Be it known unto you that the very great sphere168 is the straight sphere. Ill-informed persons have given many erroneous opinions concerning it, for they declared that, since it is the highest and loftiest and swiftest of the spheres, it is the origin of the universe. It completes its course in a day and a night, and contains in itself three hundred and sixty degrees of the Zodiac, and the sphere of the fixed stars moves in a contrary direction to this from the west of the world to the east and is one hundred years travelling one degree169. Each of the spheres of the planets completes its course according to its narrowness or wideness.
Moreover, the very great sphere, which surrounds all the other spheres on every side, controls them and causes them to revolve from the east of the world to the west; and this is the cause of night and day, light and darkness, and of the changes of the seasons, of spring and summer, autumn and winter.
Inside of this sphere everything is protected and controlled and set in motion, lest at anytime they might change their state or position or order, and this causes the planets to revolve so easily while the earth is immovable. For, if the earth were movable, day or night could not preserve their own course, as they do now, and the course of the planets and spheres of the firmament could not be determined, as they now are. There are no stars in that sphere. The ill-informed have said that it has life and that everything receives life from it; but I declare however that great its powers over everything I have mentioned, that it receives these powers from its own creator. As a proof that it is soit is not known what work anybody performs until it has taken effect. Then, since we know every action that is effected by the very great sphere before it has been performed, those actions are performed by some other being, and are not of itself170.