Why do tides bulge on both sides of earth?
Discussion
I’ve tried reading up on this but the explanations given don’t seem like explanations.
For the oceans near the moon the water is being pulled towards the centre of the earth but that gravitational pull is being countered slightly by the gravitational pull of the moon. Which could explain the water bulging on that side. However for the water on the opposite side of the planet the gravitational forces are working in unison, so at face value I would expect the water to be squeezed closer to the centre of the planet, not bulging further away.
Clearly there is something wrong with the analysis above but what?
For the oceans near the moon the water is being pulled towards the centre of the earth but that gravitational pull is being countered slightly by the gravitational pull of the moon. Which could explain the water bulging on that side. However for the water on the opposite side of the planet the gravitational forces are working in unison, so at face value I would expect the water to be squeezed closer to the centre of the planet, not bulging further away.
Clearly there is something wrong with the analysis above but what?
I'm probably going to be wrong! But I belive the moon is lifting some water up but the main effect is because water wants the path of least resistance it flows to each side pushing higher tides on either sides of the earth by the flow of water sideways rather than up.
If I'm horrendously wrong please let me know!
If I'm horrendously wrong please let me know!
louiechevy said:
I'm probably going to be wrong! But I belive the moon is lifting some water up but the main effect is because water wants the path of least resistance it flows to each side pushing higher tides on either sides of the earth by the flow of water sideways rather than up.
If I'm horrendously wrong please let me know!
Presumably the water is not moving around the planet with the moon (otherwise it would be circumnavigating the world every month), rather the bulge is the difference in depth of the ocean. Maybe I’m not following your point.If I'm horrendously wrong please let me know!
Don't forget the earth rotates every twenty four hours! From what I remember reading the moons lift is not very high at all, imagine a swimming pool and raising the middle a small amount the water wants to stay flat so flows of to the edges. The moons effect is to create two high points either side and the earths rotation within them causes two high tides a day.
I could still be horribly wrong but it makes sense in my head!
I could still be horribly wrong but it makes sense in my head!
I think this is a good explanation.
https://oceanservice.noaa.gov/education/tutorial_t...
The bulge on the far side from the moon is due to the inertia of the Earth-Moon system "throwing" (my word) water away from the surface of earth.
Earth-Moon common centre of gravity is actually a few thousand kilometres below Earth's surface, quite a ways from the earth's centre.
https://oceanservice.noaa.gov/education/tutorial_t...
The bulge on the far side from the moon is due to the inertia of the Earth-Moon system "throwing" (my word) water away from the surface of earth.
Earth-Moon common centre of gravity is actually a few thousand kilometres below Earth's surface, quite a ways from the earth's centre.
Wayoftheflower said:
I think this is a good explanation.
https://oceanservice.noaa.gov/education/tutorial_t...
The bulge on the far side from the moon is due to the inertia of the Earth-Moon system "throwing" (my word) water away from the surface of earth.
Earth-Moon common centre of gravity is actually a few thousand kilometres below Earth's surface, quite a ways from the earth's centre.
Thanks for the link. I sort of figured it out last night that the earth and moon are orbiting each other and that the centre of the orbit will be under the earth’s surface on the side of the earth facing the moon. As the earth rotates around that point, the water wants to go straight on but is accelerated towards the centre of the earth. Like swinging a ball at the end of a stretchy rope, as you swing it the rope would stretch a bit.https://oceanservice.noaa.gov/education/tutorial_t...
The bulge on the far side from the moon is due to the inertia of the Earth-Moon system "throwing" (my word) water away from the surface of earth.
Earth-Moon common centre of gravity is actually a few thousand kilometres below Earth's surface, quite a ways from the earth's centre.
Not only is the moon pulling the water on the side facing the moon towards it, it is also pulling the earth itself towards it. This means that the earth itself moves slightly towards the moon, leaving a bulge of water on the opposite side of the earth - hence two tidal bulges.
Also, gravity strength decreases under the inverse square law, so the effect of the moon's gravity on the side of the earth facing the moon is greater than on the side of the earth facing away from the moon. This causes a stretching affect on the earth turning the earth into a very slight rugby ball shape i.e. with two bulges on each side of the planet.
It's these types of gravitational effects that cause the interior of Jupiter's four Galilean moons to remain molten and hot - and in the case of the moon Io, create the most volcanically active body in the solar system.
Tidal forces can be quite dramatic. If a body strays too close to a larger body, tidal forces will rip it apart. The point where this happens is known as Roche's Limit (a chap called Roche worked it out in the 19th Century).
The rings of Saturn partly exist because of tidal forces preventing the particles that make up the rings from coalescing into a small moon. All the particles in the rings are inside the Roche's Limit for Saturn.
