# NavList:

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

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Re: Sunrise, Sunset, LAN, LMT
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 Huxtable  wrote:
...
>
> 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.
>

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