system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know what the Milky Way looks like?

If we're part of the Milky Way how do we know what it looks like? How do we observe a galaxy that we're part of?
gpt
Three things
1 - We understand the orbital mechanics of galaxies.  We understand what shapes are possible and what are not possible due to the current laws of physics.

2 - We can look at other galaxies and see examples.  For example, we see that spiral is a common shape for a galaxy.  

3 - Most importantly, by looking at the stars surrounding us in the night sky, we can get an approximate picture of what the galaxy looks like.  If you're on a highway and see a lot of cars in front of you and few behind you, you can estimate what the scene might look like to a helicopter even if you're on the ground yourself.  Then using #1 and #2 we can match up what we see with what we expect to see and get our final picture.