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    Re: New Moon, Perigee, and Solstice
    From: Trevor Kenchington
    Date: 2003 Dec 22, 21:02 +0000

    You wrote:
    > But suppose the solar component has a very
    > different resonant response. In extreme cases, this can lead to
    > complicated tides that follow no simple rule. In the case of northwest
    > Europe, the result is that the solar tides lag the lunar tides and
    > Spring Tides occur about three days after New Moon and Full Moon.
    Interesting. I had always supposed that the Age of the Tide (to give
    this lag its name) resulted from the resonant systems taking some time
    to respond to the extra energy being poured into them when the Sun and
    Moon are aligned -- a matter of friction bleeding away the energy as
    gravity pours it in, with some inertia in the resonating system (an
    explanation which will not satisfy any physicists!). However, on
    checking the Admiralty Manual, I see that it favours your explanation:
    "The "age" of the tide is due to the fact that on average over the earth
    the solar tide lags behind the solar forces by a greater amount than the
    lag of the lunar tide behind the lunar forces."
    Which I think is the same as your explanation albeit in different words.
    However, the Manual continues:
    "It is possible that this phenomenon [meaning the greater lag of the
    solar tides, not the Age of the Tide directly] is due to dissipation of
    energy in the coastal fringes."
    Which approaches my supposed explanation, without being quite the same.
    However, the Manual is over 60 years old now and there must have been a
    lot of relevant research in that time. Can you suggest somewhere I could
    go for a fuller account of the cause(s) of the Age of the Tide?
    > Speaking of tides, has anyone encountered boaters who use an azimuth
    > rule for tide prediction? There was an article last summer in "Sail"
    > magazine that suggested using an azimuth trick as a rule-of-thumb for
    > predicting the tides. The author claimed that you could check a tide
    > table for the time of low or high tide and then just record the Moon's
    > compass bearing at the time of the given tide phase, and the Moon's
    > altitude would not matter. He claimed that whenever the Moon returned to
    > that compass bearing, the tide phase would be the same (e.g. Moon at
    > azimuth 240... it must be low tide).
    I guess some things take a long time to die. That method of tidal
    prediction was current in Elizabethan times and likely back into the
    Middle Ages if not before. But how inaccurate is it when used in
    temperate latitudes? Or, more usefully, how precise does your estimate
    of the Moon's azimuth need to be before constancy of estimated azimuth
    is an insufficient approximation to constancy of LHA?
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@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

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