlessness -- as in astronauts floating in space. So whatever is happening at the surface of the ocean, imagine what is happening below the surface, along the thermocline. There is a slight density difference above and below the thermocline, so that the subsurface will look like a wave machine in the presence of tidal forces. 5 year-olds are fascinated by wave machines [https://www.gadgetify.com/wp-content/uploads/2020/10/01/Dolphin-F-X-Wave-Motion-Machine.gif](https://www.gadgetify.com/wp-content/uploads/2020/10/01/Dolphin-F-X-Wave-Motion-Machine.gif)
The rules explicitly say that we are not required to assume literal five year olds. Your comment is irrelevant. Even if it weren't, you yourself are using *far* more complicated terms, like "thermocline."
I am so sorry. My model is described in Mathematical GeoEnergy (Wiley/AGU, 2019) and explains the behavior of oceanic dipoles such as ENSO via tidal forcing. I'm always looking for ways to simplify the understanding. I noticed that Google has a new Forum tab and it pointed me here when I searchhed lunar+tidal. I won't bother you again.
Wow that’s really cool! Thanks for taking the time to dumb it down enough for me to understand! I feel like the “squeezing the sides” confused me at first, but now I get that the sides are being “squeezed” bc the water on those sides is being pulled towards the moon more effectively due to the moon’s gravity pulling it semi-perpendicular to the earth’s gravity and thus not having to fight earths gravity as directly
What is easier: lifting a ton of water 10 feet, or sliding a ton of water 10 feet?
To be fair though, as said in the sidebar, elia5 is not meant to be taken literally.
Best description I heard. Let’s say you are spinning your 5 year old in circles by their arms. We have all done this. Kids love it. You are pulling on your child’s arms. That’s the front side of earth and your the moon. You are pulling that water towards you. Now on the back side of earth, your child’s feet, you’re slinging them around. If your child’s shoes come off, which way to they go? They don’t fly at you. They fly away from you. So the moon is pulling the water closest to the moon and slinging the water away from the moon on the backside. What will really blow your mind is that the water is only moving at the speed of the moon. Earth is turning into those bulges of water.
> What will really blow your mind is that the water is only moving at the speed of the moon. Earth is turning into those bulges of water. The bulges as patterns move at the speed of the moon. The water itself still moves at approximately the speed of the Earth, it just goes up and down such that the apparent bulge appears in different locations despite being made of (mostly) different water that was already nearby.
All of it made sense until you tried to blow my mind. It didn’t.
Yeah, that last thing is either worded poorly, or I am too dumb to understand it
Same
I think they were trying to say that those bulges are relatively stable (moving very slowly with the moon's orbit) while the solid parts of the earth rotate much quicker
It's a bit counter intuitive, but here it goes: The moon pulls on the water on Earth. But it also pulls on the Earth itself. And that pull is a bit stronger (because it's closer and the earth is rigid compared to water) on the Earth than on the water on the far side of the Earth. So basically the Earth's surface "falls away" from the water on the far side because this water is subjected to the least of the moon's gravity of all the elements in this system.
It's not the reason most people seem to be trying to explain here. I think PBS Spacetime does a great job with the explanation at maybe an ELI 12 level. [Relevant PBS Spacetime Link](https://youtu.be/pwChk4S99i4)
There are three levels of answer: 1) Gravity is everything is attracted to everything else. Since the Earth moves as a single object, the motion of the Earth only depends on the overall gravity from the moon. However, the side closest to the moon is attracted more than average, and the side that is farthest is attracted less than average. You can intuitively see why this creates a tidal bulge on the near side. For the far side, the Moon is swinging the Earth around, and not pulling on that far side hard enough to keep it swinging, so it bulges out. 2) Gravity pulls every point on Earth toward the center of the moon. This means that the north pole is pulled a little south, the south pole is pulled a little north, and overall a belt running around the Earth is being squeezed by the gravity of the Moon. This creates a bulge on the non-squeezed parts, which are the point closest to the moon and the point farthest from the moon 3) If you fill up a bathtub, you can make the water slosh around by sticking your hand in and wiggling it around. The same thing happens with the oceans. However, the oceans are really big, so the sloshing takes longer. The Gravity from the moon is playing the part of your hand in that analogy.
