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    Re: Latitude + Longitude @ Noon
    From: Henry Halboth
    Date: 2005 Jun 5, 15:11 -0400

    Forwarded for your further comment is the following posting of 31 Jul
    2004, on the subject. I must say that your failure to respond, and for
    that matter the failure of this list to do so in general, was most
    dissappointing. I try only to post on practical navigation matters that I
    have tried or experimented with at sea and can only say that this list
    appears disinterested in such matters.
    On Sat, 31 Jul 2004 22:56:10 -0400 "Henry C. Halboth" 
    > I have recently returned from a sojourn at the North Carolina
    > Beaches,
    > and there had the good fortune of staying literally on the beach,
    > with an
    > unobstructed view of the sea horizon from almost east to west
    > thought
    > south. This stay afforded the opportunity for a real "navigation
    > holiday"
    > - unfortunately, I was plagued with an almost constant "Gulf Stream
    > horizon", i.e., hazy to an extent that impacted on the accuracy of
    > my
    > observed sextant altitudes. Regardless, an effort was made to "try
    > out" a
    > few of the old favorites sometimes here spoken about. First, let's
    > take a
    > look at Latitude + Longitude determination at noon by equal
    > altitudes -
    > actually determination of Longitude by equal altitude + Latitude by
    > reduction to the meridian. In this example. a Plath vernier sextant
    > was
    > used;  IC = 0, and height of eye = 20-Ft.
    > On Tuesday, July 20, 2004 - Chronometer considered accurate
    > 1. For the Longitude
    > AM obs {at} Chro time = 17-09-30 GMT - Sun's LL {at} 75-38-20
    > PM obs {at} Chro time = 17-19-00 GMT - Sun's LL {at} 75-38-20
    > Mean Chro time of obs = 17-14-15 GMT = time of LAN
    > GHA by NA for 17-14-15 GMT = 76-58-12 W = Long {at} LAN
    > 2. For the Latitude
    > Alt, Sun's LL {at} AM observation = 75-38-20
    > GHA Sun by NA {at} AM obs = 75-46-54 West
    > Long by equal altitudes {at} LAN = 76-58-12 W
    > Therefore Meridan Angle {at} AM observation = 1-11-18 E
    > Alt corr to meridian - Bowditch Tabs 29 + 30 =  +2'-12"
    > Declination by Nautical Almanac = + 20-30-06
    > Corr Alt {at} 17-09-30 = 75-38-20 + 02-12 + 11-24 = 75-51-56
    > Lat {at} 17-09-30 = 89-59-60 - 75-51-56 + 20-30-06 = 34-38-10 N
    > Position by obs = Lat 34-38-10 N + Long 76-58-12 W
    > Certain parameters/limitations must be recognized
    > 1. Lat by reduction to the meridian is for the time of sight while
    > the
    > Long by equal altitude is tor the time of LAN, necessitating
    > correction
    > to a common time for a vessel underway.
    > 2. If ship movement were involved, an adjustment in the second equal
    > altitude would be necessary to allow for any N/S component of the
    > distance made good between sights. Although this may be easily
    > accomplished, the simplicity of the equal altitude solution becomes
    > somewhat more complicated. Essentially the Long is determined
    > without a
    > knowledge of instrument error or dip. I suppose the refraction could
    > change in the short interval involved.
    > 3. Recognize that a navigator, dependent on celestial navigation,
    > who has
    > been without sights for a number of days, as was frequently not
    > unusual
    > on the North Atlantic in winter, finds it necessary to use every
    > trick in
    > his bag, and must evaluate the results according to his best
    > judgement of
    > conditions at the time of observation. Actually, if in dire need of
    > a
    > position, the vessel might be hove to for the few minutes necessary
    > so as
    > to obviate any concern as to altitude change by ship movement.
    > 4. What was my position at the time of  observations? It was Lat
    > 34-40.073 N + Long 77-00.097 W; by a map program, giving an error of
    > about 2' each in Latitude + Longitude, assuming the map program to
    > be
    > correct. Given the horizon conditions at the time of sights, this
    > margin
    > of error is entirely possible, however, successive Latitudes
    > obtained on
    > surrounding dates all produced results of 34-38-40 N + Longitude
    > 76-59-00
    > W
    > 5. I used an on line Nautical Almanac to obtain declination + GHA,
    > and
    > frankly do not know how these compare with the printed version of
    > the NA.
    > Interpolations were made by inspection, with no effort at extreme
    > accuracy - the emphasis was in evaluating a method. I hope that I
    > have
    > not made some fool mistake in the transcription.
    > 6. A rigorous analysis will demonstrate the PM meridian angle to
    > actually
    > be 1-11-12 W indicating that the true PM equal altitude was probably
    > missed by some 6-seconds of meridian angle arc too early - thus
    > placing
    > the Longitude found slightly more to the west. Calculating the
    > actual
    > meridian angle at the time of each sight provides a check on
    > accuracy.
    > 7. If you are unsure of your chronometer, and know your Longitude
    > accurately, this method may be employed to ascertain chronometer
    > error -
    > I also experimented with this methodology and will report
    > subsequently.
    > 8. For those interested in such as this, it would probably be more
    > appropriate if presented in the form of a proper calculation. This
    > cannot, however be done unless transmitted as an attachment.

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