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ELI5: How do tides work? (self.explainlikeimfive)
submitted 4h ago by Erik_B_28
fixed_grin 1 points 4h ago
Gravity weakens with distance. This means the side of the Earth closest to the moon is getting pulled the most, and the opposite side is pulled the least. This *very slightly* stretches the Earth in that direction, but it also pulls the oceans, which flow way easier than rock. This is why there's usually a ~12 hour cycle, high tide comes when the moon is either pulling the ocean around you up towards it, or pulling the ocean down just a little less than the Earth.
There are a bunch of other factors. At full and new moon, you have spring tides because the pull of the Sun is in line with the moon, so the tides are bigger. At quarter moons, the moon and Sun are at 90 degrees, and we have lower neap tides. The Earth's rotation skews things a little bit. The Moon's orbit isn't quite circular, so when it's closer it's gravity is a little stronger than when it's further away.
Then there are effects from the land as the tides hit them and strengthen in some spots and cancel out in others.
There are a bunch of other factors. At full and new moon, you have spring tides because the pull of the Sun is in line with the moon, so the tides are bigger. At quarter moons, the moon and Sun are at 90 degrees, and we have lower neap tides. The Earth's rotation skews things a little bit. The Moon's orbit isn't quite circular, so when it's closer it's gravity is a little stronger than when it's further away.
Then there are effects from the land as the tides hit them and strengthen in some spots and cancel out in others.
Otherwisuhii 1 points 21m ago
The tides move around the earth because the earth rotates, shifting the relative position of the moon and sun.
AelixD 1 points 1h ago
Tides are caused by the gravitational pull of the Moon (mostly) and Sun (less). And then affected by the local geography and hydrography.
The moon is pulling the Earth towards it. The rocky part of the Earth is pretty rigid, so we don’t typically notice the pull of the moon there. But the oceans, being made of water, are more fluid. So where the moon is directly overhead, the oceans are pulled the highest. As the Earth rotates towards the moon, the oceans are being pulled towards the moon, causing the tide to rise.
However, there is also a high tide on the side of the Earth opposite the moon. This is caused by inertia. If the rocky part of the earth was a perfectly smooth ball, it would be entirely covered in ocean. And there would be just one high tide, directly under the Moon (not completely true, due to friction, but we’ll ignore that). Now, remember what happens in a bathtub when you push all the water to one side. It goes up to a height based on how hard you pushed (if you’re my younger brother, this was hard enough to get a lot of water out of the tub and onto the floor, but the moon is more disciplined). Once it was done rising, it comes back down. But it doesn’t stop immediately at its lowest level. Instead it sloshes to the other side of the tub. The oceans are shaped like bath tubs, and the moon can only pull them so far up the coast. Once that part of the Earth rotates past the moon, the pull lessens, and the water rushes to the other side of the ocean. (This is over simplified, but you get the general idea).
So there is a really high tide (the Higher High tide) under the moon, and a lesser high tide (the Lower High tide) opposite the moon. If you imagine a simple drawing of an egg you could picture a circle inside of an oval, with the pointier bulge of the egg in the direction of the moon. This roughly represents the rocky earth inside of the oceans.
The combination of the Moon’s movement and the Earth’s rotation means that the Moon rises, on average, 54 minutes later each day. So the tides change time each day by about the same amount. So a high tide every 12 hours 27 minutes (roughly) and a full tidal cycle every 24 hours 54 minutes.
Now, all of what we said about the moon applies to the sun, but not as strongly. So make another drawing of an egg, but this time aligning the oval in the direction of the sun. These two eggs work together or against each other. When the moon snd sun are in line with each other (new moon and full moon) the ovals combine and create the highest tides, called Spring Tides (nothing to do with the season… think more like a spring in your step). When the sun and moon are 90 degrees apart, the bulging ovals are working against each other, and you get the lowest high tides, called Neap Tides.
Low tides are 90 degrees or 6ish hours offset from the high tides.
