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    Re: New Moon, Perigee, and Solstice
    From: Trevor Kenchington
    Date: 2003 Dec 22, 14:16 +0000

    First off, Sharon Blackwood wrote:
    
    > Here are tide tables for the Bay of Fundy.
    > Living in downeast Maine, I occasionally reference them,
    > but will allow someone else to calculate any lag.
    >
    >  
    http://www.lau.chs-shc.dfo-mpo.gc.ca/cgi-bin/tide-shc.cgi?queryType=showZone&language=english®ion=5&zone=30
    
    
    That link leads to predictions for various Secondary Ports around the
    head of the Bay but they are only estimated using differences on the
    Saint John -- fine if you need an approximate prediction of the time of
    high water over the gridiron at Hantsport but of no use in describing
    the subtleties of tidal phenomena.
    
    For the latter, the only published tide tables are those for Saint John
    (New Brunswick). I suspect (though I cannot be sure) that that is also
    the only place in the Bay for which a proper run of 19 years of tidal
    data has ever been collected. (Why CHS doesn't put more time into
    gathering detailed tidal data, I do not know.) Fortunately, the tides in
    the Bay are particularly simple and what is true of Saint John is
    probably very close to the truth for everywhere from Grand Manan and
    Brier Island up to the Minas and Chignecto basins (aside, of course,
    from amplitude which increases up the Bay).
    
    George wrote:
    
    > Frank Reed made the interesting comment-
    >
    >
    >>These tides bring to mind something I noticed in old editions of Bowditch. If
    >>I am remembering correctly, Bowditch from c.1800 says that the Spring Tides
    >>(tides with maximum range) occur about three days after New Moon and three days
    >>after Full Moon. That's not accurate in New England where the difference is
    >>less than a day, but it is fairly accurate in northwest Europe which has unique
    >>resonances. Bowditch, of course, copied earlier works and carried over lore
    >>regarding European tides. So when did they fix it? Is there a 19th century
    >>edition of Bowditch that has better basic descriptions of tidal phenomena?
    >>
    >
    > Response from George-
    >
    > I agree with Frank that coastal resonances must play a significant part in
    > this tidal lag, where in many parts of Europe the highest tides follow a
    > couple of days after the time of maximum tide-generating force. But are
    > those in Europe "unique resonances"? I doubt it. What about that resonance
    > toward the head of the Bay of Fundy, which produces the biggest tide-range
    > in the World? I would guess that the spring tides, at the head of the Bay
    > of Fundy, might well lag behind new-moon and full-moon just as much as they
    > do in parts of Europe. But I don't have tide-tables for that part of the
    > world. Does anyone have data to confirm (or disprove) that guess?
    
    
    I am not sure but I think that the resonance processes in the various
    areas of extreme tidal range [Fundy, Ungava (recently promoted from 3rd
    to 1st equal by CHS), Bristol Channel -- with Cook Inlet, Alaska being
    4th?] are different from one another. In the case of Fundy, the system
    is really very simple. The combination of the Bay and the Gulf of Maine
    has a resonant frequency very closely tuned to that of the semi-diurnal
    lunar tides (a point I will return to shortly). I don't think that the
    same can be said of the Bristol Channel, for example.
    
    But to answer George's specific question: New Moon this month fell on
    the 23rd. Astronomic High water at Saint John was predicted to reach 8.5
    metres on the mornings of the 23rd and 24th, that being the highest of
    the month. To pick another example, Full Moon last August fell on the
    11th. Predicted High Water reached 7.6 metres on the 13th and 14th. Thus
    the lags are small -- though they are also variable.
    
    
    What is interesting about the Fundy tides (other than their height) is
    that the resonant amplification works specifically for the lunar
    semi-diurnal tide, not the solar one. Thus, the old rule-of-thumb that
    the range of neap tides is about one half that of springs breaks down
    and the spring/neap cycle is not that pronounced. On the other hand, the
    increase in the semi-diurnal lunar tide-generating force at perigee is
    itself amplified. Hence there is an unusually pronounced perigee/apogee
    cycle in the ranges of the tides in Fundy. Where most places have a
    clear spring/neap cycle with weak modification by the perigee/apogee
    one, within the Bay the two interact with rather complex effects. I
    suspect, but cannot say for sure, that that is why the spring tides at
    Saint John have a variable lag behind the dates of Full and New Moon.
    
    Things get even worse away from the Bay itself, where we seem to get the
    odd effects of Fundy's resonance without them being overwhelmed by the
    strong semi-diurnal rise and fall seen within the Bay itself. Members of
    this list will be familiar with the old rule that High Water Springs at
    a given port always occurs at the about same time of day -- something
    known and tabulated from Medieval times. Yet at Halifax, where HWS
    usually occurs around 0930,  this month the predicted HW (2.1 metres) on
    the 23rd and 24th are to happen at 0725 and 0815 respectively. Previous
    HWS have been equally early in the day since the beginning of October,
    though earlier this year they were all in the range of 0830 to 1000. I
    have never seen it confirmed by a tidal specialist but I think that this
    is a matter of perigee a few days before New/Full Moon interacting with
    the Bay of Fundy amplification system to increase the range of the tides
      to a maximum a few days earlier than the spring/neap cycle would place
    that maximum. Thus the "spring" (really perigean) tide is one that comes
    earlier in the day.
    
    
    I don't have a sufficient collection of old tide tables but I think that
    the extent of this effect is further controlled by the 18-year cycle in
    the nodes of the Moon's orbit, affecting its maximum declination and
    hence the diurnal inequality in the lunar semi-diurnal tide. My memory
    (which I cannot immediately confirm) is that in certain years the time
    of "spring" HW can fall anywhere within the 24 hours.
    
    
    
    Trevor Kenchington
    
    
    --
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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