A compass, magnetic compass or mariner's compass is a navigational instrument for determining direction relative to the earth's magnetic poles. It consists of a magnetized pointer (usually marked on the North end) free to align itself with Earth's magnetic field. The face of the compass generally highlights the cardinal points of north, south, east and west. The compass greatly improved maritime trade by making travel safer and more efficient. A compass can be used to calculate heading, used with a sextant to calculate latitude, and with a marine chronometer to calculate longitude. It thus provides a much improved navigational capability, that has only been recently supplanted by modern devices such as the gyrocompass and the Global Positioning System (GPS).
Fundamentally, the classic compass is any magnetically sensitive device able to indicate the direction of the magnetic north of a planet's magnetosphere. Often compasses are built as a stand alone sealed instrument with a magnetized bar or needle turning freely upon a pivot, or moving in a fluid, thus able to point in a northerly and southerly direction. An early form of the compass (a magnetized needle floating in water) was invented in ancient China sometime before 1044. The dry compass was invented in medieval Europe around 1300. This was supplanted in the early 20th century by the liquid-filled magnetic compass.
Other, more accurate, devices have been invented for determining north that do not depend on the Earth's magnetic field for operation (known in such cases as true north, as opposed to magnetic north). A gyrocompass or astrocompass can be used to find true north, while being unaffected by stray magnetic fields, nearby electrical power circuits or nearby large masses of ferrous metals. A recent development is the electronic compass, which detects the magnetic directions without requiring moving parts. This device frequently appears as an optional subsystem built into GPS receivers.
The artifact itself is part of a lodestone that had been polished into a bar with a groove at one end (that Carlson suggests may have been used for sighting). The artifact now consistently points 35.5 degrees west of north, but may have pointed north-south when whole. It has been suggested that the artifact was in fact used as some constituent piece of a decorative ornament. No other similar hematite artifacts have yet been found.
Due to disagreement as to when the compass was invented, it may be appropriate to list some noteworthy Chinese literary references offered as possible evidence for its antiquity, in chronological order:
Thus, the use of a magnetic compass as a direction finder occurred sometime before 1044, but evidence for the use of the compass as a navigational device did not appear until 1119.
The typical Chinese navigational compass was in the form of a magnetic needle floating in a bowl of water. According to Needham, the Chinese in the Song Dynasty and continuing Yuan Dynasty did make use of a dry compass, although this type never became as widely used in China as the wet compass. Evidence of this is found in the Shilin guangji ("Guide Through the Forest of Affairs"), first published in 1325 by Chen Yuanjing, although its compilation had taken place between 1100 and 1250. The dry compass in China was a dry suspension compass, a wooden frame crafted in the shape of a turtle hung upside down by a board, with the lodestone sealed in by wax, and if rotated, the needle at the tail would always point in the northern cardinal direction. Although the European compass-card in box frame and dry pivot needle was adopted in China after its use was taken by Japanese pirates in the 16th century (who had in turn learned of it from Europeans), the Chinese design of the suspended dry compass persisted in use well into the 18th century.
However, according to Kreutz there is only a single Chinese reference to a dry-mounted needle (built into a pivoted wooden tortoise) which is dated to between 1150 and 1250, but there is no indication that Chinese mariners ever used anything but the floating needle in a bowl until the 16th-century European contacts. Additionally, it must be pointed out that, unlike Needham, other experts on the history of the compass make no mention of an indigenous dry compass in China and reserve the term for the European form which became later worldwide standard.
The first recorded use of a 48 position mariner's compass on sea navigation was noted in a book titled “The Customs of Cambodia” by Yuan dynasty diplomat Zhou Daguan, he described his 1296 voyage from Wenzhou to Angkor Thom in detail; when his ship set sailed from Wenzhou, the mariner took a needle direction of “ding wei” position, which is equivalent to 22.5 degree SW. After they arrived at Baria, the mariner took "Kun Shen needle" , or 52.5 degree SW. Zheng He's Navigation Map, also known as "The Mao Kun Map", contains a large amount of detail "needle records" of Zheng He's travel.
Zheng He's Navigation Map, also known as "The Mao Kun Map", contains a large amount of detail "needle records" of Zheng He's travel.