Also, gravity strength decreases under the inverse square law, so the effect of the moon's gravity on the side of the earth facing the moon is greater than on the side of the earth facing away from the moon. This causes a stretching affect on the earth turning the earth into a very slight rugby ball shape i.e. with two bulges on each side of the planet.
It's these types of gravitational effects that cause the interior of Jupiter's four Galilean moons to remain molten and hot - and in the case of the moon Io, create the most volcanically active body in the solar system.
Tidal forces can be quite dramatic. If a body strays too close to a larger body, tidal forces will rip it apart. The point where this happens is known as Roche's Limit (a chap called Roche worked it out in the 19th Century).
The rings of Saturn partly exist because of tidal forces preventing the particles that make up the rings from coalescing into a small moon. All the particles in the rings are inside the Roche's Limit for Saturn.
Bill said:
The main tidal rise is due to the sun - hence the twice daily pattern.
The sun only goes round once a day, at least it does on my planet.I think the bulge at the back is like a wave - ie once water has been pulled up or down, it keeps on going up and down for a bit.
I have a theory of dinosaurs too...
Bill said:
Simpo Two said:
The sun only goes round once a day, at least it does on my planet.
Knew I should have been clearer. I meant it's not mainly due to the moon as most seemed to be suggesting ...The sun's overall tidal effect on the earth is much less than that of the moon's. Even though the sun is obviously very much larger and more massive than the moon, it is also much further away (93 million miles compared to 250,000 miles).
With tides, although size matters, distance matters even more.
Effectively, the moon's tides distort the shape of the earth - not just the water but the land as well. Conversely, the earth has the same effect on the moon. In fact, because the earth is 81 times more massive than the moon, the tides created on the moon by the earth are much more severe than what the moon does to the earth.
Once we started getting images back from the far side of the moon, we discovered that the far side has quite a different geology (selenology to be more accurate) to the side facing the earth.
The far side contains virtually none of the huge lava plains (the Lunar Mare) that are such a feature of the near side. This is almost definitely due to the effect of the earth's gravity and tides affecting the side of the moon facing the earth. The earth's tides actually stopped the moon rotating in respect of the earth which is why it keeps the one side facing us. The moon's tides are doing the same to the earth - gradually slowing the earth's rotation by a few microseconds every year.
With tides, although size matters, distance matters even more.
Effectively, the moon's tides distort the shape of the earth - not just the water but the land as well. Conversely, the earth has the same effect on the moon. In fact, because the earth is 81 times more massive than the moon, the tides created on the moon by the earth are much more severe than what the moon does to the earth.
Once we started getting images back from the far side of the moon, we discovered that the far side has quite a different geology (selenology to be more accurate) to the side facing the earth.
The far side contains virtually none of the huge lava plains (the Lunar Mare) that are such a feature of the near side. This is almost definitely due to the effect of the earth's gravity and tides affecting the side of the moon facing the earth. The earth's tides actually stopped the moon rotating in respect of the earth which is why it keeps the one side facing us. The moon's tides are doing the same to the earth - gradually slowing the earth's rotation by a few microseconds every year.
The moon's gravity pulls strongest on the water nearest it, less on the earth itself, and even less on the water furthest away. So the earth is accelerating towards the moon slower than the "near" water and faster than the "far" water, hence the two bulges. (Acceleration in the vector sense).
Super Sonic said:
Eric Mc said:
Think of the earth as being distorted by the moon's gravity to the shape of a rugby ball and you can see how the two bulges appear.
Obviously, the earth doesn't quite look like a rugby ball but the distortion is there.
I think this is the effect, rather than the cause.Obviously, the earth doesn't quite look like a rugby ball but the distortion is there.
Tides on earth are caused by tidal EFFECTS exerted by our moon.
The moon causes the earth to become slightly oblate, creating a bulge on the side facing the moon AND a bulge on the opposite side of the earth. During the 24 hours of the earth day, the earth rotates through those two bulges - giving us two tides spaced 12 hours apart.
Eric Mc said:
Super Sonic said:
Eric Mc said:
Think of the earth as being distorted by the moon's gravity to the shape of a rugby ball and you can see how the two bulges appear.
Obviously, the earth doesn't quite look like a rugby ball but the distortion is there.
I think this is the effect, rather than the cause.Obviously, the earth doesn't quite look like a rugby ball but the distortion is there.
Tides on earth are caused by tidal EFFECTS exerted by our moon.
The moon causes the earth to become slightly oblate, creating a bulge on the side facing the moon AND a bulge on the opposite side of the earth. During the 24 hours of the earth day, the earth rotates through those two bulges - giving us two tides spaced 12 hours apart.
Check out this animation to see how the earth is orbiting the centre of mass
https://www.youtube.com/watch?v=7hMfCCqSdFc
Water is not solid like a ball so the pull on the oceans near the moon would not result in a tail on the other side as far as I can see.
Edited by Skeptisk on Tuesday 8th August 15:30
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