More Astronomy Questions!!!?

Question

Here's the question:
Explain why there are two high tides and 2 low tides each day.
i get the whole gravity pull thing, but wouldn't there only be one high and low tide per day? would the earht need to rotate a second time for a second tide?

Answer 1

Tides are caused because the gravitational pull from the Moon is stronger on one side of the earth than on the other. This "stretches" the Earth, since one side is being pulled harder. That gives two points on the Earth with high tide -- the side nearest the Moon, and the side opposite. That's why you get two high tides a day instead of one.

This diagram shows what I mean: http://www.lhup.edu/~dsimanek/scenario/tides02.gif

Answer 2

That's a very good question.

Not only does the moon go around the earth, but the earth goes around the moon! The two heavenly bodies rotate around a common point, which is not at the centre of the earth.

It's not as dramatic as it sounds - the point around which they rotate is inside the earth, just not at its centre. But it does mean that the earth is orbiting around this point, as well as spinning on its axis. This has the effect of stretching the oceans out, so that there is a bulge on each side - one way to think of it is that the bulge on one side of the earth, facing the moon, is pulled up by the moon, while the bulge on the opposite side to the moon is thrown outwards by the centrifugal effect of the earth's orbit around the earth-moon centre.

So the earth is a sphere but the ocean is ideally a sort of ellipse. Since the earth spins once each 24 hours, the two bulges cross each point in the ocean once every 24 hours each, making two tides a day.

Because the moon has moved on a bit after 24 hours, it is nearer 25 hours before the tide comes around again, but because there are two of them a day, we see the tide coming roughly every 12 hours and 25 minutes.

Answer 3

Gnomon really has the correct answer ☺

Doug