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: New Moon, Perigee, and Solstice
    From: Trevor Kenchington
    Date: 2003 Dec 24, 20:58 +0000

    I'm afraid you were badly off-base with this latest message.
    You wrote:
    > It's a very simple issue. The ocean basins are a complex set of
    > interacting oscillators. There are MANY resonant frequencies. A small
    > difference in the frequency of the driving force *can* lead to a large
    > difference in the amplitude of the response AND the phase lag of the
    > response. Northwest Europe has a SPECIFIC phase lag that Bowditch copied
    > from European sources (or maybe simply "knew" from the common culture of
    > European sea lore). The specific phase lag for Europe leads to a
    > specific lag of the Spring Tides there. That phase difference applies to
    > much of northwest Europe, but it does not apply to the Americas generally.
    Northwest Europe most certainly doesn't have a "specific phase lag": the
    times of HW literally span right round the clock just within the
    Southern Bight of the North Sea, for example). Besides, if there was one
    single phase lag for the entire region, it would not explain a
    roughly-constant "age" of the tides throughout. That would need a
    roughly-constant difference in the phase lags of the lunar and solar
    semi-diurnal tides. Maybe that difference is roughly constant from
    Ushant to the Shetlands but, if so, that seems a bit of a surprising
    outcome from the complex of resonant systems over there.
    You continued:
    > Another error in early accounts of the tides: In western Europe, the
    > tide "wave" progresses up the coast starting in Spain and working its
    > way north towards Ireland. Early observers "guessed" that this was a
    > general property of the tides worldwide, but it's not. For example, on
    > the East Coast of the US, the tide cycle is almost simultaneous all up
    > and down the coast --if it's high tide in New Jersey, it's high tide in
    > South Carolina.
    Any notion that the tides of northwest European seas are dominated by
    progressive waves (whether passing Spain or otherwise) should have been
    laid to rest by 1900. On both sides of the Atlantic, the tides are
    mostly dominated by amphidromic systems. It just so happens that the
    system centred somewhere off Newfoundland produces a co-tidal line
    roughly parallel to the coast from Cape Cod to Florida. Certain parts of
    the Irish coast experience much the same thing, though on a far more
    localized scale.
    And finally:
    > No. Laplace's theories were only barely beginning to address the real
    > ocean tides and would not have provided the kind of practical knowledge
    > that Bowditch required for his navigation manual. The early hints from
    > Laplace could explain differences along an idealized rectangular
    > east-west channel (hence the reference to the English Channel you
    > quoted), but they could not yet address the overall phase lag.
    You are right that LaPlace's tidal theories have little relevance to the
    real tides in the oceans. However, you are way off base in supposing
    that they have anything to do with the tides in the English Channel.
    Besides much else, LaPlace modelled the tidal waves as progressing
    westerly, following the apparent motion of Moon and Sun. The times of HW
    in the England Channel progress eastwards from Scilly to Dover, Ushant
    to Calais.
    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

    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