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    Re: Use of Sun Sights for Local time, and Lunars for Longitude
    From: Arthur Pearson
    Date: 2002 Oct 23, 22:37 -0400

    Thanks to Bruce, Herbert, Bill and George for this dialogue. This use of LAT from sun sites and the elegant process of stepping to acceptably accurate calculated altitudes opens the door for the navigator to use the night time moon and stars for lunars, even though no horizon is available for altitudes.  Given the relatively few days when the moon simultaneously has both a horizon and a comparing body, I would think this was a very important advantage for traditional navigators without accurate chronometers or GPS units. Is there documentation anywhere that confirms this was common practice during the heyday of lunars?

     

    Again, thanks to all who contributed to this thread. I need to sit quietly with some diagrams and an outline of steps to be sure I have absorbed the techniques.

     

    Arthur

     

    -----Original Message-----
    From: Navigation Mailing List [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM] On Behalf Of Bruce Stark
    Sent: Wednesday, October 23, 2002 6:37 PM
    To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    Subject: Re: Use of Sun Sights for Local time, and Lunars for Longitude

     

    It seems to me we're all pretty much in agreement that calculated altitudes

    are OK for clearing a distance. Now, perhaps, more lunars will be taken. On

    land it's so much more convenient to used calculated altitudes, and it only

    takes five or ten minutes to get a set of distances.

     

    I appreciate William Noyce and George Huxtable taking time to explain where

    the thirty-to-one reduction in error comes from. I also appreciate them

    pointing out that in most cases there's no need to repeat the calculations.

     

    Actually, there may be a better way of getting that thirty-to-one reduction

    in the error of the moon's hour angle than the one we've been discussing.

    Here's an excerpt from a posting William Noyce made last April:

     

    >I don't think you need to make any special "local apparent time"

    observations or calculations. Assuming the navigator >has been using

    celestial observations all along, but has an incorrect clock, he will have

    determined a celestial "fix" >whose longitude is off by almost exactly 15'

    for every minute of time error. These two errors will cancel out to >reduce

    errors in computed altitudes, the same way as Bruce Stark's procedure using

    local time. The remaining errors >come from the change in declination (pretty

    fast for the moon), and the difference in rate of change of GHA between >the

    sun, planets, and stars.

     

    Maybe some list members will check this out. Working from local time and

    shifting back and forth between arc and time isn't everyone's idea of fun.

     

    Bruce

       
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