A sextant is a navigation instrument used to measure the angle between a celestial body and the horizon. Sailors have used it for centuries to find their position at sea when landmarks are not visible. On ships and submarines, it connects careful observation with geometry, astronomy, and timekeeping.
Its main job is to turn the sky into useful information about location.
Key Facts
- Altitude angle = angle between a celestial body and the visible horizon.
- At local noon, latitude can be found from Sun altitude using latitude = 90° - corrected altitude + declination, with signs chosen for hemisphere.
- A sextant uses two mirrors so the observer can see the horizon and the celestial body in the same line of sight.
- Index error = sextant reading when the true angle should be 0°.
- Corrected altitude = observed altitude + instrument correction + horizon correction + atmospheric correction.
- Earth rotates 15° per hour, so accurate time is essential for finding longitude.
Vocabulary
- Sextant
- A sextant is an optical instrument that measures the angle between two objects, usually a celestial body and the horizon.
- Altitude
- Altitude is the angular height of an object above the horizon, measured in degrees.
- Horizon mirror
- The horizon mirror is the partly silvered mirror in a sextant that lets the observer see the horizon and the reflected celestial body together.
- Index arm
- The index arm is the movable arm of a sextant that rotates the index mirror and sets the measured angle.
- Celestial navigation
- Celestial navigation is the method of finding position using observations of the Sun, Moon, planets, or stars.
Common Mistakes to Avoid
- Reading the scale before the celestial body touches the horizon is wrong because the measured angle must be taken when the image is aligned with the horizon.
- Ignoring index error is wrong because even a small offset in the sextant can shift the calculated position by several nautical miles.
- Using local clock time instead of a standardized time reference is wrong because longitude depends on comparing observations with precise time.
- Forgetting atmospheric and horizon corrections is wrong because refraction, eye height, and the apparent horizon make the observed angle different from the true altitude.
Practice Questions
- 1 A navigator measures the Sun's altitude as 52.0°. The total correction is +0.4°. What is the corrected altitude?
- 2 At local noon, the corrected Sun altitude is 61.5° and the Sun's declination is 20.0° north. If the observer is in the Northern Hemisphere and the Sun is due south, estimate the latitude using latitude = 90° - altitude + declination.
- 3 Explain why a sextant can still be useful on a ship even when electronic navigation systems exist.