Astronomy for Astrologers
The Horizon
In the Horizon 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, which is the horizontal angle—A in the
diagram—measured from an arbitrary reference
direction—East in our case—counterclockwise to the
object circle), and by Altitude (a), which is the elevation
angle measured upward from the horizon to the object.
The great circle through the north and south points, and
through the zenith, is the Meridian, and the great circle
through the east and west points, and through the
zenith, is the Prime Vertical. Circles of parallel altitude to
the horizon, that are not great circles, are called
Almucantars.