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See also: 
How to
apply the Compass Error |
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 Before the
development of sophisticated electronic and sound
detection systems, navigators calculated directions from
objects in the sky the sun, the North Star, and the moon.
A much more reliable guide for finding direction is a
magnetic compass, which works at all times and in most
places. When a piece of magnetized iron is placed on a
splinter of wood and floated in a bowl of water, the wood
will swing until the iron is pointing north and south.
Any other direction can be found. In China and Europe the magnetized iron found in the lodestone, a naturally occurring magnetic ore, was used to make a floating compass in the 12th century. Soon afterward it was discovered that an iron or steel needle touched long enough by a lodestone also had the tendency to align itself in a north-south direction. A small pocket compass works on the same principle as the first crude compass: instead of a lodestone and a wood splinter, it has a magnetized needle that swings on a pivot to indicate north. Larger compasses have two or more parallel needles attached to the underside of a disk called a compass card. The compass works because the Earth itself is a huge magnet. Its magnetic poles are oval areas about 1,300 miles (2,100 kilometers) from the geographic North and South poles. Irregular lines of force connect the magnetic poles, and the compass needle simply aligns itself with these lines of force. In a few places, where lines of force happen to lie along meridians (that is, where magnetic north and true north coincide), the compass points to true north. Near the magnetic pole the magnetic compass is useless because there the lines of force are vertical straight down into the Earth. In other areas iron ore deposits affect the compass's accuracy. Generally, however, the magnetic compass points a little east or west of true north. The angle between true north and magnetic north is called variation or declination. A compass rose, or graduated circle, is used to measure this angle on charts. A compass card usually has direction pointers consisting of 32 points. The four principal, or cardinal, points are north, east, south, and west. They are marked N, E, S, and W. Between these lie the intercardinal points, such as northeast (NE). Further division gives such points as north-northeast (NNE). A final division is by points, such as north by east (N by E). Naming all the points of a compass in their order is called boxing the compass. Surveyors, navigators, and similar technicians need more exact directions they use degrees. The compass card has 360 degrees marked on it. North is 000° (or 360°); East, 090° ; South, 180° ; and West, 270°. On ships the magnetic compass is usually carried in a stand called a binnacle. It holds a bowl containing the compass card with its needles mounted on a pivot and has a provision for illuminating the compass face from below. The bowl is filled with a nonfreezing liquid on which the card floats to reduce vibrations. On the forward inside edge of the bowl is a vertical line called a lubber's line. This marks the "dead ahead" of the ship. In steering, the helmsman watches the mark for his course on the compass card, keeping it always opposite the lubber's line. A compass aboard a ship is affected by the magnetic force of the ship itself, which acts like a huge magnet. The effect of this magnetism on the compass is called deviation. It is measured by the angle between compass north and magnetic north. Variation and deviation together pull the compass away from true north by an amount called compass error. Navigators remove most of the deviation by compensating the compass. They take the ship to a range where they line it up with markers indicating the four cardinal points. Then they "swing ship" by pivoting the craft so that the bow points in turn to each of the markers. They remove the deviation on each heading by placing counteracting magnets in the binnacle these magnets serve to cancel the magnetic effects of the metal in the ship. In an effort to develop a navigational instrument whose accuracy would be unaffected by stray magnetic fields, the gyrocompass, which does not use magnetism, was developed. Gyrocompasses are often used in modern navigation systems because they can be set to point to true north rather than to magnetic north. Today large ships carry both magnetic compasses and gyrocompasses. Special compasses have also been developed for airplanes. Gyroscopic systems are especially useful in such applications because, unlike magnetic compasses, their accuracy is not affected by rapid alterations of course or speed. The aperiodic compass is a magnetic compass whose needle is extremely stable under most flying conditions for aircraft. The magnesyn compass is a remote-indicating magnetic compass. Readings from its pickup coil are transmitted to repeaters in other parts of the airplane. Both the gyro flux gate compass and the gyrosyn compass are remote-indicating, gyrostabilized compasses. For its indications, the obsolete Earth-inductor compass used current generated in a coil revolving in the Earth's magnetic field. The astrocompass is an astronomical instrument by which the air or sea navigator finds the true heading by sighting a celestial body. A form of astrocompass is the sun compass, which utilizes the shadow of a pin. 
