Why are there two high tides a day?

07 January 2014



If the tides are caused by the gravity of the moon, why is there a high tide on the side of the Earth furthest from the moon as well as on the closest side?

Peter Conway


Dominic - Tides certainly are caused by the gravity of the moon which is always pulling the Earth very gently towards the moon. Now, the side of the Earth which is faced directly towards the moon is slightly closer to the moon from the middle of the earth. That means it feels a slightly stronger pull because gravity decreases with distance from the moon. And so, that's being pulled more strongly towards the moon and so, you can understand why you get a high tide there. Water is being pulled there more strongly towards the moon. Now, on the far side, the pull towards the moon is weaker than anywhere else on the Earth, just because it's further away from the moon. And that means the moon is pulling down on that water on the far side less strongly than elsewhere. And so, it rises up away from the moon in the opposite direction from the moon to form this second high tide. So, you've got two, one on either opposite side of the Earth.

Chris - As the planet turns, it's turning through both of those bulges of water, so you get high tide number 1, then it takes 12 hours to get round to the other side which is half a rotation, half a day, and there's the second bulge, second high tide.

Dominic - Exactly, so. Those two bulges stay in the same place in space more or less on the line through the Earth to the moon, and the Earth is rotating once every 24 hours, so we move through one of those two bulges every 12 hours as you say.

Chris - And just very briefly, Dominic, the difference between a spring and a neap tide. How does that happen and why?

Dominic - The sun also produces tides. They're about half the strength of the lunar tides. So, that's another signal on top of the lunar tides and sometimes the sun and moon tides coincide and sometimes they don't. They coincide at full moon and new moon, and then you have tides which can be 30%, 40% higher than at other times of the month when the moon and the sun are ninety degrees apart in the sky when you have what are called neap tides which are much lower.


The explanation at cosmos magazine is more or less correct. You have to take into account the centrifugal force operating on the earth-moon system spinning around its centre of mass. The proposed explanation by Chris and Dominic is wrong because it does not take this centrifugal force into account. In stating that the moon pulls down far less strongly on the far side of the earth from the moon, Chris and Dominic ignores the gravitational pull of the earth which is an important factor. In fact the gravitational pull of the moon and the earth combined is stronger on the far side of the earth from the moon than it is on the near side of the earth from the moon.

As a student, I didn't completely understand the explanation here, but this site clarified my understanding.

This entire explanation is cobblers.

The explanation given above is absolutely correct to account for why we have tides and why there are two tides per day. Please go and read a text book of physics.

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