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    Re: Lunars: Jupiter's BIG.
    From: Jan Kalivoda
    Date: 2003 Dec 24, 13:57 +0100

    Maybe you are right.
    I have the copy of a German paper from 1889 that deals with errors of common 
    astronavigational methods. For lunar distances it gives following average 
    errors, gained from observations at sea. Sets of observations consisted from 
    five shots in every case, the error was obtained by comparing the results 
    with chronometer times interpolated for every day at the end of the voyage. 
    (Which is much more accurate than working with the assumed rate during the 
    voyage, of course.) In these days, vernier sextants were used.
    The average errors were:
    31" for one distance of the Moon and a star or planet
    22" for one distance of the Moon and the Sun
    9" for the average result from two distances measured eastward and westward from the Moon
    You should multiplicate these numbers by (approximately) 2.5 to obtain the 
    error of 95% of observations and by 3.5 for 99% of observations, isn' it?
    It should be noted that after cca 1880, the errors of lunar tables were 
    negligible compared with the errors of measurements, which wasn't the case 
    before. In cca 1830, the errors would have been two times greater, owing to 
    the unreliability of the contemporary theory of the lunar motion.
    Jan Kalivoda
    ----- Original Message -----
    From: "Fred Hebard" 
    Sent: Wednesday, December 24, 2003 2:42 AM
    Subject: Re: Lunars: Jupiter's BIG.
    Good point.
    In answer to various claims about the accuracy of lunars, it would seem
    to me that the error in lunars should approach the precision of the
    sextant, given enough measurements of decent quality and decent
    reduction procedures.  That would be 0.1 to 0.2' of arc, or 12-24
    On Dec 23, 2003, at 5:40 PM, Frank Reed wrote:
    > Fred you wrote:
    > "I believe that to _rate_ a chronometer one needs at least three
    > lunars spread over at least three days. "
    > Just one lunar will do. When you leave port, you know your
    > chronometer's error (assuming it's a port with a well-established
    > longitude). Let's suppose it's sixty seconds slow as you depart. After
    > five months at sea, you get some measure of your longitude. This could
    > be from speaking another ship, from visiting a port, OR from shooting
    > a lunar. Suppose your chronometer now appears to be 4 minutes fast.
    > That means it's gaining 1 minute per month. That's the rate.
    > On the other hand, there is some benefit in doing lunars for a few
    > days in a row. Primarily, this would assure you that you didn't get an
    > accidental close match from your first trial. You could also get the
    > same result by having several navigators shoot lunars each working up
    > an independent longitude. This was certainly the case on at least some
    > American ships at mid-century.
    > Frank E. Reed
    > [X] Mystic, Connecticut
    > [ ] Chicago, Illinois
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