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    Re: tides, did you know...
    From: Chuck Taylor
    Date: 1998 Jul 09, 10:41 PM

    hhew{at}juno.com wrote:
    >
    > Before computer-predicted tide tables,  the old-time seafarers used what
    > is called the luni-tidal interval to predict tides in places all around
    > the world.  This interval is the amount of time that elapses between the
    > meridian passage of the moon and high tide. ...
    >
    > The  tides follow the moon at that interval every day. so if you figure
    > from the almanac when the moon will be on your meridian you can predict
    > the tide just  as well as any government computer in a basement in Idaho.
    ,,,
    
    As a matter of historical interest, the mean high-water lunitidal
    interval for a given location was also called, "the Establishment of the
    Port." Another name used was the "time of high water on full and change
    days", or more simply, "High Water Full and Change", abbreviated "H.W.F.
    & C." This was because, quoting from Bowditch 1926, "on such days [when]
    the moon's two transits (upper and lower) over the meridian occur about
    midnight and noon, the vulgar [uncorrected] establishment then
    corresponds closely with the local times of high water." Technically,
    one could distinguish between the vulgar [uncorrected] establishment,
    which is the same as H.W.F. & C., and the corrected establishment or
    mean lunitidal interval, which is an average of the vulgar establishment
    over a period of time. Bowditch also points out that once you know the
    time of high water, the time of low water would be about 6 hrs 13 min
    later. Bowditch also mentions that there usually isn't much difference
    between uncorrected and corrected intervals.
    
    The establishment of the various ports was sometimes printed on charts
    as well as in tables such as Bowditch. As was pointed out earlier, armed
    with that information and an almanac to find the meridian passage of the
    moon, one could predict the timing of the tides. I decided to try out an
    example:
    
    From the 1926 Bowditch, the high water interval for Port Townsend, WA,
    is 3 hrs 47 min, and the longitude of Port Townsend is 122 d 45 m West.
    From the Nautical Almanac, Meridian Passage of the Moon will be at 0021
    on 10 July 1998 (a few hours from the time I write this).
    
    	Meridian Passage of the Moon	0021
    	High water interval		0347
    	LMT to ZT correction		0011	(for 2 deg 45 min from zone
    						 meridian)
    	Longitude correction (Table 2)  0018
    	Correction for daylight time 	0100
    	Prediction for high water	0537
    
    	Correction for low water	0613
    	Prediction for low water	1150
    
    Looking in the tide tables for the same date, we find a prediction for
    high water of 0359 and of low water of 1110. For comparison:
    
    					High Water	Low Water
    	Almanac + tidal interval	0537		1150
    	Tide table			0359		1110
    	Difference		       +0138	       +0040
    
    Not perfect, but not too bad. I must admit that a 99-cent tide table is
    a much easier way to go!
    
    Cheers,
    
    Chuck Taylor
    Everett, WA, USA
    GB 32-2 Seascape
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