The shape of the land has a strong effect on how extreme the tidal height is. Islands in the middle of the ocean have a negligible height difference, because the ocean flows freely past them. Tidal range in Hawaii is about a foot or two. On the edges of the ocean are the coasts, which stop the water from moving all the way around the Earth, so the water pushes to its maximum height, then falls back away. But in a lot of coastlines, you have deep inlets or bays, and the water channels snd focuses there. So you end up with even higher tides. The Bay of Fundy is famous for this effect.
Thee’s a lot of variance to what I’ve said based on the exact orbit of the moon, shape of the coastline, and time of year (which affects which way the Earth is tilting in relation to the Sun).
The moon is pulling the Earth towards it. The rocky part of the Earth is pretty rigid, so we don’t typically notice the pull of the moon there. But the oceans, being made of water, are more fluid. So where the moon is directly overhead, the oceans are pulled the highest. As the Earth rotates towards the moon, the oceans are being pulled towards the moon, causing the tide to rise.
However, there is also a high tide on the side of the Earth opposite the moon. This is caused by inertia. If the rocky part of the earth was a perfectly smooth ball, it would be entirely covered in ocean. And there would be just one high tide, directly under the Moon (not completely true, due to friction, but we’ll ignore that). Now, remember what happens in a bathtub when you push all the water to one side. It goes up to a height based on how hard you pushed (if you’re my younger brother, this was hard enough to get a lot of water out of the tub and onto the floor, but the moon is more disciplined). Once it was done rising, it comes back down. But it doesn’t stop immediately at its lowest level. Instead it sloshes to the other side of the tub. The oceans are shaped like bath tubs, and the moon can only pull them so far up the coast. Once that part of the Earth rotates past the moon, the pull lessens, and the water rushes to the other side of the ocean. (This is over simplified, but you get the general idea).
So there is a really high tide (the Higher High tide) under the moon, and a lesser high tide (the Lower High tide) opposite the moon. If you imagine a simple drawing of an egg you could picture a circle inside of an oval, with the pointier bulge of the egg in the direction of the moon. This roughly represents the rocky earth inside of the oceans.
The combination of the Moon’s movement and the Earth’s rotation means that the Moon rises, on average, 54 minutes later each day. So the tides change time each day by about the same amount. So a high tide every 12 hours 27 minutes (roughly) and a full tidal cycle every 24 hours 54 minutes.
Now, all of what we said about the moon applies to the sun, but not as strongly. So make another drawing of an egg, but this time aligning the oval in the direction of the sun. These two eggs work together or against each other. When the moon snd sun are in line with each other (new moon and full moon) the ovals combine and create the highest tides, called Spring Tides (nothing to do with the season… think more like a spring in your step). When the sun and moon are 90 degrees apart, the bulging ovals are working against each other, and you get the lowest high tides, called Neap Tides.
Low tides are 90 degrees or 6ish hours offset from the high tides.
The shape of the land has a strong effect on how extreme the tidal height is. Islands in the middle of the ocean have a negligible height difference, because the ocean flows freely past them. Tidal range in Hawaii is about a foot or two. On the edges of the ocean are the coasts, which stop the water from moving all the way around the Earth, so the water pushes to its maximum height, then falls back away. But in a lot of coastlines, you have deep inlets or bays, and the water channels snd focuses there. So you end up with even higher tides. The Bay of Fundy is famous for this effect.
Thee’s a lot of variance to what I’ve said based on the exact orbit of the moon, shape of the coastline, and time of year (which affects which way the Earth is tilting in relation to the Sun).
rubseb 1 points 1h ago
>However, there is also a high tide on the side of the Earth opposite the moon. This is caused by inertia. If the rocky part of the earth was a perfectly smooth ball, it would be entirely covered in ocean. And there would be just one high tide, directly under the Moon (not completely true, due to friction, but we’ll ignore that). Now, remember what happens in a bathtub when you push all the water to one side. It goes up to a height based on how hard you pushed (if you’re my younger brother, this was hard enough to get a lot of water out of the tub and onto the floor, but the moon is more disciplined). Once it was done rising, it comes back down. But it doesn’t stop immediately at its lowest level. Instead it sloshes to the other side of the tub. The oceans are shaped like bath tubs, and the moon can only pull them so far up the coast. Once that part of the Earth rotates past the moon, the pull lessens, and the water rushes to the other side of the ocean. (This is over simplified, but you get the general idea).
