The measure of distance becomes complicated when you expand it to include the solar system, where distances between space bodies can be billions of miles. To simplify distance calculations, scientists invented a more basic unit of measure- the AU (or astronomical unit). One AU is equal to the average distance of the Earth from the sun, or roughly 93 million miles. You can clearly see the advantage of such a system. [Source: WNCT9 (Greenville, N.C.)]
