Magnetic Compass
A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with magnetic north. Other methods may be used, including gyroscopes, magnetometers, and GPS receivers.
Compasses often show angles in degrees: north corresponds to 0°, and the angles increase clockwise, so east is 90°, south is 180°, and west is 270°. These numbers allow the compass to show azimuths or bearings which are commonly stated in degrees. If local variation between magnetic north and true north is known, then direction of magnetic north also gives direction of true north.
Among the Four Great Inventions, the magnetic compass was first invented as a device for divination as early as the Chinese Han Dynasty (since c. 206 BC),[1][2] and later adopted for navigation by the Song Dynasty Chinese during the 11th century.[3][4][5] The first usage of a compass recorded in Western Europe and the Islamic world occurred around 1190.[6][7]
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 the North end or pole of the needle approximately toward the Earth's North magnetic pole, and pulling the other toward the Earth's South magnetic pole.[8] 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 locations of the Earth's magnetic poles slowly change with time, which is referred to as geomagnetic secular variation. The effect of this means a map with the latest declination information should be used.[9] Some magnetic compasses include means to manually compensate for the magnetic declination, so that the compass shows true directions.
History
[
](https://en.wikipedia.org/wiki/File:Southern_Song_Figure_Holding_Land_Compass_%289872744925%29.jpg)
Figurine of a man holding a compass, Song dynasty
Model of a lodestone compass from Han dynasty
Main article: History of the compass
The first compasses in ancient Han dynasty China were made of lodestone, a naturally magnetized ore of iron.[2][10] Later compasses were made of iron needles, magnetized by striking them with a lodestone, which appeared in China by 1088 during the Song Dynasty, as described by Shen Kuo.[11] Dry compasses began to appear around 1300 in Medieval Europe and the Islamic world.[12][7] This was supplanted in the early 20th century by the liquid-filled magnetic compass.[13]...
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.[46] 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.[47] Evidence of this is found in the Shilin Guang Ji ("Guide Through the Forest of Affairs"), published in 1325 by Chen Yuanjing, although its compilation had taken place between 1100 and 1250.[47] 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.[47]
Although the European compass-card in a 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),[48] the Chinese design of the suspended dry compass persisted in use well into the 18th century.[49] 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.[46]
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, the mariner took "Kun Shen needle", or 52.5 degree SW.[50] 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.[51]
Medieval Europe
A drawing of a compass in a mid 14th-century copy of Epistola de magnete of Peter Peregrinus.[52]
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,[6] written between 1187 and 1202, after he returned to England from France[53] and prior to entering the Augustinian abbey at Cirencester.[54] In his 1863 edition of Neckam's De naturis rerum, Thomas Wright provides a translation of the passage in which Neckam mentions sailors being guided by a compass' needle:
The sailors, moreover, as they sail over the sea, when in cloudy whether they can no longer profit by the light of the sun, or when the world is wrapped up in the darkness of the shades of night, and they are ignorant to what point of the compass their ship's course is directed, they touch the magnet with a needle, which (the needle) is whirled round in a circle until, when its motion ceases, its point looks direct to the north.[55]
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.[6]
In the Mediterranean, the introduction of the compass, at first only known as a magnetized pointer floating in a bowl of water,[56] 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.[57][10] 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.[58] 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.[59]
At the same time, traffic between the Mediterranean and northern Europe also increased, with the 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.[60] However, critics like Kreutz have suggested that it was later in 1410 that anyone really started steering by compass.[61]
Muslim world
Al-Ashraf's diagram of the compass and Qibla. From MS Cairo TR 105, copied in Yemen, 1293.[6]
The earliest reference to a compass in the Muslim world occurs in a Persian talebook from 1232, Jawami ul-Hikayat,[24] where a compass is used for navigation during a trip in the Red Sea or the Persian Gulf.[8] This fish shape was from a typical early Chinese design.[62] The fish-shaped iron leaf described indicates that this early Chinese design has spread outside of China.[62] The earliest Arabic reference to a compass, in the form of magnetic needle in a bowl of water, comes from a work by Baylak al-Qibjāqī, written in 1282 while in Cairo.[24][63] Al-Qibjāqī described a needle-and-bowl compass used for navigation on a voyage he took from Syria to Alexandria in 1242.[24] 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.[24] Al-Qibjāqī also reports that sailors in the Indian Ocean used iron fish instead of needles.[64]
