system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we use parsecs instead of 3.26 lightyears?

I just learned this in school so I know near nothing about parsecs, so sorry if the answer is obvious. I just don't see the reason to say parsecs instead of using lightyears everywhere, you know, using one word and not multiple ones would be less confusing. Or are there certain things you can only do with parsecs?
gpt
Parsecs are definitely the convenient unit for astronomy. For some reason, light-years have become the standard unit used in communication with the public. Light-years are an easier concept to grasp (i.e. the distance travelled by light in one year), but parsecs really are more convenient to do maths with.

The parsec comes directly from how distances to stars are measured. As the Earth goes around the Sun, we are constantly seeing stars from different angles. This causes stars to move in little ellipses in the sky. This is called "parallax". The size of this effect tells us how far away the star is. You can see a similar effect when driving in the country. The fenceposts by the road appear to be moving really fast while the distant mountains appear to the be totally stationary.

A star is one parsec away if it moves in a little circle with a radius of one arc-second (1/60th of 1/60th of one degree) over a year. If its little circle is half an arc-second, it's two parsecs away. If it's 1/10th of an arc-second, it's 10 parsecs away. So, given that astronomers like using arc-seconds to measure angles in the sky, using parsecs makes the mathematics really simple. It's a geometric unit that we can measure directly. Light-years are less direct, because we can't directly time how long it takes light to reach us from a distant star - we have to calculate it from the speed of light and the distance.

There's also a convenient trick that 1 km/s is extremely close to 1 parsec per million years. Alpha Centauri is a bit over a parsec away. So if you know your speed in km/s, you can very easily calculate how many years it would take to reach Alpha Centauri or whatever.

So the real question is: if we actually use parsecs all the time, and they're more convenient units, why do we teach kids about light-years? It's probably because they're just a little bit easier to teach, but that's very much a social question rather than an astronomy one.