Magnetic declination
Declension, declination or even local declination ( engl. magnetic declination or variation) called, is the angle between the magnetic and geographic north, which must be taken into account especially when navigating with the compass.
- 2.1 Internal and external causes
- 2.2 history
Earth's magnetic field and declination
Geographic and magnetic pole
The geographic north pole is defined by the axis of rotation of the earth. He is the one point where the Earth's axis passes through the earth's surface.
The Arctic magnetic pole is defined as that region of the earth's surface, in the Earth's magnetic field enters perpendicularly into the soil. He is - determined by the position and shape of the earth's magnetic field - in the Canadian Arctic in 2005 was about 800 km from the geographic North Pole and changes its position every year by several kilometers. ( The magnetic polarity after he is a Pole, see compass. )
A located between the magnetic and geographic pole compass does not point to the geographic pole, but in the opposite direction to the magnetic pole - the declination is in this extreme case 180 °. However, in less near polar regions is significantly lower.
It is often assumed, the different position of magnetic and geographic pole is generally the reason why a compass does not exactly show the geographic North Pole. That this can not be the sole cause, have a glance at a Deklinationskarte: The Arctic magnetic pole is currently about 132 ° west longitude. For all the places on the meridian 132 ° West are magnetic and geographical pole in the same direction, so the declination should be zero according to this argument. In fact, it is on this meridian, even in temperate northern latitudes ( in the Pacific Ocean off the North American coast ) between ten and twenty degrees.
Earth's magnetic field and poles
Had the Earth's magnetic field such a simple and regular shape, such as the field of a bar magnet or a dipole, so compasses would always show exactly the Arctic magnetic pole. The poles of this simple magnets can be considered a good approximation as those points in the magnet, which give rise to all field lines or into which they plunge again. The field of such magnets is essentially determined by the location and the strength of the poles.
The Earth's magnetic field, however, has a more complicated structure in which the - otherwise defined - Pole play only a minor role. It is produced in an extended region of the outer Earth's core and the magnetic poles are defined as only those places on the Earth's surface where the magnetic field is perpendicular. [Note 1] you do not have to do directly with the production and the shape of the Earth's magnetic field - they are a consequence of the shape of the field.
The outer geomagnetic field can imagine produced approximated by a bar magnet inside the earth. If this spare field as well as possible correspond to the real field, so would this imaginary bar magnet in the center of the earth be very short. Its poles subject thousands of miles below the earth's magnetic field of the poles, which are, by definition, at the surface. This observation emphasizes that the poles of the earth's magnetic field are not centers of attraction as the poles of a bar magnet; they are only those points on the surface where the magnetic field is randomly vertically.
Therefore, the field lines of the geomagnetic field run mostly not from one pole to the other. They occur rather - generated deep within the earth - in all places south of the magnetic equator of the earth's surface and over the northern hemisphere again distributed in the earth's surface (see → magnetic meridian). The orientation of the compass is therefore not controlled by the magnetic poles, but only by the direction of the local magnetic field. The direction of the local magnetic field at the Earth's surface, in turn, is determined by the distribution of the field sources within the Earth and not by the position of the poles. Thus, although the compass needle points in a generally northerly direction because of the dipolähnlichen shape of the overall field, giving way to large parts of the earth's surface, the local direction of the magnetic field by 10 ° or more from the direction of the magnetic north pole.
Declination
The compass always aligns itself with the direction of the local magnetic field lines and is therefore generally not exactly at the geographical nor to the magnetic North Pole. The declination is defined as the angle between the direction of magnetic field lines at the observation and the direction of the geographic North Pole. [Note 2]
The exact global distribution of declination can be detected only by measurement. Suitable decorated magnetic observatories are able to measure the declination with an accuracy to 5. " To use for navigation with the compass (eg on ships), the measured local declinations are shown in Isogonenkarten. The drawn Isogonals are loci of equal declination. on official nautical charts or topographical maps, a representative for the displayed area of single numerical value is always given. In aviation, the declination is also called variation ( VAR) and can be seen from the aeronautical charts.
History
A first observation of the declination was probably made around the year 720 by the Chinese astronomers Yi Xing. From the years 720-1280 to Chinese sources can be found in at least nine provisions of the declination. European sources suggest that the declination in Europe was basically known since the early 1400ern. The first European measurement of the declination is probably one of Georg Hartmann in 1510 carried out in Rome observation.
Gerhard Mercator graduated in 1546 from observations of declination, that the point at which the magnetic needle points, is not located in the sky (that is not identical to the Pole Star, as partially suggested ), but lies on the ground. William Gilbert in 1600 described the earth itself as a large magnet whose poles attract the ends of the magnetic needles.
Portuguese navigators developed methods to determine the declination at sea and to be able to use the compass for navigation. João de Castro, for example, established 1538-1541 on his travels to the East Indies, along the west coast of India and the Red Sea 43 declination - the first attempt to map the world's declination. Edmond Halley's two voyages through the North and South Atlantic, 1698-1700 were the first voyages undertaken for purely scientific purposes and resulted in 1702 in a first Deklinationskarte for the entire earth.
Variability of the declination
Internal and external causes
The main part of the earth's magnetic field is generated by convection currents in the Earth's core. These operations are subject to long-term changes, which also imply changes in the declination according to the secular changes as mentioned. This play on time scales of a few years from up to a few million years. They manifest themselves in Europe at present mainly in a slow drift of the West Isogonals about 20 km per year. The secular variation of declination can be for several years approximately predict.
