The earliest adventurers became savvy navigators employing the positions of celestial bodies relative to the seasons and even time of day. At some point it was discovered directional enhancements could be made using a chunk of loadstone or even a piece of iron ore floating in a bowl of water. The first documented use is believed to have occurred in China around 1000 BC and although not initially used for navigation it did assist in alignment of buildings and monuments.
Initial use of a geomagnetic compass is believed to have occurred in the 11th century.
The earth is a rather significant permanent magnet spinning in space and is surrounded by a magnetic field. A sensitive layer of the atmosphere called the Magnetosphere is a region susceptible to solar winds and other ionizing phenomena which can have a pronounced impact on magnetically sensitive equipment. A key feature of this physical property is the ability to enable other magnets to align with the surrounding lines of flux.
The magnetic poles are considered points on the Earth’s surface where the magnetic field is entirely vertical. That is, the inclination of the Earth’s field is +90 degrees at the North Magnetic Pole and -90 degrees at the South Magnetic Pole. A compass held in a horizontal plane by Santa Claus while at home points randomly and at his off-season retreat in Ecuador it points to the poles.
Deviations in accuracy exist based on latitude and may require compensation in more sophisticated directional systems. Both of Earth’s poles wander independently and are usually not directly opposite. If the Earth’s magnetic field were perfectly dipolar, the geomagnetic poles would coincide. However, significant occurrences within our planet’s molten core and even severe solar flares will result in shift. The position of the two poles will vary and can migrate rapidly with movements of up to 22 nautical miles per year on record.
The Earth is a form of dipole whose orientation is related to the rotational axis. There are two positions where the axis best fits and are referred to as the North and South geomagnetic poles. The geomagnetic field should be placed about 270 nautical miles off the center of the Earth causing the inner reaches of the Magnetosphere to skim lower in the vicinity of the southern Atlantic ocean creating what is called the South Atlantic Anomaly.
It has been suggested that the Earth’s magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 300,000 years. The last such event, called the Brunhes-Matuyama Reversal, is believed to have occurred some 780,000 years ago.
There is no clear theory as to how the geomagnetic reversals might have occurred. Some scientists have produced models for the core of the Earth where the magnetic field is unstable and the poles can abruptly shift from one orientation to the other over the course of a few hundred to a few thousand years.
Other scientists propose that the geomagnetic dynamo turns itself off, either spontaneously or through some external action like a comet impact, and then restarts itself with the magnetic “North” pole pointing either North or South. External events are not likely to be routine factors in this reversal as there is a lack of data to correlate the age of impact craters and the timing of reversals. Regardless of the cause, the magnetic poles do periodically flip from one hemisphere to the other. Temporary variations also occur where the dipole axis crosses the equator and then returns to the original polarity and are known as excursions.
“Mag-Var” (Magnetic Variation) tables are included in the flight management system (FMS) software and are stored in memory. These charts include predicted changes in the magnetic variations between Latitudes 82 North and 82 South and are periodically revised to ensure a match with the latest available data. By the nature of the magnetic changes, these charts are never completely up to date.
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