Positional Astronomy:
Conversion between horizontal and equatorial systems

To convert between the horizontal and equatorial coordinates for object X,
we use "The" Astronomical Triangle XPZ,
where Z is the zenith, P is the North Celestial Pole, and X is the object.

Sides of the triangle:
   PZ is the observer's co-latitude.
   ZX is the zenith distance of X.
   PX is the North Polar Distance of X.
Angles of the triangle:
   The angle at P is H, the local Hour Angle of X.
   The angle at Z is 360 minus A, where A is the azimuth of X.
   The angle at X is q, the parallactic angle.

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