|Coordinate system||Fundamental plane||Poles||Coordinates||Epoch|
|Horizontal||horizon||zenith/nadir||elevation - azimuth - meridian|
|Equatorial||celestial equator||celestial poles||declination - right ascension or hour angle||B1950, J2000|
|Ecliptic||ecliptic||ecliptic poles||ecliptic latitude - ecliptic longitude|
|Galactic||galactic plane||galactic poles|
The term zenith distance is more often used in astronomy and is the complement of the elevation. That is: 0° in the zenith, 90° on the horizon, up to 180° at the nadir.
Let φ be the observer's latitude.
Let El be the elevation angle and Az the azimuth angle.
Let θ be the zenith (or zenith distance, i.e. the 90° complement of Alt).
Then the equations of the transformation are:
Use the inverse trigonometric functions to get the values of the coordinates.
NOTE: Inverse cosine is dual valued, i.e. 160° and 200° both have the same cosine. The above needs to be corrected. If H < 180 (or Pi radians) then Az = 360 - Az as derived from the above equation.
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