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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