It makes sense to me why there is a tide facing the Sun and a sublunar tide, but why is there a tidal bulge on the opposite side of the Earth too?
In: Planetary Science
The Sun is pulling on the water on the near side harder than it’s pulling on the Earth, so that water is left slightly closer and therefore perceived as higher.
The sun is pulling on the Earth harder than it’s pulling on the water on the far side of the Earth, so that water is left slightly further away and therefore perceived as higher.
There is a tide on the side of the Earth facing the Moon because the gravitational pull on the water is greater than that on the Earth so the water is pulled away from the Earth.
But this is also true of the Earth and the water on the side facing away from the Moon: the gravitational pull on the Earth is greater than that of the water, so the Earth is pulled away from the water.
The center of rotation of the Earth-Moon system is offset from the center of the Earth. It is still within the Earth but closer to the Moon.
So the oceans facing the Moon are gravitationally attracted to the Moon and the oceans opposite are furthest away from the gravitational center of the Earth-Moon system.
So while the ‘earth falls away’ statement is true that usually confuses non-physics people.
It’s a lot easier to understand as centrical force. The moon and earth stay an equal distance apart because the pull of gravity and the centrifugal force of their rotation cancel out.
Moon side tide: What pulls the earth and moon together? – gravity. Where is gravity strongest? – on the side closest to the other object. Thus the tidal bulge on the side closest to the moon.
Fare side tide: What pulls the earth and moon apart? – centrifugal force. Where is the centrifugal force strongest? – on the outside of the rotation. Thus the tidal bulge on the far side from the moon.
This can also confuse because people can visualize the moon circling around and the earth not moving. The earth moving, it just the center of the earth-moon rotation is inside the earth. So rotates for the earth is more of a wobble. Think of it like holding a glass when you swirl your drink around. It is a tiny circle but still makes the liquid go up on one side.
The opposite side of the earth is pulled by the sun also. This causes a small lowering of the earth’s crust on the opposite side, this give you the answer.
When people talk about the moon orbiting the Earth, that’s not a precise statement. They actually both orbit a common centre of mass. Because the Earth is much more massive than the moon, that point is within the body of the planet but not at its centre. The fact that Earth orbits the moon the same way that the moon orbits the Earth, just less dramatically, is important here.
If you study a little bit about the math behind centripetal acceleration, you’ll find that, if two objects are going around in a circle at exactly the same rate, the one that’s farther from the centre gets a greater outward force than the one closer in. What does this have to do with what I’m talking about? Well, Earth’s orbit technically only describes the path the centre of the planet takes. The part of the Earth that’s farthest from the moon is therefore farther from the common centre of mass than the middle of the Earth. It receives a slight force away from the centre, causing an imbalance that compels fluids to gather there until that imbalance is corrected. The most convenient fluid in this case is water which then appears to us to rise and fall, depending on where the moon is in the sky.
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