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Horizon Coordinate System

Horizon Coordinate
In this system a plane through the observing point parallel to the horizon is the plane of reference. The poles are the Zenith (point overhead) and the Nadir (point underfoot). The vertical circle through a celestial object (such as a star) and the zenith is the Object Circle. The coordinates are given (for the object) by Azimuth (horizontal angle measured from an arbitrary reference direction — East in our case — counterclockwise to the object circle) and the Altitude (elevation angle measured upward from the horizon to the object). The great circle through the north and south points and the zenith is the Meridian, and the great circle through the east and west points and the zenith is the Prime Vertical. Circles of parallel altitude to the horizon that are not great circles are called Almucantars. Horizon Sphere
A heavenly sphere based on the plane of the observer's local horizon (90° from both the zenith and the nadir), but taken from the center of the Earth.

Zenith
The zenith is the point directly overhead any spot on Earth.

The nadir is the point directly beneath (opposite the zenith) any spot on Earth.

Altitude
The angular distance of any body above or below the plane of the local horizon. Altitude is measured from 0° to 90° from the plane of the horizon to either pole.

Azimuth
The angle measured around the 360° circumference of the horizon, either east or west (there are different practices). The azimuth of an object as measured from the meridian plane of the observer and a vertical plane through any body.

Prime Vertical
A great circle passing through the zenith (north pole), nadir (south pole), and the east and west points on the horizon.

Altitude Circles
Parallel circles of altitude, wither north or south of the plane of the horizon.

Almuncantar
Parallel circles of altitude, wither north or south of the plane of the horizon.

North Point
A point on the horizon to the north of the observer, where the meridian plane intersects the horizon.

South Point
A point on the horizon to the south of the observer, where the meridian plane intersects the horizon.

East Point
A point on the horizon to the east of the observer, where the prime vertical plane intersects the horizon.

West Point
A point on the horizon to the west of the observer, where the prime vertical plane intersects the horizon.

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