The Mathematics of the Longitude - Department of Mathematics

2.6 Horizon Coordinate System
The apparent position of a body in the sky is defined by the horizon
coordinate system (Figure 2.10). The altitude, H, is the vertical angle
between the horizontal plane to the line of sight to the body. The point
directly overhead the observer is called the zenith. The zenith distance, z,
is the angular distance between the zenith and the body. H and z are
complementary angles (H + z = 90°). The azimuth, AzN, is the horizontal
direction of the body with respect to the geographic (true) north point on
the horizon, measured clockwise through 360°.
Figure 2.10
Each of the following imaginary horizontal planes parallel to each other
can be used as the reference plane for the horizon coordinate system
(Figure 2.11).
The true horizon is the horizontal plane tangent to the earth at the
observer's position.
The celestial horizon is the horizontal plane passing through the center of
the earth.