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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Timed Noon sights for position
From: Doug Royer
Date: 2004 Jan 22, 16:51 -0800
From: Doug Royer
Date: 2004 Jan 22, 16:51 -0800
Bill,the lat you quoted as 32*21'36"N threw me.I went back to the original post and saw that the 1204 Ep stated that.That is an impossibility as the vessel was on a course of 209*T.I most likely switched the lats for the 1209 and the 1204 times along with hitting a 1 instead of 0.Perhaps this is adding to the confussion. Most likely(as I threw the worksheets away after posting yesterday)the 1209 EP lat = 32*20'30"N and the 1204 EP lat = 32*20'36"N. -----Original Message----- From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Noyce, Bill Sent: Thursday, January 22, 2004 13:07 To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM Subject: Re: Timed Noon sights for position Doug writes: ------------------- One gets the estimate of latitude at LAN from the reduced noon sight.One now has an idea of ones' latitude.Are you going to trust a position line 40 hrs old to get an idea of the longitude?Yes,one can advance the line.But,in reality,how accurate will the results be from this r-fix?I was taught and practiced that the optimal time to advance or retard any line was 3-6 hrs. So,one is in the position in this scenario not having the "warm,fuzzy" feeling of confidence reguarding the ships'position. At the least one can use the r-fix and the noon GHA of the DR or EP to get a better idea of where the position is or if some gross error exists in one of the parts. ------------------- I'm still confused. Suppose the procedure you suggest does give a reliable latitude (I have some questions about that, but don't want to discuss them until I'm sure I understand the procedure.) It looks to me as if you must then choose among some of the following alternatives for a longitude: 1. The longitude of the EP or DR position. 2. The longitude at which the (estimated) course line crosses the latitude determined by LAN sight. 3. A longitude at which the (estimated) distance-made-good circle crosses the latitude determined by LAN sight. #1 is equivalent to advancing your old fix to a new EP or DR and then finding the nearest position that matches the latitude determined by the LAN sight, since longitude lines are perpendicular to latitude lines. None of these methods gains any information about longitude from the sight. If there is some information that your procedure provides, I've missed it. In your example, what would have been the resulting fix if you were actually 30 miles further east at the time of your sight? Here's how I figure it, using Hc from the USNO web site: EP 32*21'36"N,118*43'45"W (True position 31*19'55"N,118*13'47"W) GMT 12:04:26 15 Jan 04 (I assume you make the sight at the same time, based on your EP.) Ho (from USNO web) 36*32.1 Latitude computes as 32*20.0" As far as I can see, the longitude computations are exactly the same as in your original post -- the fact that the true position is off by 30 miles makes no changes to the longitude derived from this observation. If this is true, in what way is the sight providing longitude information? -- Bill