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Compass

Magnetic Compass See also:Compass Error How to apply the Compass Error Top view of Gimbaled Magnetic CompassBefore 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°.