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It is customary to employ a semi-spherical scale model to describe theapparent path of the Sun across the sky, whether it be its diurnal motion orits variation throughout the year. A flat surface and three bent semi-rigidwires (representing the three solar arcs during solstices and equinoxes) willdo the job. On the other hand, since very early times, there have been famousarmillary spheres built and employed by the most outstanding astronomers forthe description of the celestial movements. In those instruments, many of themnow considered true works of art, Earth lies in the center of the cosmos andthe observer looks at the whole "from the outside." Of course, both devices,the scale model of the sky and the armillary sphere, serve to represent themovement of the Sun, and in this paper we propose to show their equivalence bya simple construction. Knowing the basics underlying the operation of thearmillary sphere will give us confidence to use it as a teaching resource inschool.

From the Scale Model of the Sky to the Armillary Sphere