The latter two are supported by evidence of the earlier mentioning of the compass in European works rather than Arabic. The first European mention of a magnetized needle and its use among sailors occurs in Alexander Neckam's De naturis rerum (On the Natures of Things), probably written in Paris in 1190. Other evidence for this includes the Arabic word for "Compass" (al-konbas), possibly being a derivation of the old Italian word for compass.
In the Arab world, the earliest reference comes in The Book of the Merchants' Treasure, written by one Baylak al-Kibjaki in Cairo about 1282. Since the author describes having witnessed the use of a compass on a ship trip some forty years earlier, some scholars are inclined to antedate its first appearance accordingly. There is also a slightly earlier non-Mediterranean Muslim reference to an iron fish-like compass in a Persian talebook from 1232.
There have been various arguments put forward concerning whether or not the European compass was an independent invention.
Arguments for independent invention:
Arguments against independent invention:
In the Mediterranean, the introduction of the compass, at first only known as a magnetized pointer floating in a bowl of water, went hand in hand with improvements in dead reckoning methods, and the development of Portolan charts, leading to more navigation during winter months in the second half of the 13th century. While 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, the prolongation of the sailing season resulted in a gradual, but sustained increase in shipping movement: 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. For instance, it enabled Venetian convoys to make two round trips a year to the Levant, instead of one.
At the same time, traffic between the Mediterranean and northern Europe also increased, with first evidence of direct commercial voyages from the Mediterranean into the English Channel coming in the closing decades of the 13th century, and one factor may be that the compass made traversal of the Bay of Biscay safer and easier. Although critics like Kreutz feels that it was later in 1410 that anyone really started steering by compass.
In 1300, another Arabic treatise written by the Egyptian astronomer and muezzin Ibn Simʿūn describes a dry compass for use as a "Qibla indicator" to find the direction to Mecca. Like Peregrinus' compass, however, Ibn Simʿūn's compass did not feature a compass card. In the 14th century, the Syrian astronomer and timekeeper Ibn al-Shatir (1304-1375) invented the compass dial, a timekeeping device incorporating both a universal sundial and a magnetic compass. He invented it for the purpose of finding the direction to Mecca and the times of Salah prayers at the Umayyad Mosque. Arab navigators also introduced the 32-point compass rose during this time.
The familiar dry compass (commonly called a mariner's compass) was invented in Europe around 1300. The dry mariner's compass consists of three elements: A freely pivoting needle on a pin enclosed in a little box with a glass cover and a wind rose, whereby "the wind rose or compass card is attached to a magnetized needle in such a manner that when placed on a pivot in a box fastened in line with the keel of the ship the card would turn as the ship changed direction, indicating always what course the ship was on".
While pivoting needles in glass boxes had already been described by the French scholar Peter Peregrinus in 1269, and by the Egyptian scholar Ibn Simʿūn in 1300, there is an inclination to honour tradition and credit Flavio Gioja (fl. 1302), an Italian marine pilot from Amalfi, with perfecting the sailor's compass by suspending its needle over a compass card, giving thus the compass its familiar appearance. Such a compass with the needle attached to a rotating card is also described in a commentary on Dante's Divine Comedy from 1380, while an earlier source refers to a portable compass in a box (1318), supporting the notion that the dry compass was known in Europe by then.
A bearing compass is a magnetic compass mounted in such a way that it allows the taking of bearings of objects by aligning them with the lubber line of the bearing compass. A surveyor's compass is a specialized compasses made to accurately measure heading of landmarks and measure horizontal angles to help with map making. These were already in common use by the early 18th century and are described in the 1728 Cyclopaedia. Later, a prism and lens was mounted on top of a compass in such a way that enabled the user to accurately sight the heading of geographical landmarks, thus creating the prismatic compass. The Bezard compass was invented in 1906, and consists of a compass with a mirror mounted above it. This enabled the user to easily see the face of the compass while also walking and viewing the surrounding landscape.
The liquid compass was generally introduced into the British Royal Navy in 1908. An early version developed by the Navy captain Creak proved to be operational under heavy gunfire and seas, but lacked navigational precision compared with the design by Lord Kelvin:
Captain Creak's first step in the development of the liquid compass was to introduce a "card mounted on a float, with two thin and relatively short needles, fitted with their poles at the scientifically correct angular distances, and with the centre of gravity, centre of buoyancy, and the point of suspension in correct relation to each other...The compass thus designed rectified the defects of the Admiralty Standard Compass,...with the additional advantage of considerable steadiness under heavy gunfire and in a seaway... The one defect in the compass as developed by Creak up to 1892 was that "for manoeuvring purposes it was inferior to Lord Kelvin's compass, owing to comparative sluggishness on a large alteration of course through the drag on the card by the liquid in which it floated...
However, with ship and gun sizes continuously increasing, the advantages of the liquid compass over the Kelvin compass became more apparent, and the device was generally adopted after its manoeuvering capabilities had been improved.
Modern compasses usually use a magnetized needle or dial inside a fluid-filled capsule (oil, kerosene, or alcohol is common). The fluid dampens the movement of the needle and causes the needle to stabilize quickly rather than oscillate back and forth around magnetic north. North on the needle or dial is usually marked with phosphorescent paint, to enable the compass to be read at night or in poor light.
Many modern hand held recreational and military compasses integrate a protractor with the compass, using a separate magnetized needle. In this design the rotating capsule containing the magnetized needle is fitted with orienting lines and an outlined orienting arrow, then mounted in a transparent baseplate containing a direction-of-travel (DOT) indicator for use in taking bearings directly from a map. Other features found on some modern compasses are map and romer scales for measuring distances and plotting positions on maps, luminous markings on the face or bezels, various sighting mechanisms (mirror, prism, etc.) for taking bearings of distant objects with greater precision, "global" needles for use in differing hemispheres, adjustable declination for obtaining instant true bearings without resort to arithmetic, and devices such as inclinometers for measuring gradients.
The military forces of a few nations, notably the United States Army, continue to utilize older lensatic card compass designs with magnetized compass dials instead of needles. A lensatic card compass permits reading the bearing off of the compass card with only a slight downward glance from the sights (see photo), but requires a separate protractor for use with a map. The official U.S. military lensatic compass does not use fluid to dampen needle swing, but rather electromagnetic induction to dampen the needle. A "deep-well" design is used to allow the compass to be used globally with little or no effect in accuracy caused by a tilting compass dial. As induction forces provide less damping than fluid-filled designs, a needle lock is fitted to the compass to reduce wear, operated by the folding action of the rear sight/lens holder. The use of air-filled induction compasses has declined over the years, as they may become inoperative or inaccurate in freezing temperatures or humid environments.
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.
Some military compasses, like the SandY-183, contains the radioactive material tritium (3H) and a combination of phosphors. The SandY-183 contained 120mCi (millicuries) of tritium. The purpose of the tritium and phosphors is to provide illumination for the compass. This illumination is a form of fluorescence, not requiring the compass to be "recharged" by sunlight or artificial light. The name SandY-183 is derived from the name of the company, Stocker and Yale (SandY).
Large ships typically rely on a gyrocompass, using the magnetic compass only as a backup. Increasingly, electronic fluxgate compasses are used on smaller vessels. However compasses are still widely in use as they can be small, use simple reliable technology, are comparatively cheap, often easier to use than GPS, require no energy supply, and unlike GPS, are not affected by objects, e.g. trees, that can block the reception of electronic signals.
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 highly calibrated internal electronics to measure the response of the device to the Earth's magnetic field.
Other specialty compasses include the optical or prismatic hand-bearing compass, often used by surveyors, cave explorers, or mariners. This compass uses an oil-filled capsule and magnetized compass dial with an integral optical or prismatic sight, often fitted with built-in photoluminescent or battery-powered illumination. Using the optical or prism sight, such compasses can be read with extreme accuracy when taking bearings to an object, often to fractions of a degree. Most of these compasses are designed for heavy-duty use, with solid metal housings, and many are fitted for tripod mounting for additional accuracy.
In the modern era, the 360-degree system took hold. This system is still in use today for civilian navigators. The degree dial spaces the compass markings with 360 equidistant points. Some nations have adopted the "grad" system instead, which spaces the dial into 400 grads or points.
