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I wrote earlier:
" Yes, you can get a rough  value for LATITUDE by watching zenith stars. But
there's nothing in that  technique that can supply longitude.

Peter, you replied:
"This may  well be correct. But I'm still not entirely happy with it (how do
you like  this new diplomatic approach?).

:-)

And wrote:
"Only if  you are in a unique
position on the earth's surface - defined by latitude AND  longitude - will
the star be at the zenith. This is the whole idea. If you  are at some
unknown spot in the Pacific, observing a zenith star, then you  cannot be,
say, in the Baltic Sea because if you were that star would not be  at your
zenith."

This is true, but only for one instant of absolute  time. Let's take your
case of a star that passes straight overhead in the  Baltic. I think Mizar in the
Big Dipper will do. If I see it straight up, then I  know I am in latitude 55
North. If I pick a day, let's say three months ago,  March 28, it will pass
straight overhead at just about 0200 local time in the  Baltic. Does this prove
I'm in the Baltic? No. Because it will ALSO pass  straight overhead in the
northern Pacific at 0200 local time on this same date.  In order to turn this
observation into a longitude, I need to know some absolute  time. Seeing Mizar
in the zenith on this date tells me the local time is 0200.  If I then turn to
a chronometer (or shoot a lunar, or listen to the radio, or  observe some
other equivalent signal) and find out that the time in Greenwich is  0100, THEN I
can decide that I must be in the Baltic and not the Gulf of  Alaska.

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

   

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