sextant

A sextant is a handheld 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. It relies on the law of reflection, using two mirrors: a fixed horizon mirror and a movable index mirror attached to an index arm. Light from the horizon and the celestial object is reflected into a telescope, and the observer adjusts the arm until the two images coincide. The altitude is read from a graduated arc, often with a vernier or micrometer drum for increased precision. The instrument's arc spans 60 degrees, hence the name sextant, which denotes a sixth of a full circle.

Construction and operation: A brass or alloy frame supports the 60-degree arc. The index arm carries the index mirror, while the horizon mirror is mounted to allow simultaneous sighting of the horizon and the celestial image. A small telescope aided by shading and filters helps with brightness and accuracy. The reader notes the altitude, then applies corrections for index error, dip of the horizon (depending on eye height), atmospheric refraction, and the apparent semi-diameter of the Sun or Moon.

History: The sextant was developed in the early 18th century, with John Hadley in England and Thomas Godfrey in North America independently contributing to its design around the 1730s. It improved upon earlier quadrants and octants by enabling a larger angular range and greater precision for celestial navigation at sea.

Use and significance: In maritime navigation, the sextant, combined with precise time and a nautical almanac, allows sight reductions to determine latitude and longitude. Although largely supplanted by electronic systems, it remains a reliable backup tool and is still valued by navigators and hobbyists for its enduring precision and independence from power sources.