NavList:

   

Reply

Doug,

You should take elevation into account.  I also live at the same
elevation.  The actual air pressure declines by about 2 inches of
mercury at 2100 feet elevation.  The unusual conditions corrections
table (A4) inside the front cover of the Nautical Almanac ranges down
to 28.5 inches, which is pretty close to our "normal", just a bit under
28 inches at 2100 feet.  The corrections amount to 0.1 to 0.3 minutes
of arc under spring and summer temperatures.

Other than the effect on refraction, there are no significant effects
of 2100' of elevation on celestial altitudes.  The parallax of the moon
might change by about 0.01 minutes of arc.

It doesn't matter much what bodies you use.  If you can arrange for one
to be at fairly low altitude, that can reduce contortions such as lying
on your back and twisting sideways, which can be very tiring.  If you
use a liquid horizon, other than mercury, you will be limited to
brighter stars and planets, such as Jupiter or Sirius.  Bruce Stark
recommends in his book that you ignore taking the altitudes and just
focus on the distance, if you know where you are.  The methods in his
book can accommodate either.  For use at sea, you would need to have
altitudes to go along with the lunar, which then makes twilight
observations desirable (but not necessary since the altitudes only need
to be accurate to a minute or so, and you could advance a line of
position).

Take about eight distances, pretty much as fast as you can.
Theoretically, you should get a straight line when plotting distance
against the time of observation.  George Huxtable recently posted data
of a very nice lunar observed about 150 years ago in Australia, to give
you an idea of what an excellent set of data look like.

Fred

   

Reply