Local Magnetic AnomaliesIn various parts of the world, magnetic ores on or just below the seabed may give rise to local magnetic anomalies resulting in the temporary deflection of the magnetic compass needle when a ship passes over them. The areas of disturbance are usually small unless there are many anomalies close together. The amount of the deflection will depend on the depth of water and the strength of the magnetic force generated by the magnetic ores. However, the magnetic force will seldom be strong enough to deflect the compass needle in depths greater than about 1500 m. Similarly, a ship would have to be within 8 cables of a nearby land mass containing magnetic ores for a deflection of the needle to occur. Deflections may also be due to wrecks lying on the bottom in moderate depths, but investigations have proved that, while deflections of unpredictable amount may be expected when very close to such wrecks, it is unlikely that deflections in excess of 7° will be experienced, nor should the disturbance be felt beyond a distance of 250 m. Greater deflections may be experienced when in close quarters with a ship carrying a large cargo such as iron ore, which readily reacts to induced magnetism. Power cables carrying direct current can cause deflection of the compass needle. The amount of the deflection depends on the magnitude of the electric current and the angle the cable makes with the magnetic meridian. Small vessels with an auto-pilot dependent upon a magnetic sensor may experience steering difficulties if crossing such a cable.
The Effect of Magnetic
and Ionospheric Storms on the Compass NeedleDisturbances on the sun may cause disturbances of the magnetic compass needle and interference with radio communications. At the time of an intense solar flare or eruption, a flash of ultra-violet light and a stream of charged particles are emitted from the sun. The flash of ultra-violet light takes only 8 minutes to reach the Earth, where it produces great ionisation (electrification) at abnormally low layers of the upper atmosphere. Short radio waves which travel round the Earth by being reflected from a higher layer of the upper atmosphere cannot penetrate this barrier of ionisation and a radio 'fade-out' is experienced. Long radio waves however may be reflected more strongly from the base of the lower layer of ionisation. Since these short range radio fade-outs and long wave enhancements are caused by the effects of ultra-violet light from the sun, they are confined to the sunlit side of the Earth and are almost simultaneous with the flare, lasting on the average for about 20 minutes. The stream of charged particles, travelling much more slowly than light, arrives at the Earth, if it is suitably directed, at from 1 to about 3 days after it leaves the sun; it visibly signals its arrival by producing a bright and active aurora. It too causes great ionisation in the upper atmosphere, which is much more prolonged than that caused by the ultra-violet light. There is again deterioration in short wave radio communications, which may be a complete 'black-out' in higher latitudes. At this time currents of the order of a million amperes may circulate in the upper atmosphere. The magnetic field of the fluctuating currents is appreciable at the Earth's surface and may deflect a compass needle noticeably from its normal position. The effects on these so-called magnetic and ionospheric storms, which may persist with varying intensity for several days, are usually greatest in higher latitudes. Radio 'black-outs' and simultaneous deviations of the magnetic compass needle by several degrees are not uncommon in and near auroral zones. When a great aurora is seen in abnormally low latitudes, it is invariably accompanied by a magnetic and ionospheric storm. Unlike the fade-out which occurs only on the sunlit side of Earth, the interference with radio communications which accompanies an aurora and magnetic storm may occur by day and at night. All these effects occur most frequently and in most intense forms at the time of sunspot maximum; maxima are likely to occur in 2001-02. Increases in solar activity could affect the reliability of GPS and other satellite systems; for further details see the relevant Admiralty List of Radio Signals. See the
ALRS
Publications
Charting and describingLocal magnetic anomalies are depicted by a special symbol on Admiralty Charts and are mentioned in Sailing Directions. The amount and direction of the deflection of the compass needle is also given, if known. |
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