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There are several traditions of navigation.
The Polynesian navigators routinely crossed thousands of miles of open ocean, to tiny inhabited islands, using only their own senses and knowledge.
In Eastern Polynesia, navigators, in order to locate directions at various times of day and year, memorized extensive facts concerning:
These, and canoe construction methods, were kept as guild secrets. Generally each island maintained a guild of navigators who had very high status, since in times of famine or difficulty, only they could trade for aid or evacuate people. The guild secrets might have been lost, had not one of the last living navigators trained a professional small boat captain so that he could write a book.
The first settlers of the Hawaiian Islands were said to have used these navigation methods to sail to the Hawaiian Islands from the Marquesas Islands. In 1973, the Polynesian Voyaging Society was established in Hawaii to research Polynesian navigation methods. They built a replica of an ancient double-hulled canoe called the Hokule'a, whose crew, in 1976, successfully navigated the Pacific Ocean from Hawaii to Tahiti using no instruments.
There are several different branches of navigation, including but not limited to:
Knowing the ship's current position is the main problem for all navigators. Early navigators used pilotage, relying on local knowledge of land marks and coastal features, forcing all ships to stay close to shore. The magnetic compass allowing a course to be maintained and estimates of the ship's location to be calculated. Nautical charts were developed to record new navigational and pilotage information for use by other navigators. The development of accurate systems for taking lines of position based on the measurement of stars and planets with the sextant allowed ships to navigate the open ocean without needing to see land marks.
Later developments included the placing of lighthouses and buoys close to shore to act a marine signposts identifying ambiguous features, highlighting hazards and pointing to safe channels for ships approaching some part of a coast after a long sea voyage. The invention of the radio lead to radio beacons and radio direction finders providing accurate land-based fixes even hundreds of miles from shore. These were made obsolete by satellite navigation systems.
Traditional maritime navigation with a compass uses multiple redundant sources of position information to locate the ship's position. A navigator uses the ship's last known position and dead reckoning, based on the ship's logged compass course and speed, to calculate the current position. If the set and drift, due to tide and wind, can be determined, an estimated position can also be calculated.
Periodically, the navigator needs confirm the accuracy of the dead reckoning or estimated position calculations using position fixing techniques. This is done by correctly identifying reference points and measuring their bearings from the ship. These lines of position can be plotted on a nautical chart, with the intersection being the ship's current location. Addition lines of position can be measured in order to validate the results taken against other reference points. This is known as a fix.
Traditional celestial navigation systems were based on observation of the relative position of the Sun, Moon and stars. Navigators could determine their latitude by measuring the sun's angle over the southern horizon (if the ship was north of the sun's declination) at noon, and comparing that to the known angle at the same date at their home port. Conceptually they could determine their longitude by measuring the angle over the eastern or western horizon at noon, but to do so would require a much more accurate determination of "noon". The sun moves to the west at 15 degrees per hour, but then moves north and south only a degree or so per hour near noon, making it considerably more difficult to determine when it reaches zenith. The determination of longitude thus became a technological issue, requiring the development of an accurate shipborne chronometer that could tell them exactly when noon was. The need for accurate navigation led to the development of progressively more accurate clocks.
In modern celestial navigation, a nautical almanac and trigonometric sight-reduction tables permit navigators to measure the Sun, Moon, visible planets or any of 57 navigational stars at any time of day or night. From a single sight, a time within a second and an estimated position, a position can be determined within a third of a mile.
Conceptually, the angle to the celestial object establishes a ring of possible positions on the surface of the Earth. A second sighting on a different object establishes an intersecting ring. Usually the navigator knows his position well enough to pick which of the two intersections is the current position. The math required for sight reduction is simple addition and subtraction, if sight-reduction tables are available. The numerous celestial objects permit navigators to shoot through holes in clouds. Most navigation is performed with the sun and moon.
Accurately knowing the time of an observation is important. Time is measured with a chronometer, a quartz watch or a short wave radio broadcast from an atomic clock.
A quartz wristwatch normally keeps time within a half-second per day. If it is worn constantly, keeping it near body heat, its rate of drift can be measured with the radio, and by compensating for this drift, a navigator can keep time to better than a second per month.
Traditionally, three chronometers are kept in gimbals in a dry room near the center of the ship, and used to set a watch for the actual sight, so that no chronometers are ever risked to the elements. Winding the chronometers was a crucial duty of the navigator.
The angle is measured with a special optical instrument called a " sextant." Sextants use two mirrors to cancel the relative motion of the sextant. During a sight, the user's view of the star and horizon remains steady as the boat rocks. An arm moves a split image of the star relative to the split image of the horizon. When the image of the star touches the horizon, the angle can be read from the sextant's scale. Some sextants create an artificial horizon by reflecting a bubble. Inexpensive plastic sextants are available, though they have less accuracy than the more expensive metal models.
Automated navigation systems are almost all based on measuring the time-of-flight of radio waves using the well-known speed of light to measure distance from a number of points. This is possible because of the widespread availability of clocks with high precision and stability.
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