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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: Fred Hebard
Date: 2004 Feb 20, 09:43 -0500
From: Fred Hebard
Date: 2004 Feb 20, 09:43 -0500
One other point about APs for use with tables. You use a different AP for each body, because they almost always have different LHAs. This is assuming a more-or-less simultaneous round of sights on more than one body. On Feb 20, 2004, at 9:25 AM, Joel Jacobs wrote: > 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@jimthompson.net >> www.jimthompson.net >> Outgoing mail scanned by Norton Antivirus >> ----------------------------------------- >> >>> -----Original Message----- >>> From: Fred Hebard [mailto:Fred@acf.org] >>> Sent: Thursday, February 19, 2004 11:43 PM >>> To: jim2@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. >> >