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    Re: Plumb-line horizon vs. geocentric horizon
    From: Trevor Kenchington
    Date: 2005 Feb 9, 23:49 -0400

    You wrote:
    > I have looked at Jim Thompson's new diagrams and the
    > 2002 thread posts by Paul Hirose and others.
    I hope that you have also looked at the more recent thread of a couple
    of weeks ago.
    > Below I
    > have summarized what I think I'm reading.  Can you let
    > me know if I have indeed understood?
    > Due to the oblateness of Earth, the plumb line differs
    > from the actual center point of Earth by a
    > navigationally significant amount, up to 12 minutes of
    > arc.
    Fair enough.
    > But this fact does not effect the practice of
    > celestial navigation because the lat-lon grid itself
    > and the Almanac's GP data are based on the plumb line.
    Not quite.
    Longitude doesn't come into it. The axis of any ellipsoid used to
    describe the oblate Earth is the same as the axis that a theoretical
    spherical Earth would have. Thus longitude is not affected by the
    "oblateness". (Is that a word?)
    The parallels of latitude are not based on the orientation of a plumb
    line (which responds to local gravity) but on perpendiculars to the
    ellipsoid chosen as a representation of the geoid. Those two directions
    differ (as you note below) by the deflection. However, you are right
    that the practice of CelNav is not affected, because deflection is
    (usually) far too small to matter.
    CelNav observations are related to the direction of local gravity, and
    hence to the direction indicated by a plumb line.
    However, I think you are wrong to say that the Almanac's data are
    related to a plumb line. I think that all the tabulated data would be
    correct for a spherical Earth. The major error in assuming sphericity is
    the problem with latitude and that is taken care of by using charts
    based on geoidal latitude. The remaining errors in the assumption of
    sphericity (e.g. horizontal parallax of the Moon must differ depending
    on whether you view it eastward from a point on the Equator or southward
    from a point in high northern latitudes) seem all to be too small
    to matter for navigational purposes. [I am a little unsure of the points
    in this paragraph but I'm sure someone will correct my errors.]
    > The local plumb line may differ by additional amounts
    > as well, called deflections, due to surface anomalies,
    > but not by a navigationally significant amount.
    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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