The astronomic model on which ancient Greek navigation was based, which is still in place today, was already extant in the time of Pytheas, the concept of the degrees only being missing. The model divided the universe into a celestial and an earthly sphere pierced by the same poles. Each of the spheres were divided into zones ("zonai") by circles ("kukloi") in planes at right angles to the poles. The zones of the celestial sphere repeated on a larger scale those of the terrestrial sphere.