Quadrant. Quarter of a circle. 2. Nautical reflecting instrument that preceded sextant and measured angles up to 90°. Invented by Thomas Godfray of Philadelphia and John Hadley of England in 1730, each being independent of the other. 3. A 13th century instrument for measuring the altitude of a star. It was a thin plate of metal or wood, a quarter of a circle in shape, marked with degrees round the arc and with pin-hole sights on one edge. A plumb line hung from the centre. Two observers were needed, one sighting the star, the other reading the scale. 4. Horizontal fitting, that is a sector of a circle, attached to head of rudder stock to take steering chains and form leverage for controlling rudder. Sometimes carries a toothed rack into which a pinion is meshed.