A sextant is a navigational instrument used to measure the angle of elevation of celestial bodies, usually the sun or moon, in order to determine one's location and direction. More generally, a sextant can be used to measure the angle between any two objects. The sextant was first developed around 1730 and soon after began to replace the astrolabe as the navigational instrument of choice. The sextant is still in use today, primarily in nautical contexts, as it is a good backup if more sophisticated systems, such as global positioning, fail.
Sir Isaac Newton was the first to conceive of the doubly reflecting navigational instrument, which would later become the sextant, but the instrument would not come into production until after his death. English mathematician John Hadley and American inventor Thomas Godfrey independently developed the sextant around the same time. The instrument is called a sextant because it spans 60°, or one sixth of a circle. There are similar navigational instruments of different sizes known as the octant and the quadrant.
There are two basic types of sextant. The traditional model features a half-horizon mirror, which shows the horizon on one side of the field of view and the celestial body on the other. The user must move the index mirror, which reflects the celestial body, until the bottom point of the celestial body is lined up with the horizon line reflected by the fixed horizon mirror. The indicator points to the angle of measurement of the celestial body being observed on the arc.
A newer type of sextant features a whole-horizon view, making it easier to find the point where the celestial body just touches the horizon line. The half-horizon mirror performs better in low light, but this is not often an issue. Some sextants also feature an artificial horizon, a mirror that reflects a bubble in a fluid-filled tube, which can be helpful when the real horizon is obscured by fog or other obstacles. Most sextants also offer a filter to protect the viewer's eyes from the sun and to minimize the effects of haze.
Sextants are very delicate and can easily be damaged beyond repair. Even when they are functioning well, they must often be readjusted to provide accurate measurements. If a sextant becomes warped by the weather or by being dropped, it becomes useless. For this reason, sextants often have weatherproof cases and neck lanyards, which are secured before removing the sextant from the case. Navigators are often reluctant to share their sextants, and a sextant should always be purchased new if one plans to use it for navigation, as a used one is likely not to be accurate.