What does octant mean?
An octant is fundamentally one eighth of a circle or of a complete angle. The word also names a compact reflecting instrument used to measure angles, particularly the angle between a celestial body and the horizon. Navigators used such instruments at sea to calculate latitude before modern electronic navigation became standard. In mathematical and surveying contexts, an octant can refer simply to an angular section equal to 45 degrees, since a full circle contains eight such sections. The term is less common in everyday speech but remains useful in geometry, astronomy, maritime history, and the history of navigation. Its instrument sense carries a historical and technical character, while its mathematical sense is precise and formal.
One eighth of a circle or of a complete angle.
"The compass rose was divided into eight octants."
A technical mathematical term; an octant measures an angle of 45 degrees.
A reflecting instrument for measuring angles, especially the angle between the horizon and a celestial body.
"He checked the ship's latitude with the octant."
Chiefly a historical nautical term; the instrument was widely used before electronic navigation.
The plural is formed by adding -s.
"The museum displayed several octants used by eighteenth-century navigators."
Before modern satellite navigation, an octant helped sailors determine their position by measuring the angle between the horizon and a celestial body.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does octant come from?
The word comes through Latin from octo, meaning “eight,” and ultimately from the same Indo-European root that produced English “eight.” In the mathematical sense, the name reflects the division of a circle into eight equal parts. The instrument was named for the arc of one eighth of a circle that it used, and the English word entered use in the 18th century in connection with navigation and surveying.
Related word forms
How octant is actually used
The mathematical sense is formal and technical. The instrument sense is chiefly historical and nautical, although octants remain in use in some surveying and astronomical contexts.
Easily confused with octant
An octant measures angles using one eighth of a circle, whereas a quadrant is based on one quarter of a circle; both can refer to navigational or surveying instruments.