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On 2019-07-30 8:16, Peter Monta wrote:
 > If you're observing from a fixed spot, and you know the
 > direction to a distant landmark, then that's just as good as a sea
horizon
 > (in fact better, since the landmark is likely to be at least a few
degrees
 > above the true horizon and thus in a better atmospheric path).  The
 > direction (i.e. topocentric azimuth and elevation) can be found with
(what
 > else?) celestial, using either a sextant or a theodolite.

Several years ago I had a dream in which I was observing sextant angles
between a distant mountain peak and the Sun. Upon waking I remembered
the dream with unusual clarity, and on further reflection I realized it
was possible to measure azimuths and elevations of landmarks that way.

Any sight reduction method can be used. With traditional paper and
pencil techniques, begin with a plotting sheet. True north takes the
place of the prime meridian. The estimated elevation and azimuth of the
landmark are the dead reckoning latitude and longitude.

In this application, time and position are known, so the celestial
body's azimuth and refracted altitude are also known. In the sight
reduction these become "GHA" and "declination".

For observed altitude, use 90 minus the sextant angle.

The sight reduction will yield azimuth and intercept. "Azimuth" is
actually the position angle (in a horizontal coordinate system) of the
body with respect to the landmark. But on the plotting sheet it's
equivalent to computed azimuth.

Elevation angles determined by this method are affected by terrestrial
refraction. That's not necessarily a bad thing. It might be interesting
to analyze the variability of terrestrial refraction. If the landmark is
a distant isolated light you can observe all night with a wide choice of
stars.

   

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