Sextants

Revised by BoatUS editors in April 2012

SextantsSextants have been the primary device for offshore navigation for centuries, initially developed to guide mariners on trade route passages. While they are used to measure angles between a celestial body and the horizon, they can also be used to measure the apparent height of a known landmark for determining range, or to measure the angle between two landmarks to establish a line of position. With today’s electronic equipment, finding your way offshore without the benefit of cans, markers, and buoys can be done quite well without the benefit of a sextant, unless, of course, the electrical system, hardware or software goes haywire. 

Celestial navigation isn't difficult, but it does require a little skill with numbers and familiarity with the Sight Reduction Tables. Technique should be taken into consideration. Taking a sight requires practice and, of course, an accurate sextant. A sextant's accuracy is expressed in "seconds of arc". Each minute of angular measurement represents a distance of one nautical mile, so sextants can generally read out to one-fifth or one-tenth of a minute. 

From a small boat, most navigators can expect an accuracy of within a few miles. While navigators tend to prefer traditional metal instruments, many experienced boaters rely on one of the inexpensive plastic models. 

 

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