posted
Low tide when the Moon is near either horizon. High tide when the Moon has just passed zenith. Highest highs and lowest lows when the Sun is in the same direction as the Moon and adds to the gravitational pull.
[This message has been edited by extrinsic (edited August 06, 2009).]
So, the moon CAN be below the horizon when the tide is GOING OUT.
And, at the moment the tide changes to COMING IN, where is the Moon when this is happening and it's all happening with the Moon on the side of where this beach is and not on the other side of the world -- ????
posted
The tide starts going in a different direction as soon as the drag effect pulls in a sufficiently strong, different direction. Earth tides are complicated, though, by intervening land masses and axial tilt and centrifugal forces.
Yes, tides will start to go out and the Moon will be below the horizon as the Moon comes around from its far side zenith (nadir). When the Moon is at the nadir, in some locales there's usually a lower high tide on the near side than when the Moon is above the near side zenith.
The change from one tidal maximum to a rising or falling tide occurs after the Moon has reached the tangent position of zenith, nadir, or astronomical horizon. On Earth, it takes roughly thirty minutes to overcome the water's inertia. That resting time of the tides at low or high is known as a slack tide.
posted
I'm not an expert - but I think the easiest way to think of it is to consider the earth to be a 2D circle, surrounded by a circle of water, and to let the moon orbit the circle. The gravity of the moon pulls the water towards it, wherever it is - so think of the water being pulled up towards the moon. At the point on the earth directly below the moon (in our little 2D diagram), it's high tide right there. Now, on the other side, think of the sun as acting and pulling the water towards it too. So you have a circular earth surrounded by an oval of water. The sun's side of the pull is smaller than the moon's side, b/c it is so much farther away. However, you can imagine stretching the water-oval out, and it gets thin on the sides that don't have the sun or moon on them -- these are low tides.
I don't know if that simplifies the way to think about it, but if you want a much better/detailed explanation, you can check the article on wikipedia about tides.
posted
If you really want to math out on it, Texas A&M has some nice diagrams on an oceanography class's web sitePosts: 500 | Registered: May 2008
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