# 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:**Jim Thompson

**Date:**2004 Feb 20, 09:02 -0400

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.