The astronomical unit (AU) is a unit of length used in astronomy to measure the distance between objects within the solar system. By definition, one AU is the average distance between the Earth and the Sun. This standard measurement simplifies the scale of interplanetary distances, serving as a fundamental reference point for celestial mechanics. While often approximated as 150 million kilometers, its specific value is derived from the semi-major axis of a planet’s orbit and is used to express the positions and movements of various celestial bodies, providing a consistent scale for analyzing orbital dynamics and stellar neighborhood measurements.