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: Dip uncertainty
    From: Trevor Kenchington
    Date: 2004 Dec 7, 20:27 -0400

    The problem with an experimental approach would be that we are dealing
    with a stochastic system. No two places or days will have quite the same
    anomalies. Indeed, different regions will likely have different patterns
    of anomaly, while the height affected will change through the day at one
    place. (Some may see an initial low anomaly grow as the effects of,
    e.g., a cold surface extend up through the air mass. Others may see the
    reverse for a time, as solar heating intensifies through the day.) It
    would need a huge amount of data to produce global statistics on the
    Then there is the problem of how to measure the amount of anomalous
    refraction (with a dipmeter?). Or would you rather measure the
    temperature gradient and calculate the refractive index?
    Finally: Maybe somebody has studied this but who would have sponsored
    research relating to the heights of eye typical on small boats? I would
    suspect that any research would have focused on whether to take
    observations from the main deck, the bridge or the masthead of a
    sizeable ship. But if the issue really is the first few feet of the
    atmosphere versus the next layer above, there may not be much of a
    gradient in frequency and severity of anomalies between deck and
    masthead and hence nothing to study.
    Scholar.google shows only 158 journal citations for "anomalous dip" and
    none of the ones on the first page have anything to do with "dip" as we
    understand it here nor with navigation of any kind. (Combining
    "anomalous dip" with "horizon" produced only two hits, neither dealing
    with our problem.) But perhaps that search engine doesn't recognize
    journals of navigation as sufficiently academic for its purposes.
    Trevor Kenchington
    You wrote:
    > Trevor:
    >>The position that Bruce and I have been
    >>is that anomalous dip is usually less pronounced for an observer on the
    >>bridge of a modern ship or in the top of a sailing ship than for one
    >>located near the waterline or aboard a small yacht.
    > The opinions seem to be split on this subject,
    > and I don't see how to settle it without experimental
    > data on the anomalous refraction.
    > If the whole "anomalous part" of the anomalous refraction happens
    > within few meters from the sea surface, then you and Bruce are
    > right. Surely someone studied this question, but I don't know
    > how to find these results.
    > Alex.
    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