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 26, 10:48 -0300

    Geoffrey Butt wrote:
    >  " < * .. so there are two raised ocean levels opposite one another and as the
    >  "    Earth rotates beneath one experiences locally two tides per day>
    >  "
    >  " All well and good, but how do you factor in those places where there is
    >  " only 1 tide per day? Or at least, where the locals think there is only one
    >  " tide per day? "
    >  The above referred to the idealised 'ocean only' global model which
    >  approximates to the deep ocean 'driver' for local shallow water tides.
    >  As George Huxtable wrote recently the actual tide experienced locally
    >  depends on the configuration of resonant basins around coasts - some of
    >  which will resonate with apparantly one tide per day.
    Sorry Geoff but the Newtonian model _doesn't_ approximate to deep-ocean
    tides either. As I have already noted in this thread, the ocean isn't
    deep enough for a wave to run around the planet in 24 hours and wouldn't
    be even in the absence of any land masses (though I should have noted
    that that is true at low latitudes -- clearly, it is not near the
    poles). Even if Earth were a sphere of water, with no seabed at all, the
    tides still wouldn't be able to follow the Newtonian model.
    The deep oceans tides show a series of inter-linked resonant amphidromic
    systems, no different in their basic nature from the ones long known in
    the North Sea and similar basins.
    You continued:
    >  Trevor J. Kenchington wrote:
    >  " >  * .. so there are two raised ocean levels opposite one another and as the
    >  " >    Earth rotates beneath one experiences locally two tides per day "
    >  "
    >  " All of which is correct, as far as it goes into detail, save for the
    >  " very last part: As I wrote earlier today, these two "raised" areas do
    >  " not really exist.
    >  I thought early observations from satellites had demonstrated that they do
    >  exist.
    I don't know what early observations you are thinking of but it is
    certain that no such "raised" areas exist. Satellite altimetry data has
    been crucial to modern understanding of ocean tides but it has allowed
    modelling of the amphidromic systems, has not confirmed something which
    is been known not to exist. [If anyone wants more details, try a Web
    search for "TOPEX/POSEIDON". The models and resulting charts appeared in
    the mid-1990s.]
    >  " ... but even Medieval knowledge of the tides of the English Channel
    >  " was sufficient to prove to any thinking person that there is not some
    >  " bulge moving westwards down channel at 15 degrees of longitude per hour.
    >  No, as above, the nature of flow of tides in the English Channel is a
    >  function primarily of the channel dimensions and configuration of the sea
    >  bed and not of the mechanism which instigates the driving impulses.
    Again, there is no fundamental difference between the mechanisms in
    narrow waters and those in the open ocean, though I will grant you that
    that was not established until comparatively recently. It was late in
    the 19th century that the notion of progressive tidal waves moving
    around the Southern Ocean and propagating northwards up the Atlantic was
    laid to rest. It then took another hundred years (plus the advent of
    massive computing power and a whole lot of satellite data) for the
    details of the standing-wave patterns in open ocean to be worked out.
    Still, the idea of Newtonian "bulges" moving around the world has
    _never_ been taken seriously by anyone but those who have trusted
    over-simplified written accounts over common sense and personal
    experience. Sir Isaac himself didn't think that such bulges existed. Nor
    did any of his contemporaries. They just were not able to link Newton's
    understanding of tide generating forces (which was and is correct) to
    the observed tides. Fortunately, it later proved possible to combine
    theory and observation into precise tidal predictions long before the
    linking mechanism was worked out.
    My stress on Medieval knowledge was a reaction (on over-reaction,
    perhaps) to the renowned textbooks used by North American yachtsmen
    which assure them that high-water springs always occurs when the Moon
    crosses your meridian (or 180 degrees away). Chaucer's shipman knew that
    that was not true and had assorted means of remembering the vulgar
    establishments of all the ports he frequented. It is tragic that, so
    many centuries later, common knowledge has actually regressed while
    specialists have moved so far forward.
    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