Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Global oceanic tides,
    From: Trevor Kenchington
    Date: 2003 Aug 27, 09:39 -0300

    Keith Williams wrote:
    > Trevor has emphasised inter-linked resonant amphidromic oscillations and
    > de-bunked the Newtonian model of two bulges. So it seems to me one
    > problem is why do the periods of these sloshing bowls all average out to
    > produce two high tides a day virtually all over the world?
    That's an easy one.
    We are not looking at the true resonant periods of the various basins,
    except in rare cases. (The Gulf of Maine - Bay of Fundy system has
    natural resonance at almost exactly the period of the lunar semi-diurnal
    tide, which explains the great amplification of that tide in the Bay.)
    These are forced oscillations and the water in every basin oscillates
    with every tidal constituent to some degree. The semi-diurnal tide
    generating forces are the largest, so semi-diurnal tides predominate
    unless a particular basin is especially prone to oscillating at the
    frequency of some other tidal constituent or, more likely, the basin's
    semi-diurnal tides produce a node. I have already mentioned the Magdalen
    Islands, which lie at an amphidromic point for the semi-diurnal tide and
    so have no rise or fall of tide on that frequency, which means that all
    that is left is a very weak diurnal tide (and a bunch of
    negligible tides on other frequencies).
    As to the strengths of the tide generating forces: Relative to the
    semi-diurnal lunar tide (call it 100%), the semi-diurnal solar tide is
    47%. The diurnal tides (lunar, solar and soli-lunar -- don't ask me to
    explain the third of those!) rate at 42%, 19% and 58%. So the ratio of
    semi-diurnal to diurnal is 147:119, while the ratio at single
    frequencies is 100:58. Those aren't huge differences but they are enough
    to make semi-diurnal tides dominant in most places.
    > AND I feel I am being led into confusing the measurement of the
    > phenomenon (ie the identification of the various sub-oscillations) with
    > their cause - an oscillation needs some force to create it in the first
    > place.
    There shouldn't be any doubt about the force: It is the cyclical variation in 
    gravitational attraction that has been explained previously.
    The thing to remember about tides is that there are tide generating forces, 
    which can be exactly calculated by astronomers (and represented for 
    simplicity by the Newtonian model of "bulges"). Then there are the astronomic 
    tides at any one port, which can be accurately predicted on the basis of past 
    observations of how sea level at that port varies in response to the tide 
    generating forces. The only doubts are over how the known forces result in 
    the particular tides at some port. That doubt can now be resolved by a lot of 
    computer modelling but, up until the 1980s, we had the theory and the 
    observation, we could combine those to make accurate predictions, but the 
    physical links between forces and water levels were only sketchily known.
    And, for further clarity, on top of tide generating forces and astronomic 
    tides, we also have real water levels, which differ from the astronomic tides 
    as a result of a bunch of awkward effects, mostly meteorological in origin.
    > I also would assume that, as these oscillations appear not to be
    > decaying (any historical records to prove this?), then the creating
    > force still exists.
    Of course it still exists. The Earth/Moon gravitation system is steadily 
    bleeding energy into the oceans, which are losing that energy as heat 
    generated by the friction of tidal streams running over the seabed.
    > There appears to be no doubt that we are looking at
    > a netting of the influences of sun, moon, gravity, centripetal and
    > centrifugal forces (is there a centrifugal force? I was taught that it
    > was a confusion for the previous-named force)
    My understanding is that centrifugal force is the equal and opposite reaction 
    created when you impose a centripetal force on a moving body. The body 
    attempts to maintain its momentum in a straight line, as any moving body 
    must, and that attempt is perceived as a centrifugal force.
    > but so far I at least have
    > not been attending closely enough to the various themes to identify a
    > grand unification of the forum's consensus as to why and how....
    It is not for me to express the group's consensus but if there is still
    doubt as to what makes the tides do what they do, we can keep on
    plugging away at the topic. There are lots of interesting tidal
    phenomena, all with rather simple physical causes (though the details
    are never simple, of course).
    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

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site