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Old 10-26-2008, 06:16 PM
pseudotriton ruber ruber pseudotriton ruber ruber is offline
Join Date: Sep 2002
Location: Outer Control
Posts: 10,394
How fast does the tide move?

Not in and out, but up (or down) the coastline? There are various times given for high and low tides all along Long Island, and I presume there's some rhyme or reason to it, though this tides chart doesn't seem to suggest any real pattern offhand. Is there some pattern I'm not seeing based on geography generally, or is it all a matter of factors others than geography (depth of bay, the mood that Poseidon is in that morning, stuff like that).

If you want a specific inquiry, when I go running on the beach I often choose the direction I run in so I won't be staring straight into the sun as I run, but I've been wondering if the low tide could be moving along the shoreline with me, or against me, and if so, how quickly--much faster than I run, much slower, etc. or whether this is impossible to predict on a regular basis.

Last edited by pseudotriton ruber ruber; 10-26-2008 at 06:16 PM. Reason: fix'd link
Old 10-26-2008, 06:41 PM
sailor sailor is offline
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Join Date: Mar 2000
Location: Washington dc
Posts: 16,441
Every location is different. The attraction of Moon and Sun are only the primary forces which cause the tides but then the water is like water sloshing around in a pan and no general rule can be made. Only specific rules for specific places.

Take the Chesapeake bay. It runs from N to S and the primary tidal forces would pretty much cross at right angles. If it were a lake there would be no appreciable tides. But at the mouth there are tides which create tidal waves which progress up the Chesapeake. So the tides on the Chesapeake are not directly caused by any attraction of the moon but by a wave at the mouth which then moves up the bay (and reflects etc.).

Depending on the shape of the bay or inlet the tidal wave can reinforce and the tide level difference can be great. This is what happens in the bay of Fundy.
Old 10-26-2008, 10:36 PM
Napier Napier is offline
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Join Date: Jan 2001
Location: Mid Atlantic, USA
Posts: 8,935
Ditto sailor. There is a major tidal effect that is, I think, a couple feet in size that runs around the earth every 24 hours (more precisely there is a bigger effect from the Moon that takes something like 25 hours and a smaller effect that is 24 hours from the Sun). However, in many places on the coast there are sloshes or echos or resonances that are much bigger, and how their timing relates to the open ocean tides depends strongly on the local geography. Broadly speaking, there is a whole category of these that are in sounds that communicate with the open ocean only through a few small inlets, and the big effect this geometry has is to make the tides in the sounds be smaller and lag behind the open ocean tides. There is also a tide of about half a foot in the continents, if I remember right, because the crust shifts up and down. Of course, there's nothing to compare it to, like you can compare the bigger tides on water to the relatively more stationary land.


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