The deviation of a transit-instrument from the plane of the meridian at the horizon; it is also termed the azimuthal error.
I think, to the azimuthal Arch of the Horizon included between the point of sunrise and that of sunset!
The position of a celestial object along an observer's horizon. Azimuth is a horizontal angle measured clockwise in degrees from a reference direction, usually the north or south point of the horizon, to the point on the horizon intersected by the object's line of altitude (a line from the observer's zenith through the object to the horizon). If north is the reference point (0°), then east has an azimuth of 90°, south is 180°, and so forth through 360°. See more at altazimuth coordinate system.