NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Time of meridian passage accuracy
From: Antoine Cou�tte
Date: 2009 Sep 25, 23:43 -0700
From: Antoine Cou�tte
Date: 2009 Sep 25, 23:43 -0700
George is right : Time of Meridian passage may be significantly different from time of Culmination. Well known formula (although first order approximation). Let : - "Delta Time" be the difference is seconds of time between culmination and Meridian Transit, and - "Lat" the Observer's Latitude at Transit time, with "vLat" his hourly Latitude speed is arcminutes per hour, and - "Dec" the Body's Declination at Transit Time, with "vDec" the hourly Declination change in arcminutes per hour, then Delta Time = 48/Pi * (tan. Lat - tan. Dec) * (vLat - vDec) . First coefficient often published as 15.35 or 15.3 Nothing new : this formula has already been posted here and it has been taught for probably (almost and maybe more) one century now. Let us take a somewhat 'extreme' but still realistic example : Lat = North 15? with vLat = -30 and Dec = 15? South with vDec = + 12 (Moon case), then we find that Delta Time = 22.5 seconds (actual value does differ by just a few seconds of time) What are the practical implications on position accuracy if and when Meridian transit time not equal to Culmination time ? If the vessel has a significant north/south speed component, such as in the extreme case listed here-above, then : Body declination may have changed by up to 1/15 arc minute during that time period, so it is (almost) negligible for conventional Celnav, and Vessel's latitude will have changed by 0.2 ' during that time period. You may start wanting to take it in account. Antoine Antoine M. "Kermit" Couette --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---