sextants

A sextant is a navigational instrument used to measure the angle between two visible objects, typically the horizon and a celestial body such as the sun, moon, or a star. By combining this angle with precise time, navigators determine their position at sea. The instrument relies on a reflective optical system and a fixed arc.

The core features a 60-degree graduated arc with a movable index arm carrying an index mirror. A fixed horizon mirror splits the light so the observer can see the horizon directly while a reflected image of the celestial body is viewed through a telescope. The mirrors are arranged to bring the two images together, and the angle is read from the arc using a vernier or a micrometer for fine measurements. Many sextants include shading or filters for glare and, in higher-quality models, a micrometer drive for improved precision.

History and use: The sextant was developed in the early 18th century by John Hadley and independently by Thomas Godfrey. Its accuracy, ruggedness, and ability to measure large angles made it indispensable to maritime navigation for centuries. Sailors used it to determine latitude by measuring the sun or a star’s altitude and, with precise time, to determine longitude. In the late 20th century electronic systems largely supplanted it for routine navigation, though sextants remain valued as backups and for celestial navigation training. Lunar-distance methods historically used sextants to help determine longitude, requiring accurate time sources.

Variants and accuracy: A sextant can measure angles up to about 120 degrees, and readings are typically taken to a fraction of a minute of arc under favorable conditions. Higher-grade instruments use refined verniers or micrometer screws. The octant is an earlier predecessor with a 45-degree arc.