16. The Stars

Yesodei HaTorah 3:7

The ninth sphere does not itself have any divisions, nor are the constellations actually in this sphere. Rather, the stars of the constellations are in the eighth sphere. The signs of the Zodiac correspond to the divisions of the ninth sphere as things were at the time of the flood. Since then, the constellations have migrated a little because the stars are in motion, albeit very slow motion. It would take a star about 70 years to move the same distance that the sun or the moon could move in a day.

Yesodei HaTorah 3:8

Among the stars that we can see, there are small ones (which were believed to be smaller than the Earth) and large ones that are much larger than the Earth. The Earth is about 40 times the size of the moon; the sun is about 170 times the size of the Earth. Therefore, the sun is about 6,800 times the size of the moon. None of the other stars is as large as the sun, nor as small as the planet Mercury.

Editor’s note: I am not able to explain the Rambam’s astronomy. In Hilchos Kiddush HaChodesh (17:24), he says that it is based on the science of Greek astronomers. In Moreh Nevuchim, he is willing to disregard Talmudic dicta (3:14) and even Biblical verses (2:25) in favor of observable science when necessary. Readers are free to assume either that the Rambam was sharing the best science of his day, or that he is describing spiritual rather than physical phenomena.