In 1757, John Bird invented the first sextant. This replaced the Davis quadrant and the octant as the main instrument for navigation. The sextant was derived from the octant in order to provide for the lunar distance method. With the lunar distance method, mariners could determine their longitude accurately. Once chronometer production was established in the late 18th century, the use of the chronometer for accurate determination of longitude was a viable alternative. Chronometers replaced lunars in wide usage by the late 19th century.