Like the planets, the stars are in motion, some of them at tremendous speeds, but they...
Just based on the above, how can we tell which noun the pronoun they refers to: planets or stars? Is there any rule to determine which is which?
The sentence finishes like this:
...are so far away from earth that their apparent positions in the sky do not change enough for their movement to be observed during a single human existence
I know that 'intuitively' you can 100% say that they refers to stars. But, for example, if you replace planets with X and stars with Y, how can you decide what they refers to?