altazimuth coordinate system

altazimuth coordinate system or horizon coordinate system, astronomical coordinate system in which the position of a body on the celestial sphere is described relative to an observer's celestial horizon and zenith . The coordinates of a body in this system are its altitude and azimuth . Altitude is measured from the celestial horizon along the vertical circle through the body and the zenith of the observer. Azimuth is measured along the celestial horizon from the observer's south point (the point on the horizon directly south of him) to the point where the body's vertical circle intersects the horizon. Because the earth rotates on its axis, the altitude and azimuth of a celestial body are constantly changing.

coordinates, the definition of the exact position of a point on the surface of the globe in relation to two lines, latitude and longitude, which intersect at right angles. Positions on the celestial sphere are usually defined for navigational purposes using coordinates of the ecliptic system, the equinoctial system, or the horizon system.

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inertial coordinate system The system of coordinates that is used in an inertial reference frame, normally in the special theory of relativity. The three space coordinates are usually Cartesian coordinates (x, y, z), and the time coordinate is the time as measured by an observer at rest in the coordinate system. In astrometry, an inertial coordinate system is a reference frame formed by assigning coordinates to specific observable objects, such as the positions and proper motions of stars in a fundamental catalogue.