(1) The angle, in a horizontal plane, from a vertical plane through the horizontal axis of a meridian telescope to the prime vertical through the centre of that axis. Alternatively, the angle by which the east west axis of the telescope deviates from being perpendicular to the plane of the meridian. It is considered positive if the nominally west end of the horizontal axis points south of west. (2) The angle defined similarly (to 1) for any telescope with an alt azimuth mounting when the horizontal axis should be perpendicular to the meridian. (3) For any instrument for which a nearly vertical plane can be defined, the difference between the azimuth of that plane and the azimuth in which it is supposed to lie. This definition is often applied to the calibration of radio telescopes.