A compass is an instrument used for navigation and orientation that shows direction relative to the geographic "cardinal directions", or "points". Usually, a diagram called a compass rose, shows the directions north, south, east, and west as abbreviated initials marked on the compass. When the compass is used, the rose can be aligned with the corresponding geographic directions, so, for example, the "N" mark on the rose really points to the north. Frequently, in addition to the rose or sometimes instead of it, angle markings in degrees are shown on the compass. North corresponds to zero degrees, and the angles increase clockwise, so east is 90 degrees, south is 180, and west is 270. These numbers allow the compass to show azimuths or bearings, which are commonly stated in this notation.
The magnetic compass was first invented as a device for divination as early as the Chinese Han Dynasty (since about 206 BC),[1][2] and later adopted for navigation by the Song Dynasty Chinese during the 11th century.[3][4][5] The use of a compass is recorded in Western Europe and in Persia around the early 13th century.[6]
Magnetic compass[edit]
A military compass that was used during World War I
The magnetic compass is the most familiar compass type. It functions as a pointer to "magnetic north", the local magnetic meridian, because the magnetized needle at its heart aligns itself with the horizontal component of the Earth's magnetic field. The magnetic field exerts a torque on the needle, pulling one end or pole of the needle approximately toward the Earth's North magnetic pole, and pulling the other toward the South magnetic pole.[7] The needle is mounted on a low-friction pivot point, in better compasses a jewel bearing, so it can turn easily. When the compass is held level, the needle turns until, after a few seconds to allow oscillations to die out, it settles into its equilibrium orientation.
In navigation, directions on maps are usually expressed with reference to geographical or true north, the direction toward the Geographical North Pole, the rotation axis of the Earth. Depending on where the compass is located on the surface of the Earth the angle between true north and magnetic north, called magnetic declination can vary widely with geographic location. The local magnetic declination is given on most maps, to allow the map to be oriented with a compass parallel to true north. The Earth's magnetic fields are constantly changing which is referred to as geomagnetic secular variation. The effect of this means a map with the latest declination information should be used.[8] Some magnetic compasses include means to manually compensate for the magnetic declination, so that the compass shows true directions.
History[edit]
There is a proposal that this section be split into a new article titled History of the compass. (Discuss) (August 2014)
The first compasses in Han dynasty China were made of lodestone, a naturally magnetized ore of iron.[2] The compass was later used for navigation by the Song Dynasty.[9] Later compasses were made of iron needles, magnetized by striking them with a lodestone. Dry compasses begin appearing around 1300 in Medieval Europe.[10] This was supplanted in the early 20th century by the liquid-filled magnetic compass.[11]
Navigation prior to the compass[edit]
See also: Polynesian navigation
Prior to the introduction of the compass, geographical position, destination, and direction at sea were primarily determined by the sighting of landmarks, supplemented with the observation of the position of celestial bodies. On cloudy days, the Vikings may have used cordierite or some other birefringent crystal to determine the sun's direction and elevation from the polarization of daylight; their astronomical knowledge was sufficient to let them use this information to determine their proper heading.[12] For more southerly Europeans unacquainted with this technique, the invention of the compass enabled the determination of heading when the sky was overcast or foggy. This enabled mariners to navigate safely far from land, increasing sea trade, and contributing to the Age of Discovery.[13][14]
Geomancy and feng shui[edit]
The compass was invented in China during the Han Dynasty between the 2nd century BC and 1st century AD, where it was called the "south-governor" (sīnán 司南).[9] The magnetic compass was not, at first, used for navigation, but for geomancy and fortune-telling by the Chinese. The earliest Chinese magnetic compasses were possibly used to order and harmonize buildings in accordance with the geomantic principles of feng shui. These early compasses were made with lodestone, a form of the mineral magnetite that is a naturally-occurring magnet and aligns itself with the Earth’s magnetic field. People in ancient China discovered that if a lodestone was suspended so it could turn freely, it would always point toward the magnetic poles. Early compasses were used to choose areas suitable for building houses and to search for rare gems. Compasses were later adapted for navigation during the Song Dynasty in the 11th century.[2]
Based on Krotser and Coe's discovery of an Olmec hematite artifact in Mesoamerica, radiocarbon dated to 1400-1000 BC, astronomer John Carlson has hypothesized that the Olmec might have used the geomagnetic lodestone earlier than 1000 BC for geomancy, a method of divination, which if proven true, predates the Chinese use of magnetism for feng shui by a millennium.[15] Carlson speculates that the Olmecs used similar artifacts as a directional device for astronomical or geomantic purposes but does not suggest navigational usage. The artifact is part of a polished hematite bar with a groove at one end, possibly used for sighting. Carlson's claims have been disputed by other scientific researchers, who have suggested that the artifact is actually a constituent piece of a decorative ornament and not a purposely built compass.[16] Several other hematite or magnetite artifacts have been found at pre-Columbian archaeological sites in Mexico and Guatemala.[17][18]
Navigational compass[edit]
A number of ancient cultures used lodestones, suspended so they could turn, as magnetic compasses for navigation. Early mechanical compasses are referenced in written records of the Chinese, who began using it for navigation sometime between the 9th and 11th century, "some time before 1050, possibly as early as 850."[19] A common theory by historians,[20][21] suggests that the Arabs introduced the compass from China to Europe, although current textual evidence only supports the fact that Chinese use of the navigational compass preceded that of Europe and the Middle East.[3] Some scholars suggested the compass was transmitted from China to Europe and Arabs via Indian Ocean.[22] However, some scholars proposed an independent European invention.[23]
China[edit]
