I can understand a tidal bulge towards the moon, but why one on the opposite side of the earth?

Question

But the bulge on the opersite side of the earth is AWAY from the moon!

Answer 1

So the Earth is about 7000 miles thick. This means that the point closest to the Moon is 7000 miles closer than the point farthest from the Moon. This in turn means the water at that point is being pulled by the Moon's gravity about 7/230 more than the water at the point farthest from the Moon. Right?

Ah, but that's not the whole story. Because the Earth is a sphere (more or less), with water all around it (more or less), the water that's halfway BETWEEN the closest and farthest points from the Moon is being pulled with HALF the additional gravitational attraction.

So if the Moon's closest point is at 0 degrees, its farthest point must be 180 degrees.

But the water at 90 and 270 degrees is being pulled half as much, relative to the water at 180 degrees. (The actual math is a little more complex, but let's say it's "with the effect of half the distance" and leave it at that.) So it ALSO gets pulled away from the water that's hanging out there at 180 degrees.

So it isn't so much that the water opposite the Moon bulges, it's that the rest of the water on the Earth gets pulled away from the farthest point, proportionally with the nearness to the Moon. And because water is fluid (and more to the point, is bound to the Earth's gravity FAR more closely than to the Moon's), it forms an oval, or ellipse.

Want to see this in action? Blow up a balloon. Use tape to stick it to a flat surface on one side -- for example, your kitchen table -- then tape a string to the opposite side of the balloon. Pull up on the string. What happens? The baloon seems to "bulge" in the direction of the table, because the pull elongates it.

Answer 2

Scott F is pretty close. I'll try to say it a different way and between the two, you may get your answer.
The reason for the tidal bulge on the opposite side of the earth is due to a difference of gravitational attraction from the moon. The moon attracts the near side oceans more strongly than the far side. The moon also tugs on the earth itself. What you have (in an overly simplified example) is that the moon pulls on the near oceans and causes the tides to rise. The moon also pulls on the earth and the oceans on the far side, but the oceans on the far side experience a lesser pull than the earth as a whole does, so the water tends to want to lag behind as the moon pulls the earth away.
Of course the math is pretty complex and the example is oversimplified, but I hope you get a general idea.

The bottom line is that the tidal bulge on the opposite side of the earth is due to a difference it gravitational attraction of the moon.

Answer 3

The bulge on the opposite side is because the Moon pulls the Earth towards it slightly. The Earth moves a bit towards the Moon, leaving behind the water on the far side.

Answer 4

the moons gravity gets weaker the futher away you travel from it (same is true of all objects with mass and therefore gravity).

the oceans on the side of the earth closest to the moon are pulled much more strongly than the oceans on the far side of the earth.

the oceans nearest the moon are pulled up into a bulge, the oceans furthest away feel the gravity less strongly and bulge away, they are not held so tightly so can 'spread out' a bit.

when the moon and sun are both in the same direction their gravitational forces add and the high tides are higher, perhaps counter intuitively when they are directly opposite each other on either side of the earth, we still get quite high, high tides. the lowest high tides we get occur when the sun and moon are 90 degrees apart.

Answer 5

Everyone knows that the Moon orbits around the Earth. But what they don't tell you is that the Earth orbits around the Moon, or really they both orbit around a common center of gravity, which is about 2,000 miles moonward of the Earth's center. The Earth's motion around this center of gravity causes the ocean bulge on the opposite side, as kind of a centrifugal force effect.

Answer 6

My non-physicists understanding of this is that on one side the tidal bulge is caused by the pull of the moon's gravity, and on the other side by the centrifugal force due to the Earth's "orbit" around the common centre of gravity, we know that these forces are equal and opposite because the Earth and the moon are at a steady distance from one another.

Answer 7

On one end the water is closer to the moon than the solid Earth; so it is pulled more and rises. On the opposite side of the Earth, the water is farther from the moon than the solid earth, the Earth moves more toward the Moon than the water does, creating a relative rise. It's the relative positions of the water and the solid earth that creates a gravity differential. This gravity differential is the cause of the tides.

Answer 8

Ah, but is it still bulging TOWARDS the moon? Though it may be on the other side of the earth, the pull of the moon could sill make a bulge effectively "through" the body of the earth

Answer 9

Because the earth is revolving about the center of mass of the earth moon system. and the part furthest from the moon is pulled less strongly by the moon's gravity than the rest of the earth is and more by the apparent "centrifugal force" due to the motion.

Answer 10

There are two major gravitational forces on the sea - moon and sun. Both will affect tides.

The reason there are two bulges is the tide generating force and is the effect of external gravitational force (eg moon) and internal (ie the earth's) acting in combination.

It is explained on the article linked below. Good luck in understanding it!