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    Re: Datum for Nautical Almanac
    From: George Huxtable
    Date: 2004 Oct 28, 20:12 +0100

    Michael Dorl asked-
    >I've never seen it discussed here but it seems to me that sights reduced
    >using Nautical Almanac data must be referenced to some datum. I checked my
    >old NA (2000) and couldn't find any mention of what datum is used. Does the
    >current NA have any statement about the reference datum?
    >I would assume its WGS 84 (equivalent to NAD 83).
    >Or maybe this is an unnecessary refinement for celestial navigation purposes?
    When they compile the Nautical Amanac, they have no idea where in the World
    their users will be. Therefore the data has to be applicable to everyone,
    wherever they may be..
    Positions in the sky are given as though the observer is at the centre of a
    transparent Earth, without an atmosphere.
    Declinations are related to the direction of the Earth's axis in space, at
    right angles to the plane of the equator, though short-term wobbles in that
    axis (nutation) may be averaged out first.
    Hour angles (or longitudes, which are EXACTLY the same thing) are measured
    from an arbitrary meridian on the Earth's surface (Greenwich). For stars,
    they are measured as a nearly-constant offset, from "Aries", a nearly-fixed
    point in the stars, where the equator meets the Earth's orbital plane, and
    where the Sun can be found at the Spring equinox.
    In this way, the numbers in the Almanac are unrelated to the surface and
    shape of the Earth.
    To determine where they are in our sky, we have to do a bit of calculation,
    depending on our own latitude and longitude, from which we deduce the plane
    of our local horizontal.
    The ellipsoidal shape of the Earth is allowed for already in our charts.
    Instead of taking the Earth as a sphere, in which case all the lines of
    latitude would be equally spaced, they are redefined as lines where the
    direction of gravity makes an equal angle with the Earth's axis. This
    "fiddle" allows to forget about the Earth's ellipsoidal shape, though it
    complicates any exact calculation of the distance from A to B on the
    Earth's surface.
    On top of that, the Earth's gravity has lots of local bumps and knobbles,
    which shift the apparent positions of objects in the sky with respect to
    the local vertical that a plumb-bob, or our horizon, provides. All these
    are neglected as being so small as to be insignificant to navigation,
    which, fortunately, they are.
    That leaves the navigator to make corrections for refraction in the air
    above him, and for parallax because he is not at the Earth's centre.
    WGS84 gives a picture of the Earth's water-surface (or its on-land
    equivalent) which is necessary for satellite ranging methods, or
    determining heights above sea-level. For astro-navigation purposes,
    sighting distant bodies, and not attempting to measure heights, such a
    model is simply irrelevant.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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