Astronomical Unit (AU), unit of distance used in the measurement of orbits and trajectories within the solar system. One AU is the average distance between the earth and the sun. Its value has been established as, roughly, 149,600,000 km (92,956,000 mi) by means of radar-ranging studies of nearby celestial objects such as Venus or passing asteroids; these studies have enabled astronomers to determine the scale of the solar system with great accuracy.
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