Further information: Four Great Inventions, List of Chinese inventions and History of science and technology in China
Model of a Han Dynasty (206 BC–220 AD) south-indicating ladle or sinan. It is theorized[by whom?] that the south-pointing spoons of the Han dynasty were magnetized lodestones.[24]
There is disagreement as to exactly when the compass was invented. These are noteworthy Chinese literary references in evidence for its antiquity:
The magnetic compass was first invented as a device for divination as early as the Chinese Han Dynasty (since about 206 BC).[1][2][9] The compass was used in Song Dynasty China by the military for navigational orienteering by 1040–1044,[3][4][5] and was used for maritime navigation by 1111 to 1117.[25]
The earliest Chinese literature reference to magnetism lies in the 4th century BC writings of Wang Xu (鬼谷子): "The lodestone attracts iron."[26] The book also notes that the people of the state of Zheng always knew their position by means of a "south-pointer"; some authors suggest that this refers to early use of the compass.[2][27]
The first mention of a spoon, speculated to be a lodestone, observed pointing in a cardinal direction is a Chinese work composed between 70 and 80 AD (Lunheng), which records that "But when the south pointing spoon is thrown upon the ground, it comes to rest pointing at the south."[28] Within the text, the author Wang Chong describes the spoon as a phenomenon that he has personally observed.[29] Although the passage does not explicitly mention magnetism,[30] according to Chen-Cheng Yih, the "device described by Wang Chong has been widely considered to be the earliest form of the magnetic compass."[24]
The first clear account of magnetic declination occurs in the Kuan Shih Ti Li Chih Meng ("Mr. Kuan's Geomantic Instructor"), dating to 880.[31] Another text, the Chiu Thien Hsuan Nu Chhing Nang Hai Chio Ching ("Blue Bag Sea Angle Manual") from around the same period, also has an implicit description of magnetic declination. It has been argued that this knowledge of declination requires the use of the compass.[31]
A reference to a magnetized needle as a "mysterious needle" appears in 923-926 in the Chung Hua Ku Chin Chu text written by Ma Kao. The same passage is also attributed to the 4th century AD writer Tshui Pao, although it is postulated that the former text is more authentic. The shape of the needle is compared to that of a tadpole, and may indicate the transition between "lodestone spoons" and "iron needles."[32]
The earliest reference to a specific magnetic direction finder device for land navigation 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 (武經總要, "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 [mechanical] south-pointing carriage, or the south-pointing fish."[4] This was achieved by heating of metal (especially if steel), known today as thermoremanence, and would have been capable of producing a weak state of magnetization.[4] While the Chinese achieved magnetic remanence and induction by this time, in both Europe and Asia the phenomenon was attributed to the supernatural and occult, until about 1600 when William Gilbert published his De Magnete.[33]
The first incontestable reference to a magnetized needle in Chinese literature appears in 1088.[5] 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 explicit recorded use of a magnetic compass for maritime navigation is found in Zhu Yu's book Pingchow Table Talks (萍洲可談; Pingzhou Ketan) and dates from 1111 to 1117: The ship's pilots are acquainted with the configuration of the coasts; at night they steer by the stars , and in the daytime by the sun. In dark weather they look at the south pointing needle.[25]
Thus, the use of a magnetic compass by the military for land navigation occurred sometime before 1044, but incontestable evidence for the use of the compass as a maritime navigational device did not appear until 1117.
The typical Chinese navigational compass was in the form of a magnetic needle floating in a bowl of water.[34] 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.[35] 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.[35] 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.[35] 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),[36] the Chinese design of the suspended dry compass persisted in use well into the 18th century.[37] 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, and claims that there is no clear indication that Chinese mariners ever used anything but the floating needle in a bowl until the 16th-century.[34]
Diagram of a Ming Dynasty mariner's compass
The first recorded use of a 48 position mariner's compass on sea navigation was noted in 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,[disambiguation needed] the mariner took "Kun Shen needle", or 52.5 degree SW.[38] Zheng He's Navigation Map, also known as the "Mao Kun Map", contains a large amount of detail "needle records" of Zheng He's expeditions.[39]
At present, according to Kreutz, scholarly consensus is that the Chinese invention used in navigation pre-dates the first European mention of a compass by 150 years.[3] However, there are questions over diffusion. The first recorded appearance of the use of the compass in Europe (1190)[40] is earlier than in the Muslim world (1232),[6][41] as a description of a magnetized needle and its use among sailors occurs in Alexander Neckam's De naturis rerum (On the Natures of Things), written in 1190.[40][42] The earliest reference to a compass in the Middle East is attributed to the Persians, who describe an iron fish-like compass in a talebook dating from 1232.[6] 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.[41] 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. The common shape of the early compass as a magnetized needle floating in a bowl of water is considered as an evidence of diffusion from China to Europe.[40] Another evidence supported diffusion theory is the temporal proximity of the Chinese beginning to use compass (after its first use in divination) in navigation to the first appearance of compass in Europe and Arab.[40] The Persian compass is described as fish-like, which is a characteristic of early Chinese compasses from the 11th century, suggesting transmission from China to Persia.[43] The view of some academics that compass was transmitted to Europe from China, through the Islamic world.[20][21] Some suggested the compass was transmitted to Europe and Arab through Indian Ocean from China.[22] Other scholars suggested compass was brought by crusades to Europe from China.[44] However, some scholars proposed an independent European invention of the compass:[45] The first record of a magnetic compass in Europe (1187 by Neckam) proceeded those by Arabs,[23] and the Arabic word for compass (al-kunbas) appears to be derived from Italian roots,[46] which indicates an independent European invention.