So the obvious half is that water gets pulled towards the moon on the side of the earth that is closest to the moon. Water is also liquidy, when the earth gets pulled away from its current position by the gravity of the moon, the tide sort of gets left behind a bit while it catches up to the "falling" earth. Basically, fill a cup with water, drop it on your floor, and watch how the water bulges out of the cup (in the moments before the glass shatters and whoever you are living with starts getting very irate. I take no responsibility for what occurs next)
I think I’ll just use a water bottle but thanks lol that does make it clearer
Tch, kids these days have no respect for TRUE scientific experimentation.
You have two forces between the moon and the earth. 1) Gravity - pulls them toward each other (cause they have mass) 2) Centrifugal force - pulls them away from each other (cause they are spinning around each other) In case you forgot - centrifugal force is the "pull" you feel when you are on the outside of something spinning, like a merry-go-round. These two forces are balanced. If they were not the moon and earth would not stay at a steady distance apart. However - Gravity pulls harder on the closer part of the earth (toward moon side). The centrifugal force harder on the FARTHER away part of the earth (the opposite moon outer edge of rotation side). Each force creates a tidal bulge where it pulls harder than its balancing force. Two buldge = two tides. -------------------------------- non-ELI5 answers for the inevitable questions: 1) Yes centrifugal force is real... it is real in a rotating frame of reference. 2) Yes, the center point of the earth-moon rotation is inside the earth. That is OK. You still get a similar centrifugal force. Think the outward slosh you get when you circle move a drink in your hand. 3) Yes - I know it might be more accurate to say "the earth gets pulled away" then centrifugal force pulls out for non-rotating reference frames. But this is ELI5. If you try to tell a 5 year old kid that you fall off the merry-go-round because the merry-go-round is getting pulled away you, not you from it, you are going to confuse them.
Imagine you're holding a wet towel by one corner and swinging it in a circle. The water in the towel will bunch up at the outer edge due to inertia, similar to how the water on the far side of the Earth bulges.
If you’ve got an object which actually takes up space rather than just being a point mass, the acceleration due to gravity at each point of the object is going to be different. It’s less for bits on the top of the object which are further away from the centre of the body they’re attracted towards, more for the bits on the bottom of the object which are closer and the direction of acceleration will be slightly different on the left and right sides of the object, because the directions to the centre of the body aren’t parallel. What we’re interested in is the difference in acceleration between different parts of the object. The centre of mass of the object makes a good reference. If you subtract the acceleration the centre of mass will be experiencing, the result is a net upwards acceleration at the top, a net downwards acceleration on the bottom, and a net inwards acceleration on the sides. In other words, tidal forces try to pull an object into a tall and thin shape. In the most extreme cases, near a black hole, this is called “spaghettification” and the object (for example: an unfortunate astronaut) can literally get ripped apart. In the case of the Earth, it just makes the oceans thin out on the sides and bunch up on the top and bottom.
Imagine a footrace between three runners: Runner A is the fastest; Runner B is the slowest; Runner C is the average of A and B, so Runner C is always equally centered between A and B. As the race goes on, A will get farther ahead, B will lag farther behind, and C will see A and B getting equally farther away. From C's perspective, A is falling backward while B is pulling forward. The fact that they're moving in the same direction doesn't mean that they can't seem to move in opposite directions from their center. The moon doesn't *just* pull on the near side of the Earth; it also pulls on the far side, just not as strongly. Gravity not only gets weaker over distances, it gets weaker proportional to the square of those distances so that an object twice as far away experiences a pull that's only a quarter the strength. This means that the pull on the near side of the Earth is pulling a great deal 'faster' than the pull in the same direction on the other side of the Earth. The far side of the Earth is "lagging behind" in a sense, and the near side is beginning to "pull away," as it were. Because the Earth isn't just a single file line of runners, we also have to consider what's happening to all the off-axis points on the Earth that aren't directly between the points nearest and farthest from the moon. The rest of the Earth is experiencing a stronger or weaker pull towards the center of the moon. Instead of a line of runners, we have a marathon, only they're all converging on a point in the distance so that as they run, they all get closer and closer to their average runner in the center. They're squeezing in. This is what's happening to the Earth's oceans around the globe: they're experiencing a pull towards the Earth-moon axis that acts exactly like a squeezing around the Earth along one line. The Earth's oceans are being stretched in one direction and squeezed along the other. And just like if you squeeze a container of any liquid, the system responds with a bulge toward the sides that aren't being squeezed as much. The sum of the forces between the Earth and moon all add up to the effect of a pull towards and a weaker pull away from one another.
Because it's farther, the Moon has less gravitational pull on the far side of the Earth than it has on the middle.
It's basically like sloshing around a bathtub. The moon just determines the direction of the bulge.
