Final answer:
An Astronomical Unit (AU) is not used to measure the distance between the Earth and the Moon because it is defined as the average distance between the Earth and the Sun. The AU was established by using Earth's diameter as a baseline, which is not necessary for measuring the distance to the Moon.
Step-by-step explanation:
An Astronomical Unit (AU) is not used to measure the distance between the Earth and the Moon because the AU is defined as the average distance between the Earth and the Sun, not the distance between the Earth and the Moon. The AU was established by using Earth's diameter as a baseline to measure distances to nearer planets and asteroids. However, for distances to the Moon, a much shorter baseline is sufficient, so the AU is not necessary.