A device for determining a horizontal geographical direction or bearing; invented in China in 1117, and in Europe in 1190. The magnetic compass depends on a magnet, free to rotate in a horizontal plane, locating itself in line with the Earth's magnetic field. It is subject to the irregularity and to the short- and long-period variation of the Earth's field.
A compass can be used in conjunction with a clock and a sextant to provide a very accurate navigation capability. The compass was developed in China more than seventeen hundred years ago, however the original inventor remains in dispute;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.
History of the navigational compass
Although there is suggestion that the Mesoamerican Olmecs were familiar with magnetism and even developed a compass in the 2 at present the honor of inventing the compass belongs to the Chinese. 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.
The earliest recorded use of a compass in navigation lies in Zhu Yu's book Pingzhou Ke Tan (Pingzhou Table Talks) of AD 1117.
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. 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.
Prior to the introduction of the compass, direction at sea was primarily determined by the position of celestial bodies.
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. 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.
Modern navigational compasses
Modern navigational compasses hold a magnetized needle inside a fluid-filled capsule; 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 or setting the bearing of travel, and a sighting mirror that lets the user see both the compass needle and a distant object at the same time, or a lens that permits reading the bearing off of the compass card with only a slight glance down from the sights (see photo). Further, some modern compasses include an inclinometer for measuring gradients and are adjustable to account for varying magnetic declination, a serious accuracy issue. (If the needle is heavily counterbalanced, for use near the poles, and is then used near the equator, it may also bind.)
Almost identical to the photo is a military lensatic compass that uses electromagnetic induction, by the swinging needle, of eddy currents in an imperfectly conductive, nearby metal part for damping, rather than liquid. While needle locks have been common on dry, handheld compasses, they can prove desirable on liquid filled, handheld compasses under heavy use, but are not sufficiently feasible to provide.
Mariner's compasses can have two or more magnetic needles permanently attached to a compass card. 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.
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. Using Trigonometry the correct heading relative to the compass is calculated.
Compass correction
A ship's compass must be corrected for errors, called compass deviation, caused by iron and steel in its structure and equipment. 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 binacle.
Fluxgate compasses can be calibrated automatically, and can also be programmed with the correct local compass variation so as to indicate the true heading.
Using a compass
The simpliest way of using a compass is just to know that the arrow always points in the same direction, magnetic North, which is also the top of the map. Then the edge of the compass is placed so that it connects the current location with the desired destination (some sources recommend to draw the line). The compass scale is then rotated, making "north" marking to point to the actual map's north (normally the top edge of the map).
The hand held compass usually has the additional arrow drawn on the corpus. After orienting the compass so that the north arrow matches the marking on the scale, this arrow is oriented more or less toward the chosen target, in a real world.
LiteratureCompass 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:
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Compass with 400 grades division and conversion table |
Swiss army compass with mils division |
Land surveyor compass with clinometer |
Stratum compass after Prof. Clar |
Points of the compass
The mariner's compass card is divided into thirty-two equally spaced points.
Gallery
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A simple compass typical to a small yacht |
A compass suitable for orienteering in the night |
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