In this system the plane of reference is not the horizon but is a plane through the observing point parallel to the horizon. (If the observatory is at the top of a mountain, the horizon and the horizon plane defined here are different.) The line perpendicular to this plane, through the observer, passes through the POLES of the system. They are called the ZENITH (the direction overhead) and the NADIR (the direction underneath), two words that recall Arabic work in astronomy. The great circle through the zenith and the object is called the OBJECT CIRCLE. The angle between the zenith and the object along the object circle is called the ZENITH ANGLE of the object. The angle from the horizon plane to the object circle is called the ALTITUDE or ELEVATION ANGLE of the object. The other coordinate is measured as the angle (measured from the north point towards the east) of the point where the object circle intersects the horizon plane. This coordinate is called the AZIMUTH of the object. Note that the coordinates of an astronomical object in this frame of reference change continuously as the Earth rotates on its axis.
This is why the equatorial coordinate system, described next, is much more commonly used. The horizon system is commonly used for telescopes that turn on horizontal and vertical axis (ALTAZIMUTH mounting) such as the largest steerable radio telescopes.