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... 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
... 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
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.
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