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    Re: Lunar-distance almanac errors [was; The Online Nautical Almanac- beware!]
    From: George Huxtable
    Date: 2004 Jul 13, 10:38 +0100

    This is a reply to Frank Reed's posting last week, in which he said-
    >George H wrote:
    >"Did Greenwich wait until 1848, I wonder, before instituting a comparison
    >between their own observed Moon positions and their own predictions?"
    >From what I've seen, the Nautical Almanac was significantly revised in 1834.
    >The planets were added to the lunars tables, and the Moon's position was
    >improved. Before that date, it is not uncommon to find lunar distance
    >errors up to
    >1.0 minutes. After that date, they are occasionally as large as 0.5 minutes.
    >After 1883 (?) the Moon's position and the lunar distances derived from it are
    >accurate to less than 0.1 minutes of arc in almost every case.
    >"It's easy enough, now,  to calculate the precise positions, then, using
    >modern data and technology; but a comparison with the published data of
    >that era
    >would involve a lot of dreary transcribing from old almanacs, so would have to
    >be something of a labour-of-love."
    >It IS interesting to do occasional spot checks, and you can do that very
    >easily using my online lunars predictions, but why bother with an extensive
    >analysis of the old almanac data at all?? If you're studying a set of lunar
    >observations from 200 years ago, what good does it do to compound the
    >errors of
    >observation with errors in almanac data?
    My response-
    There's a very simple reason. Often, indeed I would say usually, those
    navigators have not left us their raw data, or anyway, not in a form that's
    usable to us nowadays. What they have left us is more often just a deduced
    longitude. That deduced longitude is affected by
    1. any errors in their measurements.
    2. any errors in their data reduction process.
    3. any errors in the nautical almanac tables.
    We can not do much about the first two, but if we know those errors in the
    tables, it would allow us to adjust out their effect on the deduced
    positions by a corresponding amount, without knowing any details of how the
    longitude was arrived at.
    That's the point.
    Frank added-
    >Just curious: anyone know what old Nautical Almanacs (c.1800? c.1850?) are
    >worth on the open market?
    A few years ago I bought an 1864 almanac for about ?15 ($25, say). It's
    been a really useful reference, but needed a rebind within its old boards
    which cost about the same again.
    contact George Huxtable by email at george@huxtable.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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