Simplify the following text for a Grade 5 native English-speaking person.
Original: An astronomical unit ( abbreviated as AU , au or a.u. ) is a unit of length equal to about 149,597,870.7 kilometres ( 92,955,807.27 miles ) or approximately the mean Earthâ `` Sun distance .
Simplified: The astronomical unit ( AU or au or a.u. or sometimes ua ) is a unit of length about the same as the mean distance from the center of the Earth to the center of the Sun .