Astronomy for Astrologers
Horizon Coordinate System
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 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.