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On Tue, 28 Oct 1997, Jan van Puffelen wrote:

Pierre H Boucher wrote: [regarding fixing longitude with a
meridian transit of the sun, noting the times at which the sun
reaches a certain altitude just before and just after meridian
passage and splitting the difference]

> >Here you have your longitude with a pretty good precision,

Jan van Puffelen wrote;

> Not so, the sun changes declination between the two observations and the
> ship of latitude and longitude which will throw off the results of the
> observation considerably. This (and other factors) makes this method
> completely impracticable for the use at sea. As far as I know this method
> (which pops up once in a while in this discussion group) has therefore
> never been used at sea.

In the U.S. Power Squadrons course on navigation, this method is
taught under the heading of "Emergency Navigation", suitable for
use in a lifeboat situation, but not for routine navigation (for
the reasons stated by Jan van Puffelen). The advantage of the
method is that it can be used without a nautical almanac. All you
need to know is the time, the Equation of Time, and the
declination of the sun. The latter two items can be approximated
from an Analemma Curve, which should be packed away in your
emergency kit. You can also approximate the declination of
the sun by knowing the date and interpolating between the
solstices and equinoxes. With approximate data you can get an
approximate position.

The method is also described in the brochure that comes with
Davis plastic sextants, but without caveats as I recall.

Chuck Taylor
Everett, WA, USA
ctaylor@premier1.net

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