Sextant navigation tool

The sextant is an essential navigation instrument for determining latitude at sea, used since the 18th century.

About this subject

The sextant is an optical instrument that measures the angle between two celestial objects, such as the Sun and the horizon, enabling calculation of a vessel's position. It was independently invented by John Hadley in England and Thomas Godfrey in the United States around 1730, replacing the astrolabe and quadrant. Its name derives from the 60-degree arc (one-sixth of a circle), though some modern models have larger arcs.

During the 18th and 19th centuries, the sextant was crucial for maritime exploration and global trade. It allowed navigators like James Cook to conduct precise voyages to the Pacific, mapping unknown regions. The operating principle uses mirrors to align the horizon with the celestial body, compensating for the ship's motion. Typical accuracy is 0.2 arcminutes.

Despite the advent of GPS, the sextant is still used as a backup instrument on ships and aircraft, especially on long-duration missions. It is also taught in navigation schools as part of traditional training. Interestingly, the sextant can be used for terrestrial distance measurement or object height estimation, beyond celestial navigation.

Frequently Asked Questions

How does a sextant work?

The sextant measures the angle between a celestial body and the horizon using movable mirrors. The navigator aligns the celestial image with the horizon and reads the angle on a graduated scale, which is then corrected and used with nautical tables to determine latitude.

What is the difference between a sextant and an astrolabe?

The astrolabe is older and less accurate, measuring angles with a heavy disc suspended vertically. The sextant is more precise and practical at sea, compensating for the ship's motion using two mirrors.

Is the sextant still used today?

Yes, it is maintained as a backup instrument on ships and aircraft, especially on ocean routes. It is also used by enthusiasts and in traditional navigation training.