Encyclopedia · Glossary
Astronomical Unit
The average Earth–Sun distance, about 149.6 million km.
ScienceBeginner
Definition
An astronomical unit, abbreviated AU, is the average distance between Earth and the Sun — about 149.6 million kilometers (about 93 million miles). It provides a natural yardstick for distances across the Solar System.
In context
Using the AU keeps Solar System distances manageable: Mercury orbits at about 0.4 AU, while Neptune lies about 30 AU from the Sun. Light takes roughly eight minutes to travel one astronomical unit.
Related ideas
For the far greater distances between stars and galaxies, the astronomical unit becomes too small to be practical, and astronomers switch to the light-year instead.