The sextant was the first navigational instrument to have a telescopic sight attached. The instrument takes its name from its shape: a sixth of a circle. Astronomical sextants had been in use since the 16th century, but a marine version was developed in about 1757 by Captain John Campbell with the help of the London instrument maker, John Bird. Following its introduction, the sextant became one of the principal navigational instruments used on ships throughout the world.