NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sunrise, Sunset, LAN, LMT
From: Chuck Taylor
Date: 2004 Aug 4, 21:17 -0700
From: Chuck Taylor
Date: 2004 Aug 4, 21:17 -0700
George Huxtable and Zorbec Legras pointed out that the method of finding longitude by equal altitudes before and after LAN is inexact. I said as much, by describing it as falling into the category of "lifeboat navigation", by which I meant a method to be used when better methods are not available. I also pointed out that it assumed minimal motion of the observer between observations. Despite the inexactness of the method, I find it interesting as an exercise in understanding the basic concepts involved. Best regards, Chuck Taylor North of Seattle --- George Huxtablewrote: ... > > Chuck Taylor wrote- > > >In theory, if you had an accurate timepiece and > could > >observe the exact instant that the sun crosses your > >meridian (when it reaches its highest point), you > >could then calculate your longitude by converting > >time to arc. > > The exact instant at which the Sun crosses your > meridian is NOT the same as > the moment that it reaches its highest point. > There's a correction to be > made, to account for any North-South component of > the observer's speed, and > also to allow for changing declination of the Sun. > For the non-critical > timing of the moment to measure altitude (for > latitude), that correction is > unnecessary, but for any determination of longitude > it has to be included. > > ================= > > Zorbec Legras wrote- > > >Attention de greatest sun altitude is Not the > instant of transit for a > >mobile observer. > > Nor is the instant of greatest Sun altitude (quite) > the same as the instant > of meridian transit, even for a stationary observer > (except at the > solstices), due to the changing declination of the > Sun. But it's close. > __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com