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Q & A: how do tides work?

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Gravity
Earth & Moon
Q:
If the tides are affected by the gravitational forces of the moon and sun how do they do it? I have never believed the moon's gravity pulls the water but more that it pulls the land up and down thereby causing the water to ebb and flow giving the false impression the water is being pulled back and forth. Our sun was once thought to orbit the Earth owing to a similar illusion. Many have tried to ridicule my conviction but so far none have conclusively disproved it for me. Any takers??
- John (age 57)
UK
A:
If the gravitational acceleration toward the moon was the same regardless of where you were, then the whole earth+ocean system would accelerate together, without leaving any internal evidence of the gravitational effect. That's sometimes called the principle of equivalence. However, the gravitational pull toward an object is bigger the closer you are to the object. So the part of the ocean closest to the moon is pulled a little extra, compared to the average of the whole earth. It bulges up. The part farthest from the moon is pulled a little less than average, so it bulges the opposite way- which is also up, on that side of the earth.

The same thing happens from the pull of the sun, although not quite as much. During full and new moons, the moon and sun effects add together to make extra-big tides. During half-moons, they tend to cancel, making smaller tides.

What could have possibly led you to claim "I have never believed the moon's gravity pulls the water..."? If the sun didn't pull on the water, wouldn't it keep going in a straight line and leave the sun-orbiting earth behind? If the earth didn't pull on the water, wouldn't any little splash launch water into space? Why would there be any water left here? And if the sun and earth pull on the water, why wouldn't the moon also do so? Do you seriously think that Newton and every other physicist and geologist since then has been completely clueless?

Mike W.

(published on 03/10/2011)

Follow-Up #1: two tides

Q:
If the moon is the primary cause of a high-tide, how is it that there is an equally high tide on the opposite side of the earth? It would stand to reason that since gravity is a constant and effects all things, no matter the density, the same way, the earth would be pulled with the same force the water is, not to mention the earth's own gravitational pull being much greater than the moon's, how does the moon overpower the earth's gravitational pull to create tides?
- Andrew (age 39)
Atlanta, GA, USA
A:
We tried to sort of explain the double tides before, but perhaps another version will help.

The key point is that gravity is not a constant, but varies from place to place. That variation is the sole source of tides, with the constant part being irrelevant.

The moon's gravitational field gets weaker as you go farther from the moon. Let's subtract the average of the moon's gravity on the earth and just look at the remainder. On the side toward the moon, what's left is some extra pull toward the moon. On the side away from the moon what's left is some extra pull away from the moon. So those each make high tides.

Since the moon's pull is pointed toward the moon, the direction also changes from place to place on the earth. If you look at the part on a circle at average distance from the moon, what's left after you subtract the average pull is just a little pull inwards. So that's the area where the tide is low.

Mike W.

(published on 01/01/12)

Follow-up on this answer.