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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@concentric.net
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