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Greg:
"But as a practical matter (assuming  that we've got a current almanac and
can get a reasonably accurate time  reading), even if we had no idea at all
about a DR position couldn't we at least  narrow down ..."

As long as you don't mind waiting around for the stars  to come out (and
given your assumptions of an almanac and a good clock set to  GMT or some known
zone time), you can get your position "by eye" within two or  three degrees
easily and up to ten times better with careful land-based  observations simply by
looking at the zenith --no sextant required. If I look  straight up and see
Vega, for example, within a degree of the zenith at 0600 GMT  on July 1
(Greenwich Date!), then I can write down my position immediately. The  declination of
the zenith is the observer's latitude. So for the case of Vega in  the zenith,
my latitude must be close to 39 degrees North. The right ascension  of the
zenith is the observer's sidereal time. Of course navigators don't use RA  and
SidT anymore, so for modern almanac data, you use the equivalent rule: the  GHA
of the zenith is the observer's longitude (west). For Vega in the zenith,
the SHA is about 81 degrees and the GHA of Aries is about 9 degrees at 0600GMT
on this date, so my longitude is 90 west.

Declination of zenith =  Latitude
GHA of zenith = Longitude

-FER
42.0N 87.7W, or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

   

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