Which instrument is primarily used for celestial navigation?

The sextant is primarily used for celestial navigation because it allows mariners to measure the angle between a celestial body (such as the sun, a star, or the moon) and the horizon. This measurement is crucial for determining the observer's latitude and longitude at sea. The sextant operates on the principle of triangulation, where by measuring the angle of a celestial object above the horizon, navigators can refer to pre-computed tables to find their position on Earth.

A compass, while essential for understanding magnetic direction, does not provide the necessary celestial measurements for navigation. A GPS receiver is useful for modern navigation but relies on satellite technology rather than celestial observations. A chronometer is primarily used to keep accurate time, which is important in celestial navigation but does not serve as the primary instrument for taking celestial measurements. Thus, the sextant stands out as the key instrument in traditional celestial navigation methods.