The quadrant was probably the earliest astronomical instrument to be used at sea. It consists of a flat plate in the shape of a quarter-circle, with a plumb-line suspended from the apex. The hand- held quadrant is held in a vertical plane with the right angle away from the eye and the curved edge downwards. One of the straight edges is equipped with a pair of metal pinhole sights which are aligned with an astronomical object and the hanging cord reads off the altitude of this object above the horizon on the circular scale. It was used in the 15th century to measure the altitude of a celestial body, usually the pole star. The English quadrant or back-staff, first described in 1595, was a more complex instrument that measured the altitude of the Sun by the shadow the quadrant cast. The back-staff was more accurate than previous instruments, and had the advantage that the observer had his back to the Sun. It generally superseded the old
quadrant, the astrolabe, and the cross-staff in the 17th century. A larger form of quadrant, known as a mural quadrant, is mounted on a north-south wall and has a solid arm instead of a hanging cord. This obsolete instrument was superseded by the meridian, or transit, circle, a telescope free to move only in the meridian or north-south plane. Research Quadrant
Abbreviations
SciTech
Research Quadrant