# NavList:

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

**Re: AP terminology, WAS: 2-Body Fix -- take three**

**From:**Frank Reed

**Date:**2009 Nov 12, 22:49 -0800

John, you wrote: "Ahhhh Frank, see what I mean----" Not really. Could *you* explain what you mean for me? See, as I have read these posts, I see people trying to be careful with terminology, and definitely also people get confused in their intial celestial education by the fact that different AP's are chosen for different bodies in the same round of sights with certain tabular algorithms. And that aspect IS confusing for most beginners. But it's not because of the terminology, as far as I can see (so far!). And you wrote: "All these posts, and look at the confusion: The DR is an AP, it's one particular choice of an AP. And tables have nothing to do with it." John, it seems to me that this is pure semantics --differences in the way people define the "assumed position". If we define the concept of "assumed position" narrowly (as a point where we are not located chosen only for convenience of solution) then the DR is not an AP. If we define it broadly as the point from which any intercept is calculated in the St Hilaire method, then a method plotting from the DR uses the DR as an AP. Under that definition, it's the same thing. The interesting thing here is that the difference in the way one defines it, narrowly or broadly, can impact communication with people who have *already* learned celestial navigation in different ways (for example, you talking with Peter Fogg in these messages), but it's not a problem for people learning celestial navigation from one source or one instructor (since in the latter case, you will encounter only the narrow or the broad definition, not both). But maybe in the end, this is the very confusion you're talking about. Is it? And you asked: "Has anyone answered the question WHY we use an AP in the intercept method?? To phrase it another way, since we know everything needed to calculate the celestial LOP (the GP's location and the altitude), why not just calculate the LOP directly?" Well, you can do that today. Historically it was prohibitively inefficient in terms of computation cost. But today if I observe Venus 4d 12.5' above the horizon, I can easily calculate every spot on the Earth where this would be the observed altitude. I wouldn't recommend that approach even today on a computer. With normal sights, there would be no value in that direct calculation. I'm not entirely sure what you're getting at here, so it's quite possible I completely misinterpreted your question. Do let me know if that's the case! ;-) -FER --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavList+unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---