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    Re: Datum for Nautical Almanac
    From: Michael Dorl
    Date: 2004 Oct 30, 12:48 -0500

    At 11:46 AM 10/30/04 -0400, Charles Seitz   wrote:
    >This, I assume means with respect to observations from a spherical earth.
    >But, we all know the real earth is better  modeled as a geoid.  That means
    >observations made with respect to the local horizon on the real earth will
    >be slightly
    >different than those made from a spherical earth.  It follows that circles
    >of equal
    >altitude, the basis of CN,  are not perfectly circular.
    >(I suspect observations of altitude would be nearly identical for both
    >reference systems at the poles, equator and near 45 deg lat.  At these
    >locations, the center of the earth will be directly under you)
    The vertical passes through the earths center only at the pole and equator,
    not at the 45 degree mark.
    >Since there are numerous geodetic datams, your geographical position does
    >indeed depend on your assumed datum.  So, why wouldn't there also be
    >some inherent positional discrepancy between a geocentic and geoidal earth
    >I suspect that sight reduction tables would be nearly impossible to prepare
    >if all of the possible complications were considered.  We choose an earth
    >model that works best for a particular purpose.
    Mr Seitz is getting pretty close to what got me started on this thread.  I
    was wondering how the NA could show decl and sha for a object without
    assuming some model for the earth.  If you draw a few pictures and work it
    out, the shape isn't important at all provided the spin axes coincide.  It
    doesn't matter if you assume a spherical earth, an ellipsoidal earth, or
    something altogether different.  You will get the same value for
    declination.  I suspect you could even have some weird figure
    wherein the same latitude appeared at different points on a meridian (small
    black hole in lab at local university).
    The earth model does play a part however in determining the difference
    between apparent place (geocentric) and topographic place in two regards.
    These are diurnal parallax and diurnal aberration.  To calculate these you
    need the distance and direction from the earths center to the topographic
    point in question and that depends on what model you adopt.  As Mr Huxtable
    points out diurnal aberration is too small to worry about for navigational
    purposes and the NA finesses the parallax issue by adjusting the altitude
    of the object which I believe means assuming a spherical earth.
    I've been looking into datum stuff a bit and am getting the idea that one
    way of defining a datum is by specifying rectangular offsets from the WGS
    84 datum and a reference spheroid.  There are numerous tables on the web
    showing the offsets but I haven't found definitions for the references
    spheroids except in the Astronomical Almanac.  At this point, I do not know
    if most datums have spin axes parallel to the WGS datum spin axis or
    not.  If anyone can point me at a reference showing
    rotational offsets for various datums I'd appreciate it.  Also, if anyone
    know of a web site that translates one datum to another, let me know.
    So as far as my original questions...
    1) I probably should have asked what the NA reference model was
    2) As far as I know with contribution from Mr Huxtable and Mr. Herbert Prinz
    Equator and equinox of date (spin axis) - probably doesn't matter much
    which equator you pick because polar motion is so small.
    Spherical earth - radius unknown - NA provides diurnal parallax correction
    through altitude adjustment implying spherical earth model
    Diurnal aberration ignored, irrelevant for Nav purposes.
    3) Short answer
    NA provides Geocentric coordinates relative to equator and equinox of date

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