Since Hayy’s home, as I mentioned at the outset, was on the equator, the orbital planes of all the stars were perpendicular to his horizon and their orbits equally large at a given deflection north and south. What is more, both polar axes [79] were visible to him. He observed that when a star with a large orbit and one with a small one rose together, they also set together; and seeing this repeated constantly with all stars, he realized that the firmament must be spherical.
