Astronomical instruction. Instructor (right) teaching students the principles of astronomy and navigation. He is pointing to an armillary sphere. This device (and the diagram below it) models the celestial sphere. Circles around the Earth represent the horizon, the tropics, meridians, and the plane of the ecliptic (zodiac). At left, two diagrams show the geocentric cosmologies of the 16th century Danish astronomer Tycho Brahe (Tychonic system) and the 2nd century Greek-Egyptian astronomer Ptolemy (Ptolemaic system). On the wall is a compass rose, as used by mariners during the explorations that took place during the Age of Discovery.