Quadrant

Quadrant (Lat. quadrans, 'a fourth part'), literally the fourth part of a circle, or 90°; but signifying, in Astronomy, an instrument used for the determination of angular measurements. The quadrant consisted of a limb or arc of a circle equal to the fourth part of the whole circumference, graduated into degrees and parts of degrees. Picart was the first who applied telescopic sights to this instrument. Quadrants were adjusted in the same way as the mural circle. Various innate defects of the quadrant—such as the impossibility of securing exactness of the whole arc, concentricity of the centre of motion with the centre of division, and perfect stability of the centre-work—led to its being superseded by the repeating circle, otherwise called the Mural Circle (q.v.). Hadley's Quadrant is more properly an octant, as its limb is only the eighth part of a circle, though it measures an arc of 90°. Its principle is that of the Sextant (q.v.).