人教版必修5 Unit 1 Using Language
3. What had the astronomers noticed in the sky? (Para 2)They had noticed that some planets in the sky seemed to stop, move backward and then go forward in a loop. Others appeared b
righter at times and less bright at others. 4. When did Copernicus complete his theory? (Para 3) Between 1510 and 1514 he worked on it, gradually improving his theory until he felt it was complete.