system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how did the people hundreds of years ago discover planets?

If I look into the sky and I see two yellow dots it’s all stars for me. Then I hear one of them is mars and I’m like „ok, still looks the same“. How did the people hundred of years ago discover what’s a close planet and what’s a far away star?
gpt
Stars are very far away, to someone on Earth they move as one across the sky like dots on a dome being rotated overhead.

Planets are much closer, they moved in a direction that was different from the stars and could be seen over time doing so.  They even at times appear to go "backwards" against the motion of the stars.

"Planet" is itself derived from a Greek word for "wanderer"; this included the moon and sun as well as the visible planets since they also did not move with the stars.