Re: Tide article

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Sun, 21 Oct 2001, Jim Green wrote:

> John, I will not "debate" you re this -- I have stated my understanding
> from study and research -- well my student's research -- down the road from
> John Denker.   The list -- as Sam Donaldson says -- can take it for what
> they think it is worth.  I  think  that no one gives much of a damn about
> this anyway.

Jim,

It is true that I do *expect* there to be virtually perfect long
term correlation between lunar position and tidal extrema, but I'm
not trying to debate you on the physics here.  Furthermore you're
probably right about the level of interest on the list in general.
Nevertheless, I, like you, find the tides *endlessly* fascinating
and I have facts that I simply think are at odds with your
conclusions.  One thing I can say for certain on the basis of the
data is that there is *nothing* special about the Puget Sound in
this regard.

For instance, I just looked up the predictions for Atlantic City
and I find the following:

There was a highest high tide of 5.5 ft at 0909 on 18 October
2001. The next spring tide will bring a highest high of 5.0 ft at
0818 on 3 November 2001.  During this period clocks will be set
back one hour for the return to standard time.  Thus, we see 31
tidal cycles in 384.150 hours corresponding to 12.39 hours per
tidal cycle.  This is 2 minutes *less* than half a mean lunar day
and corresponds to an accumulated shift of about one hour in
phase.  Were this shift to continue, the tides would rapidly go
out of phase with the moon.

However, if we look at the next spring tide we find a highest high
of 5.4 ft at 0702 on 15 November.  During this *additional* period
we find 23 tidal cycles in 286.733 hours corresponding to 12.47
hours per tidal cycle.  This is not quite 3 minutes *more* than
half a mean lunar day and the accumulated phase shift is about the
same and in the opposite direction.

Indeed, if we add the two periods we find a total of 54 tidal
cycles in 670.883 hours corresponding to 12.424 hours per tidal
cycle, about 10 seconds (!!) more than half a mean lunar day.

Longer periods yield ever more precise agreement, especially if
they are taken at the same time of year. For instance, I find a
highest high of 5.7 feet at 0926 on 8 October 2002.  The total
elapsed time from the 18 October 2001 event is 8520.283 hours
during which 686 tidal cycles of average duration 12.420 hours
will occur.  This represents a deviation of about 2 seconds
from half a mean lunar day and a net accumulated phase shift of
around 20 minutes over the course of the entire year.

These results are essentially identical to those I find in the
Puget Sound or anywhere else on Earth.

Granted they are based on predictions rather than data as it
sounds like you may have used.  Historical data will show larger
specific deviations because meteorological effects, most notably
wind and pressure, can significantly alter the heights and times
of extreme tides.  But predictions represent a best *fit* to the
historical data; they wash out the meteorological effects and
reveal the astronomical influences more clearly.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm