How did the process of astronomical unit develop?

A.U. is simply the average distance from to . They discovered that talking about distance in terms of MILLIONS of kilometers or miles between planets was ridiculous, and needed a new unit, so they invented one for INTERPLANETARY distances (distances between planets within the SAME solar system). When you want INTERSTELLAR distances (between stars), you need a Light year, which is a much larger unit of distance than an A.U.