NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The Nautical Mile and the arc minute
From: Gary LaPook
Date: 2015 Jun 5, 23:11 +0000
From: Francis Upchurch <NoReply_Upchurch@fer3.com>
To: garylapook@pacbell.net
Sent: Friday, June 5, 2015 2:51 PM
Subject: [NavList] Re: The Nautical Mile and the arc minute
From: Gary LaPook
Date: 2015 Jun 5, 23:11 +0000
Except when the body is on the "prime vertical" azimuth 90 or 270 in which case the latitude in not needed as the resulting longitude line (LOP running 360-180) is not sensitive to errors in latitude. Other wise you use your noon sight latitude adjusted by DR to the time of the afternoon time sight or carry the noon latitude back to the time of a morning time sight. So, if the time sight was taken at 9:00 and the vessel is moving at 6 knots covering 18 NM in that time period. If we assume an uncertainty in DR of 10% (which is used in flight navigation but probably way too high for marine navigation, Frank have you any data for DR uncertainty for surface navigation) then the latitude uncertainty would be +/- 1.8 minutes used in doing the time sight computation for longitude. If the azimuth of the sun were 90 degrees then there would be no error in the longitude. If the azimuth was 135 degrees then there would be +/- 2.5 NM (cosecant times 1.8). If the azimuth was 150 degrees then 3.6 NM. This is exactly what Sumner did, assumed different latitudes and computed the resulting longitudes.
gl
From: Francis Upchurch <NoReply_Upchurch@fer3.com>
To: garylapook@pacbell.net
Sent: Friday, June 5, 2015 2:51 PM
Subject: [NavList] Re: The Nautical Mile and the arc minute
Thanks Frank,
Wonderful exposition of what we are all about.
re the issue of old time (e.g.time sights) versus Sumner/St. Hilaire ,am I correct in thinking that you need accurate latitude to get good longitude by the old methods and that is not the case for the new methods, and hence their appartent superiority? Correct me if I have this wrong.
Francis
