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Re: tides, did you know...
From: Chuck Taylor
Date: 1998 Jul 09, 10:41 PM
From: Chuck Taylor
Date: 1998 Jul 09, 10:41 PM
hhew@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 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= =-= TO UNSUBSCRIBE, send this message to majordomo@roninhouse.com: =-= =-= navigation =-= =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-=