the arithmetic mean of the greatest and least distances of a planet from the sun, used in stating the size of an orbit; the semimajor axis.
Origin of mean distance
First recorded in 1885–90
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the average of the greatest and least distances of a celestial body from its primary
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012