How does the moon cause two tides every day?

12 September 2010

Question

As I understand it, the moon, when it’s directly overhead, causes the oceans to rise and that’s giving us our high tide. We experience that twice every 24 hours, but the moon is only overhead once every 24 hours at the same spot. What causes the second high tide?

Answer

Dominic - Mike's absolutely right that it's the pull of the moon on the water in the oceans that causes the tides, and the moon is only above one point on the Earth at any given time. It's not actually the direct pull of the moon on the water which is causing the tides.

The Earth and the seas are both in free-fall towards the moon - it's rather like being on the space station where you're above the Earth and because you're falling towards the Earth and the space station and you are falling at the same rate, there's no relative speed between you and you feel like you're weightless. Similarly, the water in the sea is receiving the same pull as the rest of the Earth and so it's not pulled up above the surface of Earth by that effect. The effect is actually that the Earth is of a finite size. The water on one side is pulled fractionally more strongly than the water on the other side because it's closer to the moon and the gravitational force decreases with distance from the moon. And the water which is closer to the moon is pulled more strongly and so it's pulled up into a tide. The [water] on the opposite side is pulled slightly less strongly and so it's pulled down less strongly towards the surface of the Earth and so you get a second bulge on the far side of the Earth. Chris - So in other words, there are two blobs of water on the Earth's surface. One on the Earth's surface closest to the moon because the water is feeling the force from the moon and you've got a bulge there, and then on the opposite side of the Earth, furthest away from the moon because that water is further from the moon and it's being pulled less than the Earth is so you've got a second bulge there. And at 90 degrees you've got two dips which are the low tides, is that right? Dominic - That's exactly it!

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