What is an Astronomical Unit?

This is the distance between Earth and Sun which equals about 93 million miles or 149 million kilometers.

Kepler's Third Law of Planetary Motion lets astronomers determine the distance to each planet from the Sun, once their orbit periods are known. But this distance is in multiples of the Earth-Sun distance which has the value of exactly 1.0 in this scale model. To determine the actual physical distances, you need to know what the Earth-Sun distance is in miles. In terms of Astronomical Units, the distances to the planets from the sun are as follows:

Mercury     0.38
Venus       0.72
Earth       1.00
Mars        1.52
Jupiter     5.20
Saturn      9.54 
Uranus     19.18
Neptune    30.06
Pluto      39.53
You can make a scale model of the solar system from these numbers if you set 1 astronomical unit equal to, say, 10 feet. Pluto would then be 400 feet from the sun, and mercury would be 3.8 feet from the sun.