# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: AP terminology**

**From:**Hewitt Schlereth

**Date:**2009 Nov 13, 20:12 -0400

Hey, guys, didn't Capt. Sumner take one sight of the sun around 10 a.m. and then ASSUME three different LATITUDES (not longitudes) to plug into a time-sight formula? HS On 11/13/09, P Hwrote: > > Since LOPs are one-dimensional objects, you need precisely one parameter to > characterize them. It is this parametrization that amounts to "calculating > the LOP directly" (answer to Geoffrey Kolbe's question). Sumner used > longitude as the parameter. The parameter doesn't have to be longitude and > in the case of LOP=meridian, it is in fact unsuitable. > > The LOP is what it is, based on the Ho and GP, independent of any AP. As I > said in an earlier post, you need to have a coordinate system to do anything > practical. The AP is the origin of such a convenient local coordinate > system. We have some freedom in choosing where to place this AP=origin. > The computed intercept distance then tells us how far the chosen AP is from > the LOP, and the computed azimuth tells us the orientation/direction. That > is what the solution of the celestial triangle AP-GP-Pole is about; it gives > the relative position of the (independently existing and fixed) LOP and our > arbitrarily chosen AP. In practice St. Hilaire proceeds in reverse; we > choose the AP first, solve the triangle, and then we construct the LOP at > the appropriate distance from the AP and orientation with respect to true > North. > > On a Mercator plotting chart LOPs look like straight lines (for not too high > Hs's). A straight line lying in a plane can be defined by two points, or by > one point and one direction. Sumner uses the former; St. Hilaire uses the > latter. St. Hilaire is less work because one detail regarding the direction > part is automatic; it is the angle between the LOP and the azimuth line > toward the GP, which is always 90 degrees. We can therefore plot the LOP at > the right angle to the azimuth line (at the intercept distance) because > Mercator mapping used in plotting charts is conformal, i.e. it preserves > angles. > > > Peter Hakel > > > ________________________________ > From: John Karl > To: NavList > Sent: Fri, November 13, 2009 12:33:27 PM > Subject: [NavList 10635] Re: AP terminology, WAS: 2-Body Fix -- take three > > > No one has addressed my question of why the St Hilaire method > calculates an altitude at a location our ship is NOT at, when we've > just measured the altitude where our ship IS at. (For politically > correct reasons, I'm not using the name of this location.) > > Now lets go back to Sumner's 1837 calculation, where he picked three > different longitudes and calculated three points on the circular LOP. > This calculation is exact, and the equation for each point is the same > as the one of the two necessary in the St Hilaire method (thus each > Sumner point is half the work of a St Hilaire reduction). And he > could calculate as many exact points as he wished. > > So I'll put my question yet another way: Why is the St Hilaire method > superior to Sumner's and consequently the only one used today?? > > I claim that the answer to this question has been made confusing > because of the conventional name (names?) used for the location of the > St Hilaire altitude calculation. As evidence of this confusion I note > that some authors write that we need to assume some point because the > distance between the GP and the LOP is too great to plot, that there's > insufficient information to plot the LOP, or that iterations are > required to get exact points on the LOP. The Sumner calculation > demonstrates that none of this is correct. > > JK > > > > > > > --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavList+unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---