When is the astronomical unit used?
The Astronomical Unit (AU) is the unit of measure for distances in the solar system. Any time you express distances between objects in the solar system, it is more convenient to express it in astronomical units than in meters/kilometers. The astronomical unit is the large semiaxix of the Earth orbit around the Sun. This is exactly the average between the smallest distance and the largest distance between Sun and Earth (or, more precisely, the Earth-Moon barycenter). Due to the non-uniform motion of Earth around the Sun, the AU does not coincide with the average Sun-Earth distance, but it is close (especially since the excentricity of the Earth orbit is small) For any activity, it is useful to define a unit in the same order of magnitude as the distances in normal use. In the solar system, it was easy to find the distances in the solar system in in values relative to each other, so the distance between Sun and Earth became the natural etalon; the position of the planets and theit motion
