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    Re: Captain Schufeldt's report on marine sextants
    From: George Huxtable
    Date: 2002 Dec 27, 16:28 +0000

    Marc Bernstein asked-
    >Do you in fact get more accurate results from computer sight reduction and
    >almanac than from the printed versions? Assuming of course that the accuracy
    >of the sextant sight isn't an order of magnitude worse.
    Yes, that appears to be the case.
    In the Nautical Almanac we are informed that the tabulated quantities are
    generally correct to the nearest 0.1', except for the Sun which has been
    adjusted by up to 0.15' to simplify interpolations. When interpolating to
    an intermediate time between the tabulated values, errors increase, and the
    largest error that can occur in the interpolated GHA or Dec of any body
    except for the Sun or Moon is less than 0.2'; it may reach 0.25 for the GHA
    of the Sun and 0.3' for that of the Moon.
    The most demanding requirement a navigator is likely to make of the
    Nautical Almanac is in calculating the Lunar Distance between two bodies,
    say the Sun and the Moon, in which the errors in both bodies will to some
    extent combine. Because the lunar distance itself does not involve the
    horizon (except for minor correction purposes), then measurements of lunar
    distance can be made to great accuracy, maybe significantly greater
    accuracy than the lunar distance can be calculated using the Nautical
    For all other sextant measurements, in which the horizon plays a part, the
    position of the horizon adds sufficient uncertainty that it almost
    certainly swamps any error in the Almanac predictions.
    On the other hand, computations of the positions of most celestial bodies,
    without such interpolation, can be made to an accuracy suitable for
    astronomers (MUCH more demanding than us navigators) using the methods
    described in Meeus, Astronomical Algorithms. His Moon predictions, however,
    are quoted only as being to "approximastely 10"" or 0.17', which appear to
    be adequate for calculating lunar distances. Maximum errors are not given.
    If higher accuracies are needed, Meeus provides references for obtaining
    the data.
    Her Majesty's Nautical Almanac Office issues a PC program on CD, "AstroNav
    PC and Compact Data 2001 - 2005", also published in the UK by The
    Stationery Office as "NavPac and Compact Data 2001 - 2005".
    As well as the CD data, polynomials are printed so that positions can be
    obtained using a calculator..
    Claimed maximum errors (in GHA) are
    Sun     0.03'
    Planets 0.04'
    Moon    0.21'
    Stars   0.05'
    This accuracy is probably sufficient for calculating lunar distances.
    The conclusion is that the Nautical Almanac provides predictions of
    sufficient accuracy, but only marginally so for calculating lunar
    distances. Computed data can improve significantly on that accuracy.
    There are many astronomical prediction programs now available for PCs, and
    it would be interesting for owners to supply claimed accuracy for the
    predictions of those programs, particularly as it applies to geocentric
    position of the Moon.
    George Huxtable
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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