When describing distances on the scale of our Solar System, conventional units of measure
are not conveniently sized. Miles are too small, and light years are too large, to permit
easily compared numbers. For example, the distance between the Sun and Earth is about
93,000,000 miles; between the Sun and Pluto is about 3,700,000,000 miles. Comparing those
two numbers, it is not easily seen that Pluto is about 40 times as far from the Sun as Earth
The Astronomical Unit (AU) represents a distance of Earth's average distance to the Sun, or
about 93,000,000 miles. Giving the distance between the Sun and Pluto as 40 AU allows the
use of smaller, more easily comparable, numbers. Similarly, a table showing the distance
between the Sun and Mercury as .39 AU gives a much better idea of that planet's position in
the Solar System than does 36,000,000 miles.
So just as it is more convenient to give distances between cities in miles vs. feet, the AU
can be used for measurement in the scale of tens to hundreds of millions of miles.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
'He who finds a thought that lets us even a little deeper into the eternal mystery of nature has been granted great grace.'