# NavList:

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

**Re: 2-Body Fix -- take three**

**From:**John Karl

**Date:**2009 Nov 8, 20:56 -0800

George & all interested, Point 1: The law of sines equation does not change the fact that no set of equations derived from Fig 7.3 can determine longitude because there is no longitude information in the figure. Outside information must be added. George's sin(LHA1) equation introduces negative LHAs, departing from the commonly used convention. (And I didn't want to depart from convention in my book, nor introduce the Law of Sines.) More fundamentally, it shifts the LHA selection question to the other fix in Fig 7.3, where (A+B) must be used in Eq. 5.5d, which then becomes Sin(A+B) in George's Law of Sines equation. This makes the sensitive decision occur around LHA = 90 instead of 0/180. That is, now one has to decide between the LHA returned by the computer, and 90 + LHA. This is much harder for a navigator to do than to decide whether a body is on, or too near his meridian. (This is the same reason we don't use the Law of Sines for computing azimuth in St. Hilaire sight reductions.) Point 2: A and B are the angles enclosed in spherical triangles and have no direction associated with them. Their definitions remain the same under an interchange of GPs. E.g., A is always the interior angle between sides Dec1 and D12. Point 3: Using the equation LHA = GHA + Lon, good for all GHAs and Lons, requires only the convention that west Lons are negative and west are positive. I can't see any room for confusion here if just the simplest algebra is understood. Of course we could switch the Lon sign convention and the equation accordingly. As George points out I picked the one that's used by Cel navigators and not the one of astronomers, irritating both George's and Meeus(and maybe me, to a lessor extent). I sympathize with George and Meeus as I've also been forced to adopt another commonly used CN convention in my book that I find irritating to no end. My pet peeve is the term "Assumed Position" in the St, Hilaire sight reduction. I wanted to call it the "Reference Point" because in the St. Hilaire method there are no assumptions at all, as I've preached on this List, in my book, and in an article "Assuming Nothing" in Ocean Navigator, Nov/Dec 2008. But there are communication advantages to sticking with convention. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavList+unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---