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    Azimuth / elevation from sextant observations
    From: Paul Hirose
    Date: 2019 Jul 30, 14:26 -0700

    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
     > (in fact better, since the landmark is likely to be at least a few
     > above the true horizon and thus in a better atmospheric path).  The
     > direction (i.e. topocentric azimuth and elevation) can be found with
     > 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

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