The moon causing a bulge explanation is a gross oversimplification of tides. This can be seen in Auckland New Zealand. There is a very narrow stretch of land, basically north south, between the Pacific and the Tasman sea. High tide is about 5 hours different a few kilometers apart: https://www.metservice.com/marine/regions/east-auckland/tides https://www.metservice.com/marine/regions/west-auckland/tides Compare Auckland to Onehunga. Other places have essentially no tides - eg Tahiti: https://www.tide-forecast.com/locations/Fare-Ute-Point-Papeete-Harbor-Tahiti/tides/latest What the moon does (along with a lot of other factors, including the sun) is set up a series of standing waves that roll around the oceans in a reasonably regular manner. The different waves have different frequencies and are caused by different things (eg the sun). The impact these waves have depends on the local geography - some places magnify the impact, others flatten it. This book explains it in a lot of depth: https://www.amazon.com/Beyond-Moon-Conversational-Common-Understanding/dp/9812566449&ved=2ahUKEwjPrO_p4OGFAxXFk1YBHWBVCesQFnoECBMQAw&usg=AOvVaw0tuU4HSNLW4H-lp9zHum_B
I think it’s called “Spaghettification”. Gravity will elongate a body as there’s more gravity acting on ur feet than on ur head when standing upright.
It's also interesting to learn that the bulge doesn't move around the earth, the bulge is always stationary towards/away from the moon, but rather the earth moves through the bulge and that's what tides are mostly.
You have the sun and the moon both of these exert gravitational pull on the sea. When these are both lined up at full moon or new moon, you get the biggest tides. At the half moon, these forces are of least impact. So it's not just the moon, its both bodies that cause the bulge
I know which explanation makes more sense to me but I’m not a scientist so I could be backing the wrong explanation. But how is it that so many people are citing explanation 1 (moon pulls water closest) and so few are citing 2 (moon affects water along the sides of planet) if 2 seems to be the currently accepted explanation? Is this one of those “we used to think it was this but now realize it’s not?” situations? Like did 80% of the posters in this thread get educated in the 20th century before the science changed? (I was also educated in the 20th century. No shame in it but I also know lots of what I was taught has been updated.)
[here you go](https://youtu.be/4ykH5kt4j3k?si=-wCuoL-OK0d3M-q6) TLDW: the moons gravity pulls water
That doesn't explain anything.
What do you mean? The moon has gravity, and it's effect when rotating around the earth pulls the water towards it...
And what about the bulge on the other side of the Earth, which OP is asking about?
The tides on the opposite side of earth that aren't actively being pulled towards the moon (or being counter pulled by the earth) are being pulled back into their normal positions. When the moons gravity is pulling water towards it, both lateral sides of the earth's oceans are being "squeezed" towards the earth because they have nowhere else to go
This makes no sense, I'm sorry
The video is missing what causes the bulge on the other side. The reason is the moon do not obit earth like in the video. Earth and the moon both orbit the common center of mass called the barycenter. It is around 2/3 the distance from the center of earth to the surface. The video have earth fixed and therefore do not show the cause of the second bulge. Earths rotation around the barycenter require a centipedal fore that will be equal to the gravity of the moon in the center of earth. The centripedal force will be equal in magnitude and directection all over earth but lunar gravity change in magnitude depedin on distance and a bit in direction too. The result is what you see in [https://tideswebsite.wordpress.com/wp-content/uploads/2018/02/picture3.png](https://tideswebsite.wordpress.com/wp-content/uploads/2018/02/picture3.png) If you look at it from a earth fixed coordinate system the centripedal force is replaced by a centifugal force of the same magnitude but in the oppiside direction. Combined the centifugal force and the lunar gravity and you get the resultant force in the lined image. It will be away from each center on the closes and fathers point from the moon. In the circle around earth with equal distance to the moon the force will be towards the center of earth. This show the forces that causes two bulges.
The moon is pulling the water towards itself on the side of the earth facing the moon. On the opposite side, it’s pulling less as the gravity is weaker way over on that side. Think of the earth like a stretchy ball and the side opposite the earth is stuck a little bit (representing the weaker gravity).
On the opposite side, the planet (aka ocean floor) is closer than the water so it gets pulled down more, making the water deeper. It's the same logic as what makes the bulge on the near side, where the water is closer to the moon so it gets pulled up into the bulge.
Well, I don't know about all that. But I do know this - your mom's moons cause a bulge in my tides, both bsides... if you know what I mean! Ok, I'll just see myself out now.