A precision optical instrument used to measure the angle between a celestial body and the horizon for position fixing.
Quick facts
Regulation
SOLAS Chapter V
In practice
The sextant requires periodic checking and adjustment for index error—the small residual error read when the instrument is set to zero and the horizon appears as a straight line—and for other errors including side error, perpendicularity, and collimation. At sea, the OOW takes sights from the bridge wing or exposed upper deck, timing each observation with the chronometer. In heavy weather, sights may be impossible, reinforcing the importance of DR skills as a complement to celestial work. Under SOLAS Chapter V and STCW, all navigating officers must be capable of using a sextant and reducing observations to a position, maintaining this ancient skill alongside modern electronic systems.
Regulatory detail & full definition
A sextant is a precision optical instrument used to measure the angle between two objects—most commonly a celestial body and the natural horizon—to a precision of better than one minute of arc. The instrument contains a graduated arc spanning one sixth of a circle (hence the name), two mirrors, and a telescope; by rotating the index arm, the observer brings the reflected image of the celestial body down to coincide with the horizon seen directly, and reads the angle from the scale and micrometer drum. Bowditch (American Practical Navigator) describes the instrument's optical system, its adjustments, and the corrections applied to the observed altitude.
The sextant requires periodic checking and adjustment for index error—the small residual error read when the instrument is set to zero and the horizon appears as a straight line—and for other errors including side error, perpendicularity, and collimation. At sea, the OOW takes sights from the bridge wing or exposed upper deck, timing each observation with the chronometer. In heavy weather, sights may be impossible, reinforcing the importance of DR skills as a complement to celestial work. Under SOLAS Chapter V and STCW, all navigating officers must be capable of using a sextant and reducing observations to a position, maintaining this ancient skill alongside modern electronic systems.