Sextants 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.