A 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 compass greatly improved the safety and efficiency of travel, especially ocean travel. 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 Global Positioning System (GPS). A compass is any magnetically sensitive device capable of indicating the direction of the magnetic north of a planet's magnetosphere. The face of the compass generally highlights the cardinal points of north, south, east and west. 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. The compass was invented in ancient China sometime before the 2nd century, and was used for navigation by the 11th century. 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 masses of ferrous metals. A recent development is the electronic compass, or fibre optic gyrocompass, which detects the magnetic directions without potentially fallible moving parts. This device frequently appears as an optional subsystem built into GPS receivers. However, magnetic compasses remain popular, especially in remote areas, as they are cheap, durable, and require no electrical power supply.
History
Navigation prior to the compass
Prior to the introduction of the compass, position, destination, and direction at sea was primarily determined by the sighting of landmarks, supplemented with the observation of the position of celestial bodies. Ancient mariners often kept within sight of land. The invention of the compass enabled the determination of heading when the sky was overcast or foggy. And, when the sun or other known celestial bodies could be observed, it enabled the calculation of latitude. This enabled mariners to navigate safely far from land, increasing sea trade, and contributing to the Age of Discovery.
Olmec artifact
Based on his find of an Olmec hematite artifact in Mesoamerica, radiocarbon dated to 1400-1000 BC, astronomer John Carlson has suggested that the Olmec might have discovered and used the geomagnetic lodestone compass earlier than 1000 BC. If true, this "predates the Chinese discovery of the geomagnetic lodestone compass by more than a millennium". Carlson speculates that the Olmecs used similar artifacts as a directional device for astrological or geomantic purposes. The artifact is part of a polished lodestone bar with a groove at one end (possibly for sighting). The artifact now consistently points 35.5 degrees west of north, but may have pointed north-south when whole. Other researchers have suggested that the artifact is actually a constituent piece of a decorative ornament. No other hematite artifacts have been found.
China
The earliest Chinese compasses were probably not designed for navigation, but rather to order and harmonize their environments and buildings in accordance with the geomantic principles of feng shui . These early compasses were made using lodestone, a special form of the mineral magnetite that aligns itself with the Earth’s magnetic field..
There is disagreement as to exactly when the compass was invented. These are noteworthy Chinese literary references in evidence for its antiquity:
- The earliest Chinese literature reference to magnetism lies in the 4th century BC book Book of the Devil Valley Master (鬼谷子): "The lodestone makes iron come or it attracts it." Its author also notes that in addition to its main geomantic purpose, the compass, or “south pointer” as the Chinese called it, could be carried with jade hunters to prevent them from getting lost during their journeys.
- The first mention of the attraction of a needle by a magnet is a Chinese work composed between 70 and 80 AD ( Lunheng ch. 47): "A lodestone attracts a needle." "This passage of Louen-heng is the first Chinese text concerning the attraction of a needle by a magnet ". In 1948, the scholar Wang Chen-Tuo constructed a "compass" in the form of south-indicating spoon on the basis of this text. However, "there is no explicit mention of a magnet in the Louen-heng " and that "beforehand it needs to assume some hypotheses to arrive at such a conclusion."
- The earliest reference to a specific magnetic direction finder device is recorded in a Song Dynasty book dated to 1040-44. There is a description of an iron "south-pointing fish" floating in a bowl of water, aligning itself to the south. The device is recommended as a means of orientation "in the obscurity of the night." The Wujing Zongyao (武经总要, "Collection of the Most Important Military Techniques") stated: "When troops encountered gloomy weather or dark nights, and the directions of space could not be distinguished...they made use of the south-pointing carriage, or the south-pointing fish." This was achieved by heating of metal (especially if steel), known today as thermo-remanence , and would have been capable of producing a weak state of magnetization. While the Chinese achieved magnetic remanence and induction by this time, a similar discovery was not made in Europe until about 1600, when William Gilbert published his De Magnete .
- The first incontestable reference to a magnetized needle in Chinese literature appears in 1088. The Dream Pool Essays , written by the Song Dynasty polymath scientist Shen Kuo, 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 Kuo pointed out that a needle prepared this way sometimes pointed south, sometimes north.
- The earliest recorded actual use of a magnetized needle for navigational purposes is found in Zhu Yu's book Pingzhou Table Talks (萍洲可談; Pingzhou Ketan) of 1119 (written from 1111 to 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.
Thus, the use of a magnetic compass as a direction finder occurred sometime before 1044, but incontestable 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"), 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 sail 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 She
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