Rambam-The Size of the Celestial Bodies- Yisodei Hatorah perek 3 halacha 8

In this halacha, the Rambam discusses how the science of his time viewed the size of the celestial bodies. He says,


כל הכוכבים הנראים יש מהן כוכבים קטנים שהארץ גדולה מאחד מהן. ויש מהן כוכבים גדולים שכל אחד מהן גדול מן הארץ כמה פעמים. והארץ גדולה מן הירח כמו ארבעים פעמים. והשמש גדולה מן הארץ כמו מאה ושבעים פעמים. נמצא הירח אחד מששת אלפים ושמונה מאות מן השמש בקירוב. ואין בכל הכוכבים כוכב גדול מן השמש ולא קטן מכוכב שבגלגל השני:


"Every celestial body that is visible is either small, in the sense that the Earth is bigger than one of them, or they are large, in the sense that they are many times bigger than the Earth. The Earth is larger than the Moon by a factor of 40. The Sun is larger than the Earth by a factor of 170 times. We find that the Moon is approximately 1/6800 of the Sun's size. The largest celestial body is the Sun and the smallest is Mercury."

The Peirush (Unknown commentator) explains that the Rambam says the word visible because he admits there are some celestial bodies that are invisible because they are too small to see. I think this is an incredible statement since they were able to figure this out with very primitive instruments. Also, it is amazing that they were able to realize that Mercury is the smallest of all the planets.