What measurement forms the basis for defining the Astronomical Unit (AU)?

Average distance between the center of the Earth and the center of the Sun

The Astronomical Unit (AU) is fundamentally anchored to the relationship between our planet and its star. It is precisely defined as the average separation between the geometric center of the Earth and the geometric center of the Sun. This average value is crucial because the Earth's orbit around the Sun is not a perfect circle but rather an ellipse. Consequently, the actual distance varies throughout the year; there is a point where the Earth is closest to the Sun, known as perihelion, and a point where it is farthest, known as aphelion. By using the average distance, scientists establish a stable, fixed baseline for measuring distances across the rest of the solar system, providing consistency that a kilometer measurement, which fluctuates slightly depending on where Earth is in its orbit, cannot easily match.