# 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:**Joel Jacobs

**Date:**2004 Aug 4, 18:02 -0400

Hello Jim, I had a distraction and came back to your message and now I'm more confused than I was. I thought Chuck was talking about using equal altitudes to calculate LON. If that's correct, then after halving the time, isn't it a simple time to arc conversion to get the point at which the sun crossed the ship's meridian. All you do is plot a vertical line at this point on a previously determined LAT which was the middle of the three sight's Chuck said to take. There is no E or W prime vertical involved, no azimuths that are N or S of the ship's position. Yes, there can be some adjustments to make, but the aren't key to the procedure. So I ask you where did I go wrong in understanding what Chuck was offering? Joel Jacobs ----- Original Message ----- From: "Jim Thompson"To: Sent: Wednesday, August 04, 2004 5:14 PM Subject: Re: Clarification of Question regarding LAN > Jim Thompson > jim2{at}jimthompson.net > www.jimthompson.net > Outgoing mail scanned by Norton Antivirus > ----------------------------------------- > > > -----Original Message----- > > From: Navigation Mailing List > > [mailto:NAVIGATION-L{at}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