With the sextant he made obeisance to the sun-god, he consulted ancient tomes and tables of magic characters, muttered prayers in a strange tongue that sounded like Indexerrorparallaxrefraction, made cabalistic signs on paper, added and carried one, and then, on a piece of holy script called the Grail - I mean, the Chart - he placed his finger on a certain space conspicuous for its blankness and said, "Here we are." When we looked at the blank space and asked, "And where is that?" he answered in the cipher-code of the higher priesthood, "31 -15 - 47 north, 133 - 5 - 30 west." And we said, "Oh," and felt mighty small.
Jack London, The Cruise of the SnarkA sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight and it is an essential part of celestial navigation. The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart. A common use of the sextant is to sight the sun at solar noon and to measure the elevation (altitude) angle at night to measure the elevation angle from the horizon plane to Polaris to find one's latitude. Since the sextant can be used to measure the angle between any two objects, it can be held horizontally to measure the angle between any two landmarks which will allow for calculation of a position on a chart. A sextant can also be used to measure the Lunar distance between the moon and another celestial object (e.g., star, planet) in order to determine Greenwich time which is important because it can then be used to determine the longitude.