A sextant is a navigation instrument used to measure the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celestial navigation. The sextant allows celestial objects to be measured relative to the horizon, rather than relative to the instrument, which allows for excellent precision. Unlike the backstaff, the sextant allows direct observations of stars, which permits its use at night when a backstaff is difficult to use. For solar observations, filters allow direct observation of the sun. Sextants can be used very accurately to measure other visible angles, for example, between one heavenly body and another.
Sextants are commonly used in marine navigation and aviation. They are considered to be an eminently practical backup navigation tool for ships because they are not dependent upon electricity or anything reliant on human-controlled signals such as GPS satellites. Metal sextants offer very high accuracy considering many uncontrollable errors which may exist from such things as refraction, oblateness of the earth, and data tabulation. Generally, a minute of arc (one mile) is about the best anyone can hope to achieve. Plastic sextants commonly exhibit errors more than 5 minutes. Metal sextants are made of either aluminum, bronze, or brass. Some people feel that the heavier weight of a bronze sextant provides greater steadiness and hence more accurate readings, especially if it is windy. Others find that the lightweight models are less tiring to their wrist and arm and that the reduced fatigue gives better results.
The sextant is an expensive, precision instrument that should be handled with utmost care. It has glass optics for a clear view, and no plastic lens is used. Many sextants have an option of either the traditional “half-silvered” horizon mirror or the full silvered horizon mirror. The traditional “half-silvered” horizon mirror is used to reflect the image of the celestial body and the horizon onto the same plane. The full silvered horizon mirror is used to reflect the image of the celestial body and the horizon onto separate planes. What makes a sextant so useful in navigation is its accuracy. It can measure an angle with precision to the nearest ten seconds.
