Expert: Francesco Marchesani - 10/13/2006

Question
How did ancient peoples such as the Greeks know the order of the
planets (at least Mercury, Venus, Mars, Jupiter, Saturn) in relation to
the sun?

Answer
Hello,
the Greek knew seven "planets", that is seven celestial bodies that move in the night sky, as opposed to the stars which are fixed. They are, from the closest to Earth to the farthest: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn.
The Greek astronomers guessed the distances by calculating their apparent movement and speed, and size too. So, they made some errors, but also got right the bigger distance of Jupiter and Saturn from Earth (given their very long "revolution", or time they take to make a circular path in the sky).