system
Given the following post, respond as if you're commenting on Reddit:
human
How did early astronomers know that those slightly brighter dots of light were planets? How did they know the order of the solar system?
gpt
What distinguished the planets from the stars for ancient astronomers was not their brightness, but the fact that some objects were always in the same relative positions to each other in the sky, while others moved around and could be found near different stars at different times.  The ones whose positions were fixed were the *stars* and the ones that moved around were the *planets*.  (In fact, the word "planet" comes from the Greek word for "wanderer," reflecting this property.)

Of course, people did not know what the physical properties of the planets and stars were in terms of their composition and such, but did know that they behaved differently in the sky.