In the thirteenth century the astronomer al-'Urḍi proposed a radical change to Ptolemy's system of nesting spheres. In his "Kitāb al-Hayáh", he recalculated the distance of the planets using parameters which he redetermined. Taking the distance of the Sun as 1,266 Earth radii, he was forced to place the sphere of Venus above the sphere of the Sun; as a further refinement, he added the planet's diameters to the thickness of their spheres. As a consequence, his version of the nesting spheres model had the sphere of the stars at a distance of 140,177 Earth radii.