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    Re: Grenadine Lunar Distances
    From: Herbert Prinz
    Date: 2003 Feb 1, 21:01 +0000

    Arthur,
    
    I hope to be able to comment on your first set of observations shortly in a
    other post. In the mean time I had a quick look at your second series of
    observations. You attribute the rather large observation error of ca. 2' to the
    difficult conditions under which the observation was made. I could accept this,
    if there were a higher variance in the observations and if they would fall on
    both sides of the theoretical value. However, as you noted yourself, your
    series is fairly consistent. Deviations of the observed from the computed
    theoretical distances are as follows:
    
     0�02'37"
     0�02'09"
     0�02'44"
     0�02'08"
     0�03'11"
     0�01'58"
    
    average:  0�02'28"
    
    This is assuming that the distances that you are listing are already corrected
    for the specified index. If not, these values have to be reduced by 1.1' each
    and are on average actually only 1.4' off, not 2.3', as you say.
    
    At any rate, there is a systematic error somewhere. It could, of course, be due
    to a bias in your judgement of the contacts. But have you considered instrument
    error? If you had previously used your sextant mainly for altitude
    observations, you probably had never before dealt with angles near 90�. You
    might want to check the instrument in that range by means of the distance of a
    pair of stars observed from land
    
    Further, I noticed a discrepancy between your description and my computation
    that I can't explain. It does not seem to affect the result. You say that the
    moon was as low as 17�, but I think it was above 35� and rising. You compute
    the apparent motion as 21' per hour while I get 24'.
    
    Best regards
    
    Herbert Prinz
    
    
    Arthur Pearson wrote:
    
    >
    > SECOND SET
    > The second set of observations was a series of lunar distances alone. I
    > cleared and reduced to GMT based on calculated altitudes that were in
    > turn based on known GMT and position.  There is no point to this process
    > other than improving one's lunar technique.  This set was taken hard on the
    > wind in 25 knots and 6 foot seas just south of Bequia. My data and
    > results are as follows:
    >
    > Date: Jan. 10, 2003
    > Index correction: -1.1'
    >
    > GMT        Ds
    > 18:29:25   92� 28.4'
    > 18:32:31   92� 29.0'
    > 18:36:06   92� 30.8'
    > 18:38:47   92� 31.1' (after graphing, I used this one)
    > 18:43:44   92� 33.8'
    > 18:46:13   92� 33.4'
    >
    > Position per GPS at time of selected distance:
    > 12� 56.9' N
    > 61� 16.7' W
    >
    > GMT per Lunar: 18:42:59
    > Time error of lunar: 5m 12sec fast
    > Distance error of lunar: 2.3' too long
    >
    > COMMENTS
    > With a much lower moon (Hs~17�) and thus a much faster rate of change in
    > apparent distance (21' per hour), ...
    
    
    

       
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