Now you are aware that "universe" is the name given by most astronomers to the sphere the center of which is the center of the earth, and the radius of which is equal to the straight line between the center of the sun and the center of the earth; this you have seen in the treatises written by astronomers. But Aristarchus of Samos brought out writings consisting of certain hypotheses, in which it appears, as a consequence of the assumptions just made, that the universe is many times greater than the "universe" just mentioned. His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun in the circumference of a circle, the sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same center as the sun, is so great that the circle in which he supposes the earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.