Greetings, all. This is my first post here.

I've been doing amateur astronomy for more than four decades, and one thing that I haven't quite understood is the continuing use of distance units "AU" (149.6 gigameters), light year (9.461 petameters) and "parsec" (30.86 petameters) instead of SI units.

Long ago in the pre-web days I suggested that we use the term nautical light year (NLY) to indicate a distance of 10 petameters. (Compare with "nautical mile".) I was told another had come up with the identical idea earlier, but it gained no traction.

I have also heard the suggestion that we could us the term "sagan" for a billion billion meters (1 exameter).

These are in use because they are "natural" distance scales in astronomy.

An AU (astronomical unit) is the mean distance from the Earth to the Sun, which is a very useful length for comparing distances within the Solar system (or within others!), not to mention very relevant for research that involves the Earth's motion.

A light year is the distance that light travels in a year. Since light travel time is exactly the time lag in a signal (such as an astronomical observation), it makes sense to use "light-years" - If a thing is 10 light-years away, we are seeing it as it was 10 years ago.

A parsec is a little more than 3 light-years, and is derived from the the term "parallax-second", or an apparent motion of a star of one second of arc (1/3600th of a degree) over the course of an Earth-year. While the parsec has technically been superseded by the light-year in many ways, it does seem to be a more natural distance with in the Galaxy; the nearest star is 4.2 light-years away, but only 1.3 parsecs. Galactic astronomers (such as MilkyWay@home researchers) traditionally use "kiloparsecs" (thousands of parsecs) as their chosen distance scale.

With any unit, there is a trade-off between mathematical simplicity and human relevance. A mile was originally defined as a thousand paces, with a pace being ~5.28 feet, or the distance an average human covers in two steps. A pace is also close to the average height of a human. Personally, I don't feel like walking a kilometer is a very far distance, but a mile feels significant. I kind of wish that we were using paces and miles instead of meters and kilometers...