NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Clarification of Question regarding LAN
From: Jim Thompson
Date: 2004 Aug 4, 18:14 -0300
From: Jim Thompson
Date: 2004 Aug 4, 18:14 -0300
Jim Thompson jim2@jimthompson.net www.jimthompson.net Outgoing mail scanned by Norton Antivirus ----------------------------------------- > -----Original Message----- > From: Navigation Mailing List > [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Robert Gainer > > Chuck said, > >The more traditional way of determining longitude was > >to use a time sight at the time the sun crosses the > >Prime Vertical (i.e., the time at which the sun is due > >east or due west of you). This procedure is described > >in Bowditch and elsewhere. It requires that you know > >your latitude, which you can get from a noon sight or > >from an observation of Polaris. > > Chuck, > I don?t understand how that will work. The magnetic variation and the > latitude must be problems in that method. If you are at 23 degrees north > latitude or greater the sun is never due east or west. If you do not know > the magnetite variation with some degree of accuracy wont that have a very > large effect on the method? Is this practical at all? > All the best, > Robert Gainer Robert, I really got stuck on that too when I was learning this stuff last year. The key is to realize that the Prime Vertical Circle is not in the terrestrial coordinate system. Let's see if my explanation stands the test of this posting: See the text and diagrams at http://jimthompson.net/boating/CelestialNav/CelestNotes/Coordinates.htm#Hori zonCoordinates for a discussion of the Horizon Coordinate System, and http://jimthompson.net/boating/CelestialNav/CelestNotes/Coordinates.htm#Link ed for an explanation of how the various coordinate systems are linked through the PRINCIPAL vertical circle (not PRIME). The intersection of the horizon coordinate system's PRINCIPAL vertical circle with the celestial (and so terrestrial) equator defines the north cardinal point in the horizon coordinate system. If you then go 90 degrees clockwise from that point around the horizon, you reach the horizon coordinate system's West cardinal point. See also Figure 1527a in Bowditch. The vertical circle in the horizon coordinate system that goes through that point is the PRIME vertical circle. Now here's the tricky part, requiring a lot of reflection until it comes clear in the mind: When a body is on the PRIME vertical circle relative to your position, it is by definition also exactly west of you in the terrestrial coordinate system. Bowditch' definition of the Prime Vertical Circle includes this statement: "The intersections of the prime vertical circle with the horizon define the east and west points of the horizon.". For the practical application referred to by Chuck, see Figure 3 on http://jimthompson.net/boating/CelestialNav/NoonSunSight.htm titled, "Figure 3. Sun sight when the sun is on its prime vertical". Jim