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    Noon Sight Revisted
    From: Dan Hogan
    Date: 1998 Jul 29, 6:35 AM

    The Noon Sight is the simplest sight in celestial navigation. It can
    be used with pencil and paper and the Almanac data, no table required
    and only simple math. All the corrections made in the Noon Sight
    reduction are to account for the observer being on on the surface of
    the earth, the refraction of light from space, the difference in
    distance from the position of the observer to the geographical
    position of the body on the surface of the earth (ZD), *as if it all
    was observed from the center of the earth*.
    
    OBSERVING THE (all the same thing): Meridian Passage
                                       Local Apparent Noon (LAN)
                                       Noon Sight
    
    Start a series a sights before Noon. Normally use Lower Limb(LL). Each
    sight should be a little higher. Adjust the sextant to keep the Sun in
    contact with the horizon. The adjustments will get smaller and then
    the Sun appears to hang at the same altitude for about a minute before
    dropping a little. When the Sun Hs is lower LAN has passed. Keep the
    sextant set at the highest reading you got and record the sight Hs.
    Only the approximate time is put down. You can't tell LAN (Meridian
    Passage) time exactly.
    
     Time: HH:mm.m
    
      HH= hour
          mm.m= minutes
    
    NOON SIGHT REDUCTION
    
    You convert observed altitude(Ho) into Zenith Distance(ZD). The
    distance of the observer from the geographic position of the Sun (in
    arc measured at the center of the earth) is the same as zenith
    distance. This makes it easier to figure out whether to add or
    subtract the ZD and Dec.
    
    THE FORM
    
         sextant altitude(Hs)
         index error(+/-)
         dip of horizon(-)
         sun Correction(+)
         ------------------
         observed altitude(Ho)
    
    Next subtract from 90d to get ZD
    
         89d 60' (same as 90d)
         Ho(-)
         -------
         zenith distance(ZD in dd mm.m)
    
    Now we know, in degrees, how far we are from the geographical
    position(GP) of the sun.
    
    Name all North declinations plus(+) and all South declinations
    minus(-), this avoids the use of rules, and then algebraically
    combine with the ZD. And you have your latitude.
    
         ZD
         declination(N+/E-)
         ------------------
         LATITUDE
    
    Example Noon Sight
    
         Sextant Altitude    Hs   71d 09.0'
         Index Error         IE      -02.0'
         Dip of horizon      dip     -02.7'
         Sun correction(LL)  sun     +16.0'
         ----------------------------------
         Observed Altitude   Ho   71d 21.3'
    
         (Same as 90d)            89d 60.0'
         Observed Altitude   Ho  -71d 21.3'
         ----------------------------------
         Zenith distance     ZD   18d 38.7'
         declination         dec +21d 21.0'
         ----------------------------------
         Noon Latitude       lat +39d 59.7'
    
    
    PREDICTING THE NOON SIGHT
    
    Use your approximate longitude. divide it by 15(one hour for each 15
    degrees). At the lower RH daily page of the Nautical Almanac get the
    GMT time of meridian passage of the sun for your date. It varies from
    1144 to 1206 because of the earths eccentric orbit and speed. Take
    your longitude(label West longitudes plus(+) and East longitudes
    minus(-)) and algebraically combine with the daily page meridian
    passage time. The answer is your approximate time of meridian passage.
    Depending on the uncertainty of your longitude you can start observing
    a few minutes early.
    
         Your LAN = (Lon / 15) +/- GMT Mer Pass
    
    Example prediction LAN, July 30, 1998
    
         Lon  DR        +04d.20'
                         00:16.5' = +04d.20' / 15
         GMT Mer Pass.  +12:06
         ------------------------
         Predicted LAN   12:22.5'
    
    Dan Hogan
    dhhogan{at}concentric.net
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