This explanation of the other high tide is complete bogus. Did you just make that up or did someone tell you this?
This explanation of the other high tide is complete bogus. Did you just make that up or did someone tell you this?
AelixD 1 points 59m ago
I said its over simplified. Its due to inertia in that the ocean that had been pulled towards the moon is now trying to rush away from the moon in a straight line, before the gravity of the Earth pulls it back down. Pretty similar to centrifugal force, but not the same. But this is ELI5 so I wanted to use an example that makes sense and gets the basic point across.
When disagreeing with something said, feel free to explain what you think is correct, rather than just say “Nuh uh! Liar liar, pants on fire!”
Feel free to read more at NOAA. https://oceanservice.noaa.gov/education/tutorial_tides/tides03_gravity.html
When disagreeing with something said, feel free to explain what you think is correct, rather than just say “Nuh uh! Liar liar, pants on fire!”
Feel free to read more at NOAA. https://oceanservice.noaa.gov/education/tutorial_tides/tides03_gravity.html
rubseb 1 points 39m ago
>Its due to inertia in that the ocean that had been pulled towards the moon is now trying to rush away from the moon in a straight line, before the gravity of the Earth pulls it back down.
No, that's just plain wrong. It's not a simplification, it's just incorrect.
Your misunderstanding is shown most clearly by the fact that you think there would be only one high tide if the Earth were a perfectly smooth ball. There wouldn't (there would still be two), because the detailed shape of the Earth (or the oceans being "shaped like bathtubs") has absolutely nothing to do with it.
Yes, inertia plays a role, but you've badly misunderstood how.
No, that's just plain wrong. It's not a simplification, it's just incorrect.
Your misunderstanding is shown most clearly by the fact that you think there would be only one high tide if the Earth were a perfectly smooth ball. There wouldn't (there would still be two), because the detailed shape of the Earth (or the oceans being "shaped like bathtubs") has absolutely nothing to do with it.
Yes, inertia plays a role, but you've badly misunderstood how.
AelixD 1 points 10m ago
Rather than just saying I’m wrong, feel free to provide an actual explanation. I obviously think I know something about tides. Make me smarter, instead of just being condescending.
Nationalook401 1 points 9m ago
These high tides on opposite sides of earth cause low tide in the middle areas.
AelixD 1 points 8m ago
Right, due to a finite volume of water. An increase in one location requires a decrease in another.
KyllianPenli 1 points 4h ago
Gravity and centrifugal force (what you feel when taking a sharp turn in a car).
Gravity of the earth keeps water on earth. Gravity of the moon and sun pull at it. At the side of the sun/moon, the tide rises. Because of the centrifugal force, it also rises on the other side. Since the water has to come from somewhere, the tide falls somewhere else.
The tides move around the earth because the earth rotates, shifting the relative position of the moon and sun.
Gravity of the earth keeps water on earth. Gravity of the moon and sun pull at it. At the side of the sun/moon, the tide rises. Because of the centrifugal force, it also rises on the other side. Since the water has to come from somewhere, the tide falls somewhere else.
The tides move around the earth because the earth rotates, shifting the relative position of the moon and sun.
KaptenNicco123 1 points 4h ago
The anti-tide has nothing to do with the centrifugal force.
rubseb 1 points 1h ago
It has everything to do with the centrifugal force.
https://tidesandcurrents.noaa.gov/restles3.html
https://www.livescience.com/29621-what-causes-the-tides.html
The Earth and the Moon orbit around a common barycenter (which happens to lie within the Earth, so the Moon's orbit is much more dramatic). And as the Earth is being "flung around" by the Moon, the high tide on the side of the Moon is caused by the centripetal force of the Moon pulling on the Earth, while the high tide on the opposite side is caused by the centrifugal force, i.e. the Earth's inertia.
https://tidesandcurrents.noaa.gov/restles3.html
https://www.livescience.com/29621-what-causes-the-tides.html
The Earth and the Moon orbit around a common barycenter (which happens to lie within the Earth, so the Moon's orbit is much more dramatic). And as the Earth is being "flung around" by the Moon, the high tide on the side of the Moon is caused by the centripetal force of the Moon pulling on the Earth, while the high tide on the opposite side is caused by the centrifugal force, i.e. the Earth's inertia.
