What is the primary use of a sextant?

The primary use of a sextant is to measure the angle between a celestial object and the horizon. This essential function allows navigators to determine their position at sea through celestial navigation. By taking measurements of celestial bodies, particularly the sun, stars, or planets, sailors can establish their latitude, and, with additional calculations involving time and other observations, can also determine their longitude.

The sextant operates by allowing one to align the image of the celestial body with the horizon through the use of mirrors and a movable arm, enabling the navigator to read the angle of elevation precisely. This action is crucial for accurate navigation, particularly in open waters where other land-based navigational aids are unavailable.

While measuring the altitude of the sun and calculating longitude and latitude are important aspects of navigation and relate to the use of the sextant, the direct function of the instrument is specifically to measure the angle between a celestial object and the horizon. Determining water depth is not a function of a sextant, as this task is typically performed with a sounding line or echo sounder.