# NavList:

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

**Re: Cannot dispense with the assumed position at sea**

**From:**Joel Jacobs

**Date:**2004 Feb 20, 09:25 -0500

I have a couple of questions. 1. I'm receiving only one side of the dialog, and would like to see what "Frank" is saying. Where can I find his comments? 2. Why, at least for those in this country, is there a need to construe definitions that are different from those found in the standard U.S. published navigation texts of which most list members are aware. 3. I have sold mine, but I recall the tables such as HO 214, 229, and 249 also had sections with definitions. The Nautical Almanac had definitions. I realize many changes have taken place, but are none of these sources satisfactory? Should we ignore Bowditch as another example? Joel Jacobs ----- Original Message ----- From: "Jim Thompson"To: Sent: Friday, February 20, 2004 8:02 AM Subject: Re: Cannot dispense with the assumed position at sea > Fred, I still think we are convergent: > > One can use a precise DR position in the Ageton-Bayless table, Reed tables > and computer programs or calculators to do the same thing as a whole-degree > AP does in HO 229: determine an Hc and Zn to plot the LOP. Those 3 methods > can accept the precise longitude to determine meridian angle from a precise > LHA, and the precise latitude in calculating Hc and Zn, as in: > > Hc = arcsin [cost x cosD x cosL) + (sinD* x sinL)] > Z = arccos [(sinD* - sinL) x (sinHc / (cosHc) x cosL)] > where > t = meridian angle, precise decimal DMS. > D=declination of the body, precise decimal DMS. > L=DR latitude, precise decimal DMS. > *Note the sign (+ or -): negative if L and D are contrary in name (N or S). > > Which means that one can use a precise DR position as an "AP" in the sense > that you mean by using whole-degree AP's as an entering argument for HO 229, > except that one cannot use HO 229 for a precise DMS DR position, and so > would have to use one of the alternative methods that can. > > If a navigator puts an EP box around the point on the celestial LOP > perpendicular to the position used to create the LOP, then that EP has more > significance if the "assumed position" is part of the DR plot. Of course > the workaround using a whole-degree AP would be to subsequently drop a > perpendicular to the DR position, I think achieving the same end except with > extra plotting steps if a whole-degree AP is used as an intermediary. > > With respect to semantics, I think my understanding of AP is that I see it > as a general term for the position for which Hc and Zn are determined. Thus > in my mind any position used for that purpose is an "AP" (Henning uses > "initial position" or IP). Owing to the whole-degree history of the > entering argument AP, it seems to have traditionally acquired a more > specific meaning. > > Jim Thompson > jim2{at}jimthompson.net > www.jimthompson.net > Outgoing mail scanned by Norton Antivirus > ----------------------------------------- > > > -----Original Message----- > > From: Fred Hebard [mailto:Fred{at}acf.org] > > Sent: Thursday, February 19, 2004 11:43 PM > > To: jim2{at}jimthompson.net > > Subject: Re: Cannot dispense with the assumed position at sea > > > > Jim, > > > > This isn't semantics. As Doug said, they mean different things. An AP > > is used for sight reduction tables such as H.O. 229. One enters these > > tables at a whole degree of latitude, such as 36* N, rather than a > > fractional value, such as 36*16.5'N. You _can't_ enter the tables > > from other than a whole degree of latitude. Likewise, the longitude is > > chosen to give an LHA in whole degrees; again, one cannot enter the > > tables from a fractional LHA. One then plots the azimuths and > > distances from that AP. It also makes locating the latitude of the AP > > a bit more convenient. > > > > The EP and DR are places where you actually reckon you are, so they are > > almost never at whole degrees. In contrast, the AP is not a place > > where you reckon you are, but the closest to where you reckon you are > > in whole degrees of latitude and fractional degrees of longitude that, > > combined with the GHA of a body, give an LHA in whole degrees. >