Astronomy for Astrologers
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.
Nadir -- 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
(the North Pole) and the nadir (the South Pole), and the
east and west points on the horizon.
Altitude Circles - Parallel circles of altitude, whether
north or south of the plane of the horizon.
Almuncantar - Parallel circles of altitude, whether north
or south of the plane of the horizon.
North Point – Is a point on the horizon, to the north of
the observer, where the meridian plane intersects the
horizon.
South Point – Is 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.