KaptenNicco123 1 points 4h ago
The moon has mass. All things with mass attract all other things with mass. Therefore, the moon attracts the water that is on the surface of the Earth. The water thus gets pulled towards the moon, which means away from the surface of the Earth, which means the water level rises.
Barneyk 1 points 1h ago
This doesn't explain why the ride rises at the opposite side of the earth as well.
KaptenNicco123 1 points 1h ago
The tide rises on the opposite side of the Earth because the Earth itself is pulled by the tides as well. The Earth is pulled with a certain strength towards the moon, the water on the near side is pulled to the moon a little bit stronger than the Earth is, and the water on the far side is pulled to the moon a little bit weaker than the Earth is. The effect of this is that the water on the far side of the moon appears to be pushed away from the Moon, relative to the Earth.
boytoy421 1 points 1h ago
also as the earth rotates the rock rotates faster than the water which effects tides.
lets imagine a hypothetical planet with no moon and a tiny little island the size of like a table, small enough not to ever get in the way (and no sun either, the liquid water is from magic). and lets say you drop 2 probes on this planet, one that'll land on the tiny little island and one that'll land on the surface of the ocean. if they start out with a straight line between them over time the one on the ocean will be at a different position than the one on the island because the "connection" between the ocean and the planet isn't as stiff as the one connecting the island to the planet
lets imagine a hypothetical planet with no moon and a tiny little island the size of like a table, small enough not to ever get in the way (and no sun either, the liquid water is from magic). and lets say you drop 2 probes on this planet, one that'll land on the tiny little island and one that'll land on the surface of the ocean. if they start out with a straight line between them over time the one on the ocean will be at a different position than the one on the island because the "connection" between the ocean and the planet isn't as stiff as the one connecting the island to the planet
Impossible_Trip_8286 1 points 3h ago
I’m confused. If the moon is pulling the water towards it, wouldn’t that be low tide because the water would be moving away from the shore ? The moon acting like a vacuum?
2nds1st 1 points 3h ago
What people aren't explaining is that the bump in the water where the sun and moon are stays there. The earths rotation moves the land into the bump.
AelixD 1 points 1h ago
Kinda. When the moon is directly over the shore, its pulling the water up the shore. When its over the middle of the ocean, its pulling water away from both shores.
Impossible_Trip_8286 1 points 3h ago
So the eastern seaboard of the U.S. would experience high tide at the same time coastal Europe would be low tide? And coastal Europe wouldn’t experience high tides the way the Eastern seaboard does? Bas3d on earths rotation?
AelixD 1 points 1h ago
Close enough. The two locations are not 90 degrees apart; about 70 if I recall.
Mental_Cut8290 1 points 28m ago
I don't know where this inertia/centripital-force explanation came from.
Look... the sun/moon pulls on earth and water from wherever they are. The earth and water feels it strongly on the side close to the sun/moon, and weakly on the far side; but the earth is a relatively solid object while the water is more able to flow, so the earth only moves according to the pull at the center.
Close to sun/moon, water feels strong gravity but earth only moves "center" gravity, so the water moves more than the earth up towards the sun/moon.
On the far side, water only feels weak gravity, but earth again is pulled by "center"gravity, so the water floats up away from the sun/moon on the far side.
These high tides on opposite sides of earth cause low tide in the middle areas.
Look... the sun/moon pulls on earth and water from wherever they are. The earth and water feels it strongly on the side close to the sun/moon, and weakly on the far side; but the earth is a relatively solid object while the water is more able to flow, so the earth only moves according to the pull at the center.
Close to sun/moon, water feels strong gravity but earth only moves "center" gravity, so the water moves more than the earth up towards the sun/moon.
On the far side, water only feels weak gravity, but earth again is pulled by "center"gravity, so the water floats up away from the sun/moon on the far side.
These high tides on opposite sides of earth cause low tide in the middle areas.
Lost-Tomatillo3465 1 points 26m ago
according to flat earth, water doesn't curve, so there's no way there are tides.
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