Building a Mural Quadrant — The Wall-Mounted Instrument for Precise Star Positions

A mural quadrant is a large quarter-circle arc mounted permanently on a wall aligned precisely north-south. It is the most accurate instrument available for measuring the altitude of celestial objects without a telescope. The observer sights a star through pinholes on a moveable arm that pivots at the centre of the arc, and reads the altitude from degree markings on the curved scale. The larger the quadrant, the more finely the degrees can be subdivided and the more accurate the readings. Tycho Brahe (1546–1601) built mural quadrants with radii of nearly two metres at his observatory Uraniborg on the island of Hven, achieving accuracy of about one arcminute — a remarkable feat without optical aid. His catalogue of over a thousand star positions, measured with instruments like this, later enabled Johannes Kepler to discover that planetary orbits are ellipses. This blueprint builds a functional wooden mural quadrant with a radius of about one metre — large enough to measure star altitudes to within a quarter of a degree.