Item History: The Gunter’s quadrant is a simplified version of the elegant, but complicated, Arabic astrolabe. It is typically a brass or wood instrument with a scale of 0 to 90 degrees. It has two peepholes along one edge for sighting the sun, the moon, stars or the planets. The astrological figures on the quadrant were typically calculated for a specific latitude which limits the instrument's usefulness at sea. A plum bob on a string falls across the scale to indicate the altitude, or angle of elevation, of the celestial body above the horizon. The brass is engraved with several geometrical figures: a scale for the declination of the sun above or below the equator, a stereographic projection of the equator, the signs of the Zodiac, and the positions of several stars around the north pole. A shadow square for solving plane triangles was provided. If the quadrant was made of wood, the figures were printed on paper and pasted on the wood.
The instrument is named for Edmund Gunter, an outstanding mathematician specializing in spherical trigonometry as it applied to astronomy, who was born in Hertfordshire England in 1581. He first described the quadrant in the 1623 publication “De Sectore & Radio.” While Gunter never manufactured the quadrant it was produced by professional instrument makers. The quadrant presented here was produced by the eminent instrument maker Henry Sutton in 1657. Sutton was famous for the accuracy of his scale divisions.
The principle use of the Gunter quadrant was to tell the time of day. After the observer used the quadrant to determine the altitude of the sun; determined the declination of the sun for that date; and determined his latitude the scales could be used to determine the time.