By Mark Slingo
Updated Mar 24, 2022
A sextant is a precision instrument used by navigators to measure the angle between the horizon and a celestial body—be it the Sun, Moon, or a star. The device, whose name derives from the Latin sextus meaning “one‑sixth,” features a 60° arc that allows for precise angular measurements essential for determining longitude and latitude at sea.
This tiny but crucial screw adjusts the position of the horizon mirror, ensuring that the reflected image aligns perfectly with the direct view for accurate readings.
The frame forms the robust skeleton of the sextant, providing a stable foundation for all other components and protecting the delicate mechanisms from the rigors of maritime use.
Release levers disengage the index arm from the arc scale, permitting free movement. This feature is vital for resetting the instrument between measurements.
A colored transparent strip, similar to sunglasses, shields the observer’s eyes from direct sunlight while still allowing the celestial target to be viewed safely.
The telescope is the eye‑piece’s magnifier, directing the observer’s gaze to the horizon glass and providing the necessary magnification for precise alignment.
This reinforcing ring attaches the telescope securely to the sextant frame, maintaining a steady line of sight during navigation.
The eyepiece is the final optical element through which the navigator looks, translating the telescope’s magnified image into a clear, focused view.
Also known as the telescopic tube, this component connects the eyepiece to the telescope and can be twisted to fine‑tune the focus.
The horizon glass is a bifacial piece: one side is silvered to act as a mirror, while the other remains clear. It allows the observer to see a celestial body directly on one side and its reflection beside it, a classic “mirror and sight” configuration.
Mounted beneath the index arm, the index plate is a polished surface that reflects objects onto the horizon glass, enabling the navigator to overlay the celestial image with the reflected horizon.
Attached to the index arm, the Vernier scale provides a fine‑tuned measurement of tenths of a degree, allowing for precise angle determination.
The micrometer drum, located at the lower end of the index arm, allows for minute adjustments of the angle. It displays the minutes of a degree, completing the sextant’s precise measurement capability.
Mastering these components ensures accurate celestial navigation, a skill that has guided seafarers for centuries.
