What specific orbital characteristic necessitates using the *average* Earth-Sun separation for the AU?
Answer
Planetary orbits are ellipses, involving perihelion and aphelion variations.
Planetary motion is governed by Kepler's laws, dictating that orbits trace out ellipses rather than perfect circles. This means that the distance between the Earth and the Sun is constantly changing over the course of a year. The closest point in the orbit is termed perihelion, and the farthest point is aphelion. If the AU were based on the distance at perihelion or aphelion, its value would fluctuate depending on when the measurement was taken. Defining the AU as the *average* distance smooths out these periodic variations, yielding a stable unit of measure that remains constant and reliable for long-term astronomical mapping and comparison.
Related Questions
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