Late in the 13th century, the Yemeni Sultan and astronomer al-Malik al-Ashraf described the use of the compass as a "Qibla indicator" to find the direction to Mecca.[65] In a treatise about astrolabes and sundials, al-Ashraf includes several paragraphs on the construction of a compass bowl (ṭāsa). He then uses the compass to determine the north point, the meridian (khaṭṭ niṣf al-nahār), and the Qibla. This is the first mention of a compass in a medieval Islamic scientific text and its earliest known use as a Qibla indicator, although al-Ashraf did not claim to be the first to use it for this purpose.[6][66]
In 1300, an Arabic treatise written by the Egyptian astronomer and muezzin Ibn Simʿūn describes a dry compass used for determining qibla. Like Peregrinus' compass, however, Ibn Simʿūn's compass did not feature a compass card.[6] 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 prayers.[67] Arab navigators also introduced the 32-point compass rose during this time.[68] 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 sealed with tar or wax to prevent the penetration of water. The other instrument is a dry compass.[64]
Navigational sailor's compass rose, 1607
In the 15th century, the description given by Ibn Majid while aligning the compass with the pole star indicates that he was aware of magnetic declination. An explicit value for the declination is given by ʿIzz al-Dīn al-Wafāʾī (fl. the 1450s in Cairo).[8]
Pre modern Arabic sources refer to the compass using the term ṭāsa (lit. "bowl") for the floating compass, or ālat al-qiblah ("qibla instrument") for a device used for orienting towards Mecca.[8]
Friedrich Hirth suggested that Arab and Persian traders, who learned about the polarity of the magnetic needle from the Chinese, applied the compass for navigation before the Chinese did.[69] However, Needham described this theory as "erroneous" and "it originates because of a mistranslation" of the term chia-ling found in Zhu Yu's book Pingchow Table Talks.[70]
India
The development of the magnetic compass is highly uncertain. The compass is mentioned in 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.[71][72]
Medieval Africa
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.[73] There is evidence that Swahili maritime merchants and sailors acquired the compass at some point and used it for navigation.[74]
Dry compass
Early modern dry compass suspended by a gimbal (1570)
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".[7] Later, compasses were often fitted into a gimbal mounting to reduce grounding of the needle or card when used on the pitching and the rolling deck of a ship.
While pivoting needles in glass boxes had already been described by the French scholar Peter Peregrinus in 1269,[75] and by the Egyptian scholar Ibn Simʿūn in 1300,[6] 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.[76][10] 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),[77] supporting the notion that the dry compass was known in Europe by then.[46]
Bearing compass...
Liquid compass
A surface liquid-filled compass on a boat
The liquid compass is a design in which the magnetized needle or card is damped by fluid to protect against excessive swing or wobble, improving readability while reducing wear. A rudimentary working model of a liquid compass was introduced by Sir Edmond Halley at a meeting of the Royal Society in 1690.[87] However, as early liquid compasses were fairly cumbersome and heavy and subject to damage, their main advantage was aboard the ship. Protected in a binnacle and normally gimbal-mounted, the liquid inside the compass housing effectively damped shock and vibration, while eliminating excessive swing and grounding of the card caused by the pitch and roll of the vessel. The first liquid mariner's compass believed practicable for limited use was patented by the Englishman Francis Crow in 1813.[88][89]
Liquid-damped marine compasses for ships and small boats were occasionally used by the Royal Navy from the 1830s through 1860, but the standard Admiralty compass remained a dry-mount type.[90] In the latter year, the American physicist and inventor Edward Samuel Ritchie patented a greatly improved liquid marine compass that was adopted in revised form for general use by the United States Navy, and later purchased by the Royal Navy as well.[91]
Despite these advances, the liquid compass was not introduced generally into the Royal Navy until 1908. An early version developed by RN Captain Creek proved to be operational under heavy gunfire and seas but was felt to lack navigational precision compared with the design by Lord Kelvin.[92][93] However, with ship and gun sizes continuously increasing, the advantages of the liquid compass over the Kelvin compass became unavoidably apparent to the Admiralty, and after widespread adoption by other navies, the liquid compass was generally adopted by the Royal Navy.[92]
Design
A liquid-filled protractor or orienteering compass with lanyard.
Modern compasses usually use a magnetized needle or dial inside a capsule completely filled with a liquid (lamp oil, mineral oil, white spirits, purified kerosene, or ethyl alcohol are common). While older designs commonly incorporated a flexible rubber diaphragm or airspace inside the capsule to allow for volume changes caused by temperature or altitude, some modern liquid compasses use smaller housings and/or flexible capsule materials to accomplish the same result.[14] The liquid inside the capsule serves to damp the movement of the needle, reducing oscillation time and increasing stability. Key points on the compass, including the north end of the needle are often marked with phosphorescent, photoluminescent, or self-luminous materials[15] to enable the compass to be read at night or in poor light. As the compass fill liquid is noncompressible under pressure, many ordinary liquid-filled compasses will operate accurately underwater to considerable depths.
...
Extra Connections: