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A compass (or mariner's compass) is a navigational instrument for finding directions on the earth. It consists of a magnetized pointer free to align itself accurately with Earth's magnetic field, which is of great assistance in navigation. The cardinal points are north, south, east and west. A compass can be used in conjunction with a clock and a sextant to provide a very accurate navigation capability. This device greatly improved maritime trade by making travel safer and more efficient. The compass was developed in China more than seventeen hundred years ago, however the original inventor remains in dispute; legend has it was invented by emperor Huang Di. A compass can be any magnetic device using a needle to indicate the direction of the magnetic north of a planet's magnetosphere. Any instrument with a magnetized bar or needle turning freely upon a pivot and pointing in a northerly and southerly direction can be considered a compass. A compass dial is a small pocket compass with a sundial. A variation compass is a specific instrument of a delicate type of construction. It is used by observing variations of the needle. A gyrocompass or astrocompass can also be used to ascertain True north. History of the navigational compass Although there is tantalizing evidence that the Mesoamerican Olmecs were familiar with magnetism and even developed a compass in the 2nd millennium BC, at present the honor of inventing the compass belongs to the Chinese. The earliest recorded use of lodestone as a direction finder was in a 4th century BC Chinese book: Book of the Devil Valley Master (鬼谷子). The compass is considered one of the Four Great Inventions of Ancient China. Dream Pool Essay written by Song Dynasty scholar Shen Kua in AD 1086 contained a detailed description of how geomancers magnetized a needle by rubbing its tip with lodestone, and hung the magnetic needle with one single strain of silk with a bit of wax attached to the center of the needle. Shen Kua pointed out that a needle prepared this way sometimes pointed south, sometimes north. The earliest recorded use of a compass in navigation lies in Zhu Yu's book Pingzhou Ke Tan (Pingzhou Table Talks) of AD 1117. The navigator knows the geography, he watches the stars at night, watches the sun at day; when it is dark and cloudy, he watches the compass A pilot's compass handbook titled Shun Feng Xiang Song (Fair Winds for Escort) in the Oxford Bodleian Library contains great details about the use of compass in navigation. After this point there is much debate on what happened to the compass. Theories include its travel to the Middle East via the Silk Road, and then to Europe, direct transfer of the compass from China then Europe and then later from Europe to the Middle East, as well as independent creation of the compass in Europe and then its transfer thereafter to the Middle East. The latter 2 are supported by evidence of the Arabic word for "Compass" (al-konbas) possibly being a derivation of the old Italian word for compass. Other evidence for this includes the earlier mentioning of the compass in European works rather than Arabic. The first European mention of the directional compass occurs in Alexander Neckam's De naturis rerum (On the Natures of Things), probably written in Paris in 1190. As for the Arab world, Yemeni Sultan al-Ashraf appears to be the earliest confimed mention of the compass in 1290, though some authors assert an earlier recording, as early as 1242 for Arabic, and 1231 for Persian. In China it seems that the convention was that the compass spoon (they used a spoon instead of a needle) pointed south. Since the European convention has always been the opposite it has been suggested that independent invention may be more likely than cultural diffusion from China in this case. Prior to the introduction of the compass, direction at sea was primarily determined by the position of celestial bodies. Navigation was supplemented in some places by the use of soundings. Difficulties arose where the sea was too deep for soundings and conditions were continually overcast or foggy. Thus the compass was not of the same utility everywhere. For example, the Arabs could generally rely on clear skies in navigating the Persian Gulf and the Indian Ocean (as well as the predictable nature of the monsoons). This may explain in part their relatively late adoption of the compass. Mariners in the relatively shallow Baltic made extensive use of soundings. In the Mediterranean, however, the practice from ancient times had been to curtail sea travel between October and April, due in part to the lack of dependable clear skies during the Mediterranean winter (and much of the sea is too deep for soundings). With improvements in dead reckoning methods, and the development of better charts, this changed during the second half of the 13th century. By around 1290 the sailing