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Magnetic declination (also called magnetic variation) is the angle between magnetic north and true north at a particular location on the Earth's surface. The angle can change over time due to polar wandering .
The first is magnetic declination or variation—the angular difference between magnetic North (the local direction of the Earth's magnetic field) and true North. [1] The second is magnetic deviation —the angular difference between magnetic North and the compass needle due to nearby sources of interference such as magnetically permeable ...
The local magnetic variation is indicated on NOAA nautical charts at the center of the compass rose. The magnetic variation is indicated along with the year of that variation. The annual increase or decrease of the variation is also usually indicated, so that the variation for the current year can be calculated.
By setting the date and latitude, the true north and the magnetic variation could be read from the instrument without the need for calculation. [8] With the iron vessels introduced in the 19th century the azimuth compass would be mounted on a pedestal at a neutral point where the readings would not be affected by magnetism of the vessel.
Changes in Earth's magnetic field on a time scale of a year or more are referred to as secular variation. 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.
The International Geomagnetic Reference Field (IGRF) is a standard mathematical description of the large-scale structure of the Earth's main magnetic field and its secular variation. It was created by fitting parameters of a mathematical model of the magnetic field to measured magnetic field data from surveys, observatories and satellites ...
Because Earth's magnetic field is a global phenomenon, similar patterns of magnetic variations at different sites may be used to help calculate age in different locations. The past four decades of paleomagnetic data about seafloor ages (up to ~) has been useful in estimating the age of geologic sections elsewhere. While not an independent ...
The Bangui magnetic anomaly in central Africa and the Kursk magnetic anomaly in eastern Europe (both in red) In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by ...