Next message: Fred Hebard: "Re: Latitude + Longitude @ Noon"
George,
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" <>
writes:
> 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 @ Chro time = 17-09-30 GMT - Sun's LL @ 75-38-20
> PM obs @ Chro time = 17-19-00 GMT - Sun's LL @ 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 @ LAN
>
> 2. For the Latitude
> Alt, Sun's LL @ AM observation = 75-38-20
> GHA Sun by NA @ AM obs = 75-46-54 West
> Long by equal altitudes @ LAN = 76-58-12 W
> Therefore Meridan Angle @ 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 @ 17-09-30 = 75-38-20 + 02-12 + 11-24 = 75-51-56
> Lat @ 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.
>
