NavList:
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, 11:47 -0800
From: Doug Royer
Date: 2004 Jan 22, 11:47 -0800
Trevor,you and Bill are not missing anything reguarding this matter.I fully understand what you both are saying as to "confirming" the longitude of the DR or EP.I also wished to show that if it is the only thing one has to get an approximation of longitude one at least has an idea of where it is. Now,say the vessel is on a coarse of 103*T.The auto-pilot is set and one calculates the DR accordingly.The vessel travels through an unknown set of 170*T with a drift of 2kt. for 5 hrs with no calculations for leeway.The last sight taken was 40 hrs before.The position of the vessel is now nowhere near the DR.If the vessel has no electronic nav. equipment one won't be aware that the vessel has been pushed off the trackline by the amount it is. Conditions are such that 1 sight can be taken around the time of LAN.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. -----Original Message----- From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Trevor J. Kenchington Sent: Wednesday, January 21, 2004 14:06 To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM Subject: Re: Timed Noon sights for position Doug, I dare say that I am missing something but I have read through your original message twice and I am in the same position as Bill Noyce. As best as I can see from your logic, you first determine the time (GMT) of LAN for your EP, then you observe the Sun's altitude at that time, then you assume that your observation was taken at LAN and use its time to determine your longitude. That, however, is circular reasoning and can never do anything but "confirm" the longitude of your EP, no matter how erroneous it really is. If there is more to your method, please explain it. If there isn't anything more, I would say that the correspondence between your GPS readings and your "observed" longitudes only demonstrates that your EPs are very accurate. Trevor Kenchington Doug Royer wrote: > There are many procedures and many ways to accomplish the same goal.In my > origonal post on this matter I put forward this method as one of the methods > used.Some list members were horrified.Most times one doesn't use the Noon > sight to obtain longitude.I wished to show that when needed(deteriorating > conditions or circumstances)it can be used to give an approximation of > longitude where only an estimated latitude was calculated.One more tool. > Sometimes one doesn't have the luxury of turning the ships head from the > trackline to obtain celestial course lines(as Joel noted)or the time to > shoot double altitudes or multiple averaging sights.Are there pitfalls > useing this method?You bet. -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus