Sailors have always navigated by observing the heavens. By measuring the height of the sun, stars or planets above the horizon they could determine their position. In the 15th century mariners used quarter circles with two mirrors situating the celestial body observed in relation to the horizon line. These instruments were superseded by the nautical astrolabe, the octant (whose 45° arc enables the measurement of heights up to 90°), then the sextant (with a 60° arc). They were simpler to use and also more precise. The sextant determines the latitude to within several hundred metres. Present-day satellite navigation systems have rendered this type of instrument obsolete.