Science Questions

How does the moon cause two tides every day?

Sun, 12th Sep 2010

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Mike Jacobs, Cape Town asked:

As I understand it, the moon, when itís directly overhead, causes the oceans to rise and thatís 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?


Dominic -  Mikeís absolutely right that itís 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.  Itís 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 Ė itís rather like being on the space station where youíre above the Earth and because youíre falling towards the Earth and the space station and you are falling at the same rate, thereís no relative speed between you and you feel like youíre weightless.  Similarly, the water in the sea is receiving the same pull as the rest of the Earth and so itís 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 itís 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 itís pulled up into a tide. The [water] on the opposite side is pulled slightly less strongly and so itís 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 Earthís surface.  One on the Earthís surface closest to the moon because the water is feeling the force from the moon and youíve 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 itís being pulled less than the Earth is so youíve got a second bulge there.  And at 90 degrees youíve got two dips which are the low tides, is that right?

Dominic -   Thatís exactly it!


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But it's not a particularly helpful alternative explanation. Bored chemist, Sat, 25th Sep 2010 granpa, Sun, 26th Sep 2010

the effect of the rotation of the earth, with respect to the stars,
is canceled out by the earths bulge and can therefore be factored out and ignored.

Treating the earth as though it didnt spin with respect to the stars then
the centripetal force, due to its motion around the center of mass of the earth moon system,
would be exactly the same, in strength and direction, everywhere on its surface.
(and always points away from the moon)

to a first approximation, the change of the gravitational field of the moon from front to back of the earth is linear
and is equal to the centripetal force at the center of the earth

granpa, Fri, 8th Oct 2010

so it can be factored out. granpa, Fri, 8th Oct 2010

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