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A magnetic compass is a navigational instrument that displays directions in a frame of reference that is fixed relative to the Earth's surface. The frame of reference defines the four cardinal directions (or points) –north, south-east, and due west. Intermediate directions are also defined. Normally, a diagram called a compass rose, which shows the directions (with their names usually shortened to initials), is marked on the compass.
When the compass is in use, the rose is aligned with the real directions in the frame of reference, so, for good example, the "N" mark on the rose really points to the north. Often, in addition to the rose or occasionally instead of it, angle markings in degrees are shown on the compass. North corresponds to 0 degrees, and the angles increase clockwise, so east is 90 degrees, south is 180, and west is 270.
These numbers allow for the compass to show azimuths or bearings, which are normally stated in this notation. The magnetic compass was first invented as a device for divination as archaic as the Chinese Han Dynasty (Since about 206 before Christ. The compass was used in Song Dynasty China by the military machine for navigational orienteering by 1040-1044, and was used for nautical navigation from the year 1111 to 1117.
The use of a compass is registered in Western Europe between 1187 and 1202, and in Persian Empire in 1232.The dry compass was invented in Europe around 1300. This was superseded in the early 20th century by the liquid-filled magnetic compass.
A compass functions as a pointer to "magnetic north" because the magnetized needle at its center aligns itself with the lines of the Earth's magnetic field. The magnetic field wields a torque upon the needle, attracting one end or pole of the needle toward the Earth's North magnetic pole, and the other towards the South magnetic pole.
The needle is mounted upon a low-friction pivot point, in better types a precious stone bearing, so it can turn easily. Whilst the compass is supported level, the needle turns until, after a few seconds to allow for oscillations to die out, one end points toward the North magnetic pole.
A magnet or compass needle's "north" pole is specified as the one which is attracted to the North magnetic pole of the Earth, in northern Canada. Because opposite poles attract ("north" to "south") the North magnetic pole of the Earth is actually the south pole of the Earth's magnetic field. The compass needle's north pole is always marked off in some way: with a distinguishing color, luminous paint, or an arrowhead.
Instead of a needle, professional compasses generally have bar magnets affixed to the underside of a disk pivoted in the center so it can turn, known as a "compass card", with a "compass rose" displaying the cardinal points and degrees marked on it.
More dependable compasses are "liquid-filled"; the chamber carrying the needle or disk is filled with a liquid whose function is to dampen the oscillations of the needle in order it will settle down to point to North more rapidly, and in addition to protect the needle or disk from shock.
In navigation, directions on maps are conveyed with reference to geographical or true north, the direction toward the Geographical North Pole, the rotation axis of the Earth. Because the Earth's magnetic poles are near, but are not at the same locations as its geographic poles, a compass doesn't point to true north. The direction a compass points is called magnetic north, the direction of the North magnetic pole, located in northeastern Canada.
Dependent on where the compass is situated on the surface of the Earth the angle between true north and magnetic north, called magnetic declination can change widely, increasing the farther one is from the prime meridian of the Earth's magnetic field.
The local magnetic declination is given with most maps, to provide the map to be oriented with a compass parallel to true north. Some magnetic compasses include means to manually compensate for the magnetic declination, therefore the compass shows true directions.
In geographic regions near the magnetic poles, in northeastern Canada and Antarctica, fluctuations in the Earth's magnetic field cause magnetic compasses to have such large errors that they are ineffective, so other instruments must be applied for navigation.
The positions of the magnetic poles shift over time on a time-scale that is not extremely long by human standards. Important movements happen in a few years. A nautical magnetic compass is a navigational instrument that is an essential tool in a mariners world this is a very special gift for a mariner that they will treasure and take pride in.
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