According to Aristotle, the Sun, Moon, planets and stars - are embedded in perfectly concentric crystal spheres that rotate eternally at fixed rates. Because the celestial spheres are incapable of any change except rotation, the terrestrial sphere of fire must account for the heat, starlight and occasional meteorites. The lowest, lunar sphere is the only celestial sphere that actually comes in contact with the sublunary orb's changeable, terrestrial matter, dragging the rarefied fire and air along underneath as it rotates. Like Homer's æthere (αἰθήρ) - the pure air of Mount Olympus - was the divine counterpart of the air breathed by mortal beings (άήρ, aer). The celestial spheres are composed of the special element aether, eternal and unchanging, the sole capability of which is a uniform circular motion at a given rate (relative to the diurnal motion of the outermost sphere of fixed stars).