Treatise on the Armillary Sphere

This Judeo-Arabic text describes the construction and use of an armillary sphere, a scientific instrument that models the cosmos: the earth in the center, surrounded by rings representing the celestial equator, the ecliptic with the zodiac, and so on. Rotating the rings mimics the motion of the earth and the planets. Armillary spheres were reported in the Islamic world as early as the ninth century; unfortunately, none has survived from the early medieval period. In this brief excerpt, Dunash describes the spherical shape of the earth and the heavens.