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With a local declination of 14°E, a true bearing (i.e. obtained from a map) of 54° is converted to a magnetic bearing (for use in the field) by subtracting declination: 54° – 14° = 40°. If the local declination was 14°W (−14°), it is again subtracted from the true bearing to obtain a magnetic bearing: 54°- (−14°) = 68°.
18th-century azimuthal compass held in the National Archaeological Museum of Spain. An azimuth compass (or azimuthal compass) is a nautical instrument used to measure the magnetic azimuth, the angle of the arc on the horizon between the direction of the Sun or some other celestial object and the magnetic north.
Over hundreds of years, magnetic declination is observed to vary over tens of degrees. [13] The animation shows how global declinations have changed over the last few centuries. [34] The direction and intensity of the dipole change over time. Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century ...
Mercator suggested that the magnetic north pole was an island in the longitude of the Azores, where magnetic declination was, at that time, close to zero. These ideas were supported by Michiel Coignet in his Nautical Instruction. [55] Halley made extensive studies of magnetic variation during his voyages on the pink Paramour.
Magnetic declination from true north in 2000. Historically, the magnetic compass was an important tool for navigation. While it has been widely replaced by Global Positioning Systems, many airplanes and ships still carry them, as do casual boaters and hikers. [28] The direction in which a compass needle points is known as magnetic north.
In mid-December of 2024, scientists officially updated the World Magnetic Model (WMM), which helps keep track of our planet’s magnetic north and its chaotic and relatively unpredictable movements.
Magnetic declination map at sea-level for the year 2010 derived from WMM2010. The World Magnetic Model ( WMM ) is a large spatial-scale representation of the Earth's magnetic field. It was developed jointly by the US National Geophysical Data Center and the British Geological Survey .
The existence of magnetic declination, the difference between magnetic north and true north, was first recognized by Shen Kuo in 1088. [3] The first mention of a compass in Europe was in 1190 AD by Alexander Neckam. He described it as a common navigational aid for sailors, so the compass must have been introduced to Europe some time earlier.