Gunter's Quadrant, 1623-1700.
© Science Museum / Science & Society Picture Library
Made of bras, this horary quadrant has engraved scales to a design conceived by the English mathematician Edmund Gunter (1581-1626) in 1623. The horary quadrant was used for finding local time by sighting either the Sun or a bright star at night. Held vertically, a measurement could be taken from the position on the scale of a moveable bead on the thread of the instrument's plumb bob. The instrument was derived from the astrolabe and first appeared during Medieval times. Like its predecesor, it includes a two-dimensional depiction of the heavens that is reduced to a quarter. Gunter invented several other measuring instruments which bear his name, and also introduced the words 'cosine' and 'cotangent' into the language of trigonometry.