A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Latitude + Longitude @ Noon
From: Henry Halboth
Date: 2005 Jun 5, 15:11 -0400
From: Henry Halboth
Date: 2005 Jun 5, 15:11 -0400
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. >