Why don’t astronomers use SI prefixes?
The numbers that astronomers use regularly are, well, astronomical. The universe is so big.
Fortunately, the Systeme Internationale, SI, which defines the units which are now overwhelmingly (except in the US) used for science and engineering, has allowed for this and defined a set of prefixes which multiply the basic unit by some power of 1000, to ease the use of the unit.
These are the currently defined SI prefixes.
Why do astronomers not use them? How many times have I heard Brian Cox (bless him) on television talking about billions of kilometres? Many of the prefixes are in common use by the public because of the computer industry - mega, giga and tera certainly are in common use.
Use of the prefixes would also contribute to a better understanding of just how big astronomical distances really are. It’s not immediately clear to the layman just how big a light year is, for example, or why it isn't so simple to just send a rocket to the nearest star. It always amuses me how Star Trek, when reading from the scanners how far it is to the enemy ship, almost randomly will refer to kilometres or light years.
So here are a few common distance markers, approximately, to help the conversion.