Medieval Europe[edit]
Pivoting compass needle in a 14th-century copy of Epistola de magnete of Peter Peregrinus (1269)[47]
Alexander Neckam reported the use of a magnetic compass for the region of the English Channel in the texts De utensilibus and De naturis rerum,[48] written between 1187 and 1202, after he returned to England from France[49] and prior to entering the Augustinian abbey at Cirencester.[50] Robert Southey suggested that the Siete Partidas contained a reference from the 1250s to the needle being used for navigation.[51] In 1269 Petrus Peregrinus of Maricourt described a floating compass for astronomical purposes as well as a dry compass for seafaring, in his well-known Epistola de magnete.[48] In the Mediterranean, the introduction of the compass, at first only known as a magnetized pointer floating in a bowl of water,[52] 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.[53] 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.[54] 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.[55]
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.[56] However, critics like Kreutz have suggested that it was later in 1410 that anyone really started steering by compass.[57]
Navigational sailor's compass rose
Muslim world[edit]
The earliest reference to an iron fish-like compass in the Islamic world occurs in a Persian book from 1232.[6] This fish shape was from a typical early Chinese design.[43] The earliest Arabic reference to a compass, in the form of magnetic needle in a bowl of water, comes from the Yemeni Sultan and astronomer Al-Ashraf in 1282.[41] He also appears to be the first to make use of the compass for astronomical purposes.[58] 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 in the Arab world accordingly.[6] In addition he reports that on the Indian Ocean floating compasses with a hollow floating fish made of sheet-iron were used.[23]
In 1300, another Arabic treatise written by the Egyptian astronomer and muezzin Ibn Simʿūn describes a dry compass for use as a "Qibla (Kabba) indicator" to find the direction to Mecca. Like Peregrinus' compass, however, Ibn Simʿūn's compass did not feature a compass card.[48] In the 14th century, the Syrian astronomer and timekeeper Ibn al-Shatir (1304–1375) invented a timekeeping device incorporating both a universal sundial and a magnetic compass. He invented it for the purpose of finding the times of salat prayers.[59] Arab navigators also introduced the 32-point compass rose during this time.[60] In 1399, an Egyptian reports two different kinds of magnetic compass. One instrument is a “fish” made of willow wood or pumpkin, into which a magnetic needle is inserted and which afterwards is sealed with tar or wax to prevent the penetration of water. The other instrument is a dry compass.[23]
India[edit]
The development of the magnetic compass is highly uncertain. The compass is mentioned to fourth century AD Tamil nautical books; moreover, its early name of macchayantra (fish machine) suggest a Chinese origin. In its Indian form, the wet compass often consisted of a fish-shaped magnet, float in a bowl filled with oil.[61][62] This fish shape was from a typical early Chinese design.[43]
Medieval Africa[edit]
There is evidence that the distribution of the compass from China likely also reached eastern Africa by way of trade through the end of the Silk Road that ended in East African centre of trade in Somalia and the Swahili city-state kingdoms.[63] There is evidence that Swahili maritime merchants and sailors acquired the compass at some point and used it for navigation.[64]
Later developments[edit]
Dry compass[edit]
Early modern dry compass suspended by a gimbal (1570)
The dry 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".[10] Later, compasses were often fitted into a gimbal mounting to reduce grounding of the needle or card when used on the pitching and rolling deck of a ship.
While pivoting needles in glass boxes had already been described by the French scholar Peter Peregrinus in 1269,[65] and by the Egyptian scholar Ibn Simʿūn in 1300,[48] traditionally Flavio Gioja (fl. 1302), an Italian pilot from Amalfi, has been credited with perfecting the sailor's compass by suspending its needle over a compass card, thus giving the compass its familiar appearance.[66] 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),[67] supporting the notion that the dry compass was known in Europe by then.[34]
Bearing compass[edit]
Bearing compass (18th century)
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.[68] A surveyor's compass is a specialized compass made to accurately measure heading of landmarks and measure horizontal angles to help with map making.
