astronomical unit

(ua or au or AU)

   a unit of distance used by astronomers to measure distances in the Solar System. One astronomical unit equals the "average" distance from the center of the Earth to the center of the Sun (mathematically, it is the length of the semimajor axis of the Earth's elliptical orbit, which is the ordinary average of the Earth's minimum and maximum distances from the Sun). The currently accepted value, adopted in 1996, is 149 597 870 691 meters (1.495 978 706 91 x 10⁸ kilometers or about 92 955 807 miles), with an uncertainty of about 30 meters. The astronomical unit is a convenient yardstick for measuring the distances between objects in the Solar System. Astronomers find it particularly convenient to use astronomical units in solving the equations of planetary motion. Because these equations are the same regardless of the unit used for distance, the predictions they generate will remain correct even if future astronomers determine a slightly different length for the distance between the Earth and Sun. This unit is accepted for use with SI units. The official symbol for the unit is ua, but the symbol au is common in English-speaking countries. Note that AU is also used as the symbol for the absorbance unit (see above).