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    Re: Noon Sight Revisted
    From: Russell Sher
    Date: 1998 Jul 30, 04:03 EDT

    Dan - thanks for your LAN sight revision. I find it easier to remember
    whether to add/subtract the TZD by the following, which is similar to your
    method - but involves signs 'N' or 'S' rather than + or - .... see what you
    think:
    method:
    1) Note whether you are looking North or South in order to observe the body
    on your meridian  - name this 'N' or 'S' accordingly - i.e. sun north of you
    = 'N'.
    2) Correct for dip & refraction to obtain Ho and subtract Ho from 90 deg. to
    obtain TZD
    3) Now... name this  TZD OPPOSITE to the sign in step 1.; i.e. If you were
    looking N then you spontaneously name TZD 'S' and vice versa.
    4) Extract declination for you time of observation - Note whether it is 'N'
    or 'S' - (from almanac)
    5) Simply add (4) and (3) if they are the SAME sign (i.e. both 'N' or both
    'S') OTHERWISE (if opposite signs), subtract the smaller from the larger.
    6) The result is your latitude - named by the larger quantity (i.e. the
    differance when subtracted)  if they were subtracted in step (5) Otherwise
    if you added in step 5., the sign is then obviously the sign common to both
    when they were added in step 5.
    Russell Sher
    PLESSEY(S.A.)
    Telecomm. R+D
    (021) 710 2321
    > -----Original Message-----
    > From:	Dan Hogan [SMTP:dhhogan{at}XXX.XXX]
    > Sent:	Wednesday, July 29, 1998 2:35 PM
    > To:	navigation{at}XXX.XXX
    > Subject:	[Nml] Noon Sight Revisted
    >
    > 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}XXX.XXX
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