Twelve-inch quadrant, c 1767.
© Science Museum / Science & Society Picture Library
Quadrant made by John Bird, said to have been used by Captain James Cook (1728-1779). Astronomical quadrants were used to measure the angle of a celestial object from the zenith. Operated in conjunction with a transit instrument, they could be used to check the running rate of astronomical clocks known as regulators. At sea, a quadrant can be used to help fix a ships position and to determine latitude.