Sextant
A sextant is a precision instrument that measures angles between visible objects, especially a celestial body and the horizon, so navigators can estimate position at sea.
What a sextant is
A sextant is an angle-measuring instrument best known for celestial navigation. A navigator sights the horizon through the instrument while bringing a reflected image of the Sun, Moon, planet, or star down to meet it. The measured angle is not a position by itself, but it is the raw observation that can be turned into a line of position on a chart.
How the mirrors help
The classic marine sextant uses two mirrors. One mirror moves with an index arm; the other is partly silvered so the observer can see the horizon and the reflected celestial body at the same time. When the reflected body appears to touch the horizon, the navigator reads the angle from the graduated arc and fine adjustment drum.
From octants to sextants
Reflecting instruments improved older sighting tools because they let moving ships compare a celestial body with the horizon more accurately. Octants, with smaller arcs, became important in the 18th century. Sextants extended the useful range and became especially valuable for lunar-distance observations and for the broader practice of precise marine navigation.
The role of time
A sextant needs a trustworthy clock. For latitude, a noon sight of the Sun or a star's altitude can be compared with tables. For longitude, the navigator must know the exact time at a reference meridian, historically Greenwich, and compare the observed sky with predicted positions. Marine chronometers made that calculation far more practical at sea.
Corrections and skill
Real observations need corrections. Navigators account for index error in the instrument, height of eye above the sea, atmospheric refraction, the Sun or Moon's apparent size, and the difference between apparent and true altitude. Good technique also matters: the instrument must be held steadily, the horizon must be visible, and the reading must be timed carefully.
Modern use
Satellite navigation made sextants unnecessary for everyday positioning on most vessels, but the instrument has not disappeared. It remains a teaching tool, a backup method, and a way to understand navigation as a relationship between sky, time, Earth, and chart. Variants also appeared in aviation and spaceflight, where star sightings helped check or align navigation systems.
Why it matters
The sextant marks a practical bridge between astronomy and travel. It turned the sky into measurable information for ships crossing oceans, helped make long-distance routes more reliable, and shows why instruments, tables, clocks, and trained observers all matter in measurement science.