Re: Question about tides

"james" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]

> In a book about tides, I read : "It is crucial ro realize that spring
> and neap tides must each occur at about the same time all over the
> world because the Earth rotates within the tidal bulges.

I'm afraid that there is in fact no such thing. The Victorian or
pre-Victorian concept is still in use as a very badly oversimplified
explanation of what may not actually be the truth of the matter. Whereas
it is true that tides gain or lose amplitude with each successive "tidal
day" it is by no means clear what exactly is happening.

> So far, so good, but  a few pages after that there's a passage about
> tidal lags : "... there is a tidal lag, such that high tide commonly
> arrives some hours after the passage of the Moon overhead. Because the
> linear velocity of the surface of the Earth with respect to the Moon
> decreases polewards, the tidal lag is greatest at low latitudes ...
> but the precise lag is always constant for a particular location".

It depends on where you are. If you lived in certain parts of Fiji I
believe you would find them following the sun. The type of tides seem to
vary with latitude. There are a variety of types of tide. But talking
about this "Lag": The tides around Britain are high at pretty nearly the
same time in Liverpool and Dover but at the same time in Bristol and
Hull they are low. 6 hours and 20 minutes or so later, the reverse is
true. It is as though a three phase pulse is travelling around Britain
and Ireland.

It is a mistake (in my opinion) to directly relate the tidal pulse with
the position of the moon. After all, if it was directly following the
moon
it would be racing around the globe at nearly 1000mph.

> So after all, spring and neap tides can only occur at the same time of
> day along the same line of latitude, where the tidal lag is equal all
> around the Earth. At different latitudes, the tidal lag will make them
> occur at different times. Am I right here?

Spring tides occur at New and Full moon. They reach high or low at
different times as stated above. Neap tides occur at the quarter moons
with the same effect. Another pointer to the fact that the oceans are
not following the moon is that flotsam and boats etc are not set in
motion by the moon. If that were so, there would be a jam of detritus
swilling around the globe like ice floes.

For reasons I don't wish to go into there was an argument going nowhere
in the sci.geo.earthquakes newsgroup about this sort of thing. All the
posters with differing viewpoints were unwilling to admit that I was
right. (Fools!)




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