Why are there two high tides a day?

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Why are there two high tides a day?
« on: 08/09/2011 16:30:02 »
Kissane  asked the Naked Scientists:
   Hi Chris,
Why are there two high tides a day?
Doug Kissane
What do you think?
« Last Edit: 08/09/2011 16:30:02 by _system »


Offline graham.d

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Why are there two high tides a day?
« Reply #1 on: 10/09/2011 13:49:41 »
Tidal forces are due to the gravitational pull from the moon and, to a lesser extent, the sun. (let's forget the sun for the sake of explanation). As the earth rotates one side is closest to the moon and the other is the furthest away. The reason we see two tides is not easy to explain without maths but I'll have a go.

If we neglect the earth's rotation for a minute and think about the moon going round the earth we can see that this is stable state where the two bodies are orbiting each other. Because the earth is much bigger we often think of this as the moon going round a fairly static earth but this is not quite the case, and the earth is also moving in an orbit, albeit a smaller one than the moon. In this stable state, if you were to be at the centre of the earth you would detect no gravitational effects from the moon; technically you, at the earth's centre of gravity, are in "free fall" within the earth-moon system. You experience none of the earth's gravity, and the moon's gravity is in balance with the centrifugal force throwing you outwards in the earth's orbit with the moon. If you go up the surface on the side where the moon is, you experience a force equal to the earth's gravity pulling you to the earth's centre. However this is reduced slightly because you are closer to the centre of your orbit with the moon and the net force (trying to throw you outward, away from the moon) is less. That is the net force towards the earth's centre is less. If you go the the other side (away from the moon) the net force is higher but is away from the direction of the moon so the net force towards the centre of the earth is again less. So in such a two body orbiting system you can see that the tidal forces result in a body being stretched on both its nearside and its far side.

This means that such forces, acting on surface water, cause the water to be bulged out on both the near and far sides of the earth and reduced in depth at the poles and at the positions between the near and far sides (those roughly in line with the orbital path). Because the earth is rotating, these peaks and troughs of depth are dragged around the earth's surface every 24 hours so we see two high tides each day.

The effect of the sun makes solving the maths of this very hard but the principle is the same.