Joseph,
The Rambam states that the moon is one fourtieth the size of earth we now know it is about one fourth the size of the earth.
Rambam states that there is no star bigger then the sun we now know that there are.
Rambam (yisodei hatorah 3:4) states that the sun is located in the sphere between mercury and mars wich would mean that there should be no planets between mercury and mars except for the sun. We now know that this is simply not true (we live on a planet between mercury and mars and we arn’t on the sun!).