Celestial navigation has been used since 1845. It often involves the use of a sextant, an instrument measuring angles between stars and the visible horizon. Often used in conjunction with a sextant are a chronometer, a nautical almanac and sight reduction tables, reducing the need for a lot of mathematical computation. Using the sextant, measurements of star angles are taken at certain times of each day, then plotted on a nautical chart. Depending on the location of the "sightings" plotted on the map, a seaman using a sextant can approximate his location to within a 3-mile radius.
The U.S. Naval Academy discontinued its course on celestial navigation in 1998. Midshipmen still continue to use a sextant, though they feed the information they get into a satellite-linked computer. The computer performs all computations and pinpoints a location to within 60 feet. The U.S. Merchant Marine Academy still offers courses in celestial navigation, as it is required to pass the U.S. Coast Guard License Exam.
Terrestrial navigation is not used as much as celestial navigation, but is still an accurate method. The process involves using both a magnetic and a gyro compass together with the determination of latitudinal and longitudinal locations. The information is plotted on a map in relation to what is known as either the Greenwich Meridian or Prime Meridian. This longitudinal meridian runs from the North Pole through Greenwich, England, to the South Pole, and its location is the basis from which terrestrial navigation is derived. Buoy lights, land masses and other fixed objects are often noted on nautical maps and are used to accompany compass readings and mappings of latitude and longitude. They help accommodate for variations in calculations, such as boat drift or wind conditions, and help provide more accurate location measurements.
Private yachtsmen still use celestial navigation, particularly long-distance cruising yachts going around the world. Celestial navigation is considered an essential skill for small cruising boat crews, especially when venturing beyond the visual range of land. Although GPS technology is extremely accurate, offshore boaters often combine its use with that of celestial navigation.