system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Hubble's Law (and how he figured out the universe was expanding)

I've read articles on this, but to an astronomy layman, it's like reading greek.  ELI5 how this amazing mind using a 1930's telescope was able to determine this, please.
gpt
You know how an ambulance sounds higher pitched when it's moving towards you, and lower pitched when it's moving away from you? Light does this too, but it becomes bluer when moving towards you and redder when moving away. Hubble discovered that pretty much every galaxy in the sky was redshifted - moving away from us - and not only that, but the farther away they were, the more redshifted they were, and *not only that*, but the distance from us and the velocity away from us are directly proportional (that's Hubble's Law). It turns out that if you imagine you were observing from a distant galaxy, Hubble's Law would still hold - no matter where you are, it looks the same. Therefore, the universe is said to be expanding.