Model for explaining the very slow motion called precession of the equinoxes. The phenomenon—which sparked scientific and doctrinal controversies—was investigated for several centuries. It was one of the arguments cited by Copernicus in support of his astronomical system. The model, actuated by a series of hand-operated gears, also simulates the nutation of the Earth's axis.
If we view the board from the top toward the lower front, the row of visible sides of the painted sticks displays the portrait of Charles III, Duke of Lorraine; if, using the mirror placed in front, we observe the arrangement from the opposite side, we see the portrait of the Grand Duchess Christina of Lorraine, daughter of Charles III and wife of Ferdinand I de' Medici.
From Wikimedia Commons: The observer (right) views a star through the opposite opening (upper left) to determine the star's altitude as it passes through the meridian. An assistant (lower right) reads the time off a clock and another one (lower left) records the measurements. The area above the quadrant is filled with a mural painting showing several other of Brahe's instruments.