A sextant<span> is a </span>doubly reflecting navigation instrument<span> that measures the </span>angular distance<span> between two visible objects. The primary use of a sextant is to measure the angle between an </span>astronomical object<span> and the </span>horizon<span> for the purposes of </span>celestial navigation<span>.
The estimation of this angle, the altitude, is known as </span>sighting<span> or </span>shooting<span> the object, or </span>taking a sight<span>. The angle and the time when it was measured, can be used to calculate a </span>position line<span> on a nautical or aeronautical </span>chart. F<span>or example; sighting the </span>Sun<span> at </span>noon <span>or </span>Polaris<span> at night (in the Northern Hemisphere) to estimate </span>latitude<span>. Sighting the height of a landmark can give a measure of </span>distance off<span> and held horizontally.
A sextant can measure angles between objects for a </span>position on a chart.<span> A sextant can also be used to measure the </span>lunar distance<span> between the moon and another celestial object (such as a star or planet) in order to determine </span>Greenwich Mean Time<span> and hence </span>longitude<span>.
