This model of the lunar sphere, made by Girolamo della Volpaia, shows the system of four partial spheres in which the Moon was set, according to Georg von Peurbach. Three partial spheres, designed to resemble three spheres of the Sun, move the small lunar epicycle on an eccentric path around the Earth. A fourth partial sphere lies in a more external position. Its axis is tilted slightly away from that of the first three. The fourth sphere introduces into the model the retrograde motion of the lunar nodes. These are the points of intersection between the monthly path of the Moon and the annual path of the Sun—points at which eclipses can occur. This model broadly reflects the lunar theory expounded in Ptolemy's Almagest.

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