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The Quadrant

Quadrant Astrolabe

Lent by the National Museum
of American History
Quadrants were used to measure altitudes of celestial objects and were especially useful for mapping the sky. They consisted of a sighting device mounted on a quarter circle, or quadrant, which was engraved with degree markings. Quadrants ranged in size from small hand-held or table-mounted versions to large mural quadrants mounted on walls.

This rare instrument--only seven dating from medieval times are known to exist--combines the features of a quadrant and an astrolabe. Essentially, the circular face of an astrolabe has been "folded over" twice to create a quarter-circle. This instrument could serve as a measuring tool and perform many of an astrolabe's calculation functions as well.

Portable Mural Quadrant

Lent by the National Museum
of American History
This is a replica of a quadrant built in 1573 by Danish astronomer Tycho Brahe to measure the altitude of stars. The observer placed it on a flat surface, swiveled the frame to align the device in the north-south direction, and adjusted the leveling screws to align the vertical edge with the plumb bob. The observer sighted upward from the corner of the quadrant along the moveable sighting arm. Degrees of altitude are marked along the curved edge.

Smithsonian National Air and Space Museum