Most military defense forces have adopted the "mil" system, in which the compass dial is spaced into 6400 units (some nations use 6000) or "mils" for additional precision when measuring angles, laying artillery, etc. The value to the military is that one mil subtends approximately one metre at a distance of one kilometer. Former Warsaw Pact countries (Soviet Union, GDR etc.) used a 60° graduation (i.e., the unit is 100 Russian angular mils), often counterclockwise (see picture of wrist compass). This is still in use in Russia.
Like any magnetic device, compasses are affected by nearby ferrous materials as well as by strong local electromagnetic forces. Compasses used for wilderness land navigation should never be used in close proximity to ferrous metal objects or electromagnetic fields (batteries, car bonnets (automobile hoods), engines, steel pitons, wristwatches, etc).
Compasses used in or near trucks, cars or other mechanized vehicles are particularly difficult to use accurately, even when corrected for deviation by the use of built-in magnets or other devices. Large amounts of ferrous metal combined with the on-and-off electrical fields caused by the vehicle's ignition and charging systems generally result in significant compass errors.
At sea, a ship's compass must also be corrected for errors, called deviation, caused by iron and steel in its structure and equipment. The ship is swung, that is rotated about a fixed point while its heading is noted by alignment with fixed points on the shore. A compass deviation card is prepared so that the navigator can convert between compass and magnetic headings. The compass can be corrected in three ways. First the lubber line can be adjusted so that it is aligned with the direction in which the ship travels, then the effects of permanent magnets can be corrected for by small magnets fitted within the case of the compass. The effect of ferromagnetic materials in the compass's environment can be corrected by two iron balls mounted on either side of the compass binnacle. The coefficient representing the error in the lubber line, while the ferromagnetic effects and the non-ferromagnetic component.
A similar process is used to calibrate the compass in light general aviation aircraft, with the compass deviation card often mounted permanently just above or below the magnetic compass on the instrument panel. Fluxgate compasses can be calibrated automatically, and can also be programmed with the correct local compass variation so as to indicate the true heading.
The simplest way of using a compass is to know that the arrow always points in the same direction, magnetic North, which is roughly similar to true north. Except in areas of extreme magnetic declination variance (20 degrees or more), this is enough to protect from walking in a substantially different or even opposite direction than expected over short distances, provided the terrain is fairly flat and visibility is not impaired. In fact, by carefully recording distances (time or paces) and magnetic bearings traveled, one can plot a course and return to one's starting point using the compass alone.
However, compass navigation used in conjunction with a map (terrain association) requires a different compass method. To take a map bearing or true bearing (a bearing taken in reference to true, not magnetic north) to a destination with a protractor compass, the edge of the compass is placed on the map so that it connects the current location with the desired destination (some sources recommend physically drawing a line). The orienting lines in the base of the compass dial are then rotated to align with actual or true north by aligning them with a marked line of longitude (or the vertical margin of the map), ignoring the compass needle entirely. The resulting true bearing or map bearing may then be read at the degree indicator or direction-of-travel (DOT) line, which may be followed as an azimuth (course) to the destination. If a magnetic north bearing or compass bearing is desired, the compass must be adjusted by the amount of magnetic declination before using the bearing so that both map and compass are in agreement. In the given example, the large mountain in the second photo was selected as the target destination on the map.
The modern hand-held protractor compass always has an additional direction-of-travel (DOT) arrow or indicator inscribed on the baseplate. To check one's progress along a course or azimuth, or to ensure that the object in view is indeed the destination, a new compass reading may be taken to the target if visible (here, the large mountain). After pointing the DOT arrow on the baseplate at the target, the compass is oriented so that the needle is superimposed over the orienting arrow in the capsule. The resulting bearing indicated is the magnetic bearing to the target. Again, if one is using "true" or map bearings, and the compass does not have preset, pre-adjusted declination, one must additionally add or subtract magnetic declination to convert the magnetic bearing into a true bearing. The exact value of the magnetic declination is place-dependent and varies over time, though declination is frequently given on the map itself or obtainable on-line from various sites. If not, any local walker club should know it. If the hiker has been following the correct path, the compass' corrected (true) indicated bearing should closely correspond to the true bearing previously obtained from the map.