season could start in late January or February, and end in December. The additional few months were of considerable economic importance; it enabled Venetian convoys, for instance, to make two round trips a year to the eastern Mediterranean, instead of one. Around the time Europeans learned of the compass, traffic between the Mediterranean and northern Europe increased, and one factor may be that the compass made traversal of the Bay of Biscay safer and easier. Construction of a simple compass A magnetic rod is required when constructing a compass. This can be created by aligning an iron or steel rod with Earth's magnetic field and then tempering or striking it. However, this method produces only a weak magnet so other methods are preferred. This magnetised rod (or magnetic needle) is then placed on a low friction surface to allow it to freely pivot to align itself with the magnetic field. It is then labeled so the user can distinguish the north-pointing from the south-pointing end; in modern convention the north end is typically marked in some way, often by being painted red. Flavio Gioja (fl. 1302), an Italian marine pilot, is sometimes credited with perfecting the sailor's compass by suspending its needle over a fleur-de-lis design, which pointed north. He also enclosed the needle in a little box with a glass cover. Modern navigational compasses Modern navigational compasses hold a magnetized needle inside a fluid-filled capsule; the fluid causes the needle to stop quickly rather than oscillate back and forth around magnetic north. Other features common on modern handheld compasses are a baseplate with rulings for measuring distances on maps, a rotating bezel for measuring bearings of distant objects, and a sighting mirror that lets the user see both the compass needle and a distant object at the same time. Further, some modern compasses include an inclinometer for measuring gradients and are adjustable to account for varying Magnetic declination. Mariner's compasses can have two or more magnetic needles permanently attached to a compass card. These move freely on a pivot. A lubber line, which can be a marking on the compass bowl or a small fixed needle indicates the ship's heading on the compass card. Traditionally the card is divided into thirty-two points (known as rhumbs), although modern compasses are marked in degrees rather than cardinal points. The glass-covered box (or bowl) contains a suspended gimbal within a binnacle. This preserves the horizontal position. Large ships typically rely on a gyrocompass, using the more reliable magnetic compass for back-up. Increasingly electronic fluxgate compasses are used on smaller vessels. Compasses are available marked in mils - a unit of measurement commonly used by the military. Solid state compasses Small compasses found in clocks, cell phones (e.g. the Nokia 5140i) and other electronic gear are Solid-state electronics usually built out of two or three magnetic field sensors that provide data for a microprocessor. Using Trigonometry the correct heading relative to the compass is calculated. Often, the device is a discrete component which outputs either a digital or analog signal proportional to its orientation. This signal is interpreted by a controller or microprocessor and used either internally, or sent to a display unit. An example implementation, including parts list and circuit schematics, shows one design of such electronics. The sensor uses precision magnetics and highly calibrated internal electronics to measure the response of the device to the Earth's magnetic field. The electrical signal is then processed or digitized. Compass correction
Using a compass
Compass balancing Because the Earth's magnetic field varies at different latitudes, compasses are often balanced during manufacture. Most manufacturers balance their compass needles for one of five zones, ranging from zone 1, covering most of the Northern Hemisphere, to zone 5 covering Australia and the southern oceans. This balancing prevents excessive dipping of one end of the needle which can cause the compass card to stick and give false readings. Some different compass systems: Image:Boussole_en_grades_table_conversion.jpg|Compass with 400 grades division and conversion table Image:RECTA_full_syst.jpg|Swiss army compass with mils division Image:Boussole_avec_inclinomètre.jpg|Land surveyor compass with clinometer Image:Stratum_compass-clar_hg.jpg|Stratum compass after Prof. Clar Points of the compass The mariner's compass card is divided into thirty-two equally spaced points. Four of these - east, west, north, and south - are the cardinal points, and the names of the others are derived from these. See also Gallery Image:Compass_Fitted_To_A_Yacht.jpg|A simple compass typical to a small yacht Image:Silva_Sighting_Compass.jpg|A compass suitable for orienteering in the night Notes | |||||||||||
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