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    Re: Datum for Nautical Almanac
    From: George Huxtable
    Date: 2004 Oct 30, 13:45 +0100

    Michael Dorl made this further enquiry below, about the basis of the
    Nautical Almanac's predictions-
    I will answer his further questionas as well as I can, but am not an
    astronomer or any sort of pundit on such matters.
    >I understand that declination is independent of the reference spheroid
    >provided the spin axes agree.
    >are the following statements concerning the NA true?
    >   Decl and GHA/SHA are with respect to earth's mean equator and equinox.
    Yes, that's my understanding.
    >   Diurnal; aberrations effects are not included.
    Correct. An observer riding round with the Earth in equatorial regions is
    travelling at something like twice the speed of a commercial jet-plane, say
    800 miles in an hour. This is a tiny fraction of the speed of light, which
    is 186,000 miles in a SECOND. The tan of the angle of DAILY aberration is
    given by the ratio between the two speeds: therefore the diurnal aberration
    is no greater than 0.3 arc-seconds, so is rightly neglected in the almanac.
    Even if it happened to be much greater, the effect would still have been
    excluded, because it depends on the observer's position: there's no such
    effect at the poles, for example. So the almanac would have left it to the
    observer to correct for himself. Being completely negligibe, the question
    doesn't arise.
    >   Diurnal parallax effects are included based a spherical earth of (I
    >don't know radius).
    No, they are excluded, because they depend on the position of the observer.
    They are left for the observer to allow for, depending on the altitude, but
    information is provided to assist. The horizontal parallax is given for
    Moon and Sun, and it's included in the altitude corrections for some
    The predictions are based on a spherical Earth of equatorial radius 6378
    kilometres. For the Moon only, this may not be sufficiently accurate. It
    may need correcting for the shape of the Earth, so an extra small
    correction, "Augmentation of the Moon's parallax" can be made, which
    depends on the observer's latitude.
    >The suppositions concerning diurnal effects are made since the NA contains
    >no corrections based on position;  the NA corrections depend
    >only on observed altitude.
    Ah. But the observed altitude depends greatly on the observer's position,
    doesn't it?
    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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