- A sextant is an instrument used for celestial navigation.Stockbyte/Stockbyte/Getty Images
A sextant is a navigational instrument that measures the angle between the horizon and a given astronomical object using two mirrors. This measurement can be used to determine the latitude and longitude of the observer. Although navigating using a sextant is a complex process requiring the use of navigation tables, the basic principles behind it are quite simple. - A sextant is most commonly used to find an observer's latitude, which is a measure of how far north or south of the equator an object is located. To do so, the sextant is used to measure the angle between the sun when it is at its highest point, which is at noon, and the horizon. This measurement is used in combination with navigational tables to find what line of latitude the sun will be above at noon on that particular day.
- A sextant also can calculate an observer's longitude when it is used in combination with a highly accurate clock. Longitude is a measurement that indicates how far east or west of the prime meridian an object is located. Every hour, the earth moves by 15 degrees. Therefore, if you know what longitude the sun was directly above at noon, and if you know what time it was when the sun was directly above you, you can use another set of navigational tables to find your current longitude.
- Sextants also were used in astronomy to measure the distance between two stars and to determine the altitude of celestial objects. Today, these measurements are made using telescopes and satellites, but in the 16th century, astronomers such as Tycho Brahe used sextants to make many revolutionary astronomical observations. These astronomical sextants were much larger than navigational sextants, as they could be built onto towers and did not have to be carried on ships. They also had an artificial horizon, so that measurements could be made between two celestial objects rather than between a single celestial object and the horizon.
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