NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: UNK
Date: 2015 Jun 6, 19:16 +0100
Thanks Henry,
I never understood it from that perspective. Thanks very much. I will look into this. I have recently been interested in “Chords” in another context.(the Kelly Graphical method for lunar clearance and I have managed to find a Chord angle device.) Wonderful idea, goes back to the ancient Greeks! It sounds to my simple, non mathematical brain, that the theory of all this was well known back to the 17th century and I wonder why it took so long for Sumner etc al to re-discover it?
Oh well . History full of that sort of thing, including in my field of medicine.
Thanks again.
Francis
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Henry Halboth
Sent: 06 June 2015 19:07
To: francisupchurch@gmail.com
Subject: [NavList] Re: The Nautical Mile and the arc minute
Francis,
You are correct in that, except when observing a body on the prime vertical, an accurate Latitude is required to obtain a true Longitude by one observation utilizing the Time Sight However, the Time Sight can and was adapted to LOP navigation with results generally equivalent to the Marc St.Hilaire method, whether by the tangent or chord determination. As Capt. Sumner demonstrated, apparently by accident, regardless of the Latitude employed it, in combination with the Longitude calculated, constitutes a point on the LOP of the observer. Therefore, by plotting a line at right angles to the azimuth, the tangent method, or by recalculating another Longitude by use of a different Latitude (as Sumner did) and connecting the two positions, the chord method, a LOP is established identical to that provided by the St. Hilaire method. The argument used to be whether the chord or tangent methods provided the greater accuracy - but lets leave that for further discussion.
Henry
On Fri, Jun 5, 2015 at 5:51 PM, Francis Upchurch <NoReply_Upchurch@fer3.com> wrote:
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
