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    Re: The Online Nautical Almanac- beware!
    From: George Huxtable
    Date: 2004 Jul 7, 17:45 +0100

    Frank Reed wrote, regarding Omar Reis' Online Nautical Almanac-
    >I've puzzled out the source of the larger portion of the errors in his star
    >positions. He has apparently not included the "proper motions" for the stars.
    >Many stars, especially distant stars, have proper motions that are so small
    >that they are of on consequence even after a couple of hundred years. But for
    >those stars which have large proper motions, this is a big issue even for
    >Reis's recommended period of time: 1950 to 2050. For example, the position for
    >Arcturus in 1950 given by Reis's almanac site is off by 0.8 minutes in SHA
    >and 1.4
    >minutes in Declination, and his 1950 position for Rigel Kentaurus (alpha
    >Centauri) is off by 5.4 minutes in SHA and 0.5 minutes in Declination. If
    >you take
    >it back to 1804, his Rigil Kent position is off by nearly half a degree, and
    >the Arcturus position is out by more than 7 minutes of arc.
    >In addition, there is still a precession problem (that's a best guess on my
    >part) in Reis's star positions. This is not a serious problem in the
    >recommended period, but it leads to additional errors of up to 1.5 minutes
    >in most cases
    >and errors over 5 minutes in exceptional cases when you go back 200 years.
    >And George, if you used any data from his site to analyze those Lewis & Clark
    >lunars back in April, as I did, you can see that you may want to revisit
    >those calculations.
    >None of these errors are present in my online almanac data:
    No, I've no need to revisit the Lewis & Clark calculations on that account,
    as my own pocket-calculator predictor program allows for stars' proper
    motion, nutation, and precession.
    But I think Omar himself, at "Omar Reis" , would find
    Frank's comments useful, and if Frank has no objection I propose to pass
    them on to him together with Frank's email address, to encourage direct
    dialogue between them. On the other hand, if Frank were to pass that
    information on to Omar directly, perhaps that would be better still.
    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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