An astronomical unit is like any other unit:
completely arbitrary, but useful. Why do we use a
foot? Or a meter? Or an inch? Or a gallon? It's
just a useful way of thinking of the sizes of
things. So, we have units at all different sizes.
It wouldn't make much sense to measure the
distance from here to Europe in inches (so we use
miles), in the same way it wouldn't be useful to
measure the size of an atom using inches (we use a
unit called Angstroms
Angstroms , which are 0.0000000001 meters).
Astronomical units are just a useful way to
think about the solar system relative to the
distance from Earth to the Sun, because it's
easy to use.
For an example of how easy it is, you could
When you use AU, it is easier to understand the
relative distances, and that Saturn is about ten
times farther from the sun.
- The Earth is 93000000 miles from the sun, Saturn
is 890700000 miles from the sun.
- The Earth is 1AU from the sun, Saturn is 9.6AU
from the sun.