For Munich, the declination in 1840 was over 17 ° West and since then assumes continuous easterly values . She reached the end of the 1980s, 0 °, is currently (2014) good 2 ° East and adopts each year by 7.5 arc minutes.
Minor portions of the earth's magnetic field come from the magnetosphere and the electrically conductive E- layer of the ionosphere. These shares are subject to rapid changes on time scales of fractions of a second up to several years. Both magnetosphere and ionosphere are linked in particular interaction with the solar wind. His influence varies due to the Earth's rotation on a daily basis and leads among other things to a pronounced diurnal cycle of declination.
On magnetically quiet days ( Sq - variation ) the daily variation follows a not precisely predictable in the individual case, however, the typical pattern in the middle. The strength of the variation depends on the time of day, time of year, the geographic (or more precisely the magnetic ) width and other factors. In northern latitudes, the declination typically takes about eight or nine clock in the morning local time, an Eastern maximum, 13 or 14 against the clock follows a Western extreme views. It closes in the afternoon and at night a slow eastern drift until the next morning an eastern maximum is reached again. In southern latitudes, the western maximum is in the morning and the eastern in the afternoon. The strength of the variation is larger than in the winter and in the vicinity of a magnetic pole is larger than at the equator in the summer. In Germany, the typical daily variation in winter is about four minutes of arc, on hot summer days, about eight to ten minutes of arc.
During a magnetic storm the declination near the poles to 30 ° and more can vary, in temperate latitudes up to about 2 °.
History
The variability of the geomagnetic field was first noted by Henry Gellibrand, which in June 1634, the declination in London by more than 7 ° was smaller than William Borough had it measured in October 1580. The investigations of Edmond Halley ( including the lessons on two voyages from 1698 to 1700 magnetic measurements ) pointed to a West drift of the geomagnetic field. The London clockmaker George Graham succeeded in 1722 with a 2 ' readable magnetic needle discovering stable daily changes in direction of the geomagnetic field, which in a day, sometimes within hours, 30' could exceed. He distinguished between the first quiet and disturbed days. In 1741 Olav Peter Hiorter and Anders Celsius introduced in Uppsala established a connection between magnetic storms and auroras when they observed changes in the declination of 4 ° within 4 minutes.
John Canton exhibited in 1759 in London after that the mean motion of the needle was greater on calm days in summer ( about 13 ') than in winter (less than 7' ), which means variability with annual period had been identified for the first time ( Cassini showed in 1782 that it was not a mere influence of temperature in Paris). The organized scientific study of the earth's magnetic field began in the early 19th century with the works of Humboldt in Berlin, Arago in Paris, as well as Gauss and Weber ( the founders of the Magnetic Association ) in Göttingen.
Declination and navigation
In German-speaking declination is currently only about one to four degrees to the east and increases annually by about six to eight minutes of arc to the east. It can be ignored because of their small amount for many orientation tasks ( like walking ). For higher accuracy requirements, or in areas with larger declination ( for example, achieved in Canada, USA, South Africa and New Zealand up to 20 ° and more) but it must be taken into account.
The course angle with respect to true north and magnetic north can be converted with knowledge of the current local declination simply interlocked. Here, however, is to carefully observe the correct sign. If magnetic north is west of true north, so is referred to as the declination is west or counts them negatively; magnetic north is east of true north, so is referred to as the declination east or one of them positive.
A rule of thumb is: ". From false to true course with the true sign of the true to the wrong course with the wrong sign " The "true" rate is calculated on true north course (see right-handed ), the "wrong" course related to the magnetic north ( cf. miss -setting ).
If, therefore, be converted to a magnetic north -related course in a related geographically North Course (false → true), the declination is to be added taking into account their specified sign. If a be converted to true north oriented course in a related magnetic north course ( true → false), the declination is to be added under reversal of their signs.
Other methods for course transformation in the orientation using the compass in the terrain see → hiking compass.
For details of the course transformation in the maritime and aviation see → Course feed.
Determination of the declination
For the price conversion just described the Navigator requires the current local declination. You can refer to appropriate maps or retrieve Deklinationsrechnern (see links). These data are usually made of magnetic mathematical models, which are derived from global measurements and allow to determine also values for places and ( to a limited extent ) for times, where have been no direct measurements.
Examples of such models are the International Geomagnetic Reference Field ( IGRF ) or the World Magnetic Model ( WMM). However, these models describe only originating from the Earth's core large-scale field strength distributions. Small-scale field structures formed in the Earth's mantle and crust will not be considered. The models can therefore have in comparison with the real field localized deviations of a few degrees in declination and inclination and more. Such errors are mostly on land, on continental margins and seamounts, seamounts and trenches to find. In some cases, local geological structures, may cause deviations of 50 ° and more from the global model. At sea, give the models in general, the measurements with a scatter of about 0.5 ° again.
Naturally, are not taken into account in the models and the non - secular shorter -term changes such as the diurnal variation or magnetic storms. The accuracy of the models is limited. If the declination is not known or only with insufficient accuracy, so it can be determined for the current location, by the determined with the compass bearing of a distant target as possible ( ie, its direction relative to magnetic north ) with the removed from the map or otherwise determined direction of true north is compared with respect. The determined correction angle not only corrects the declination, but also any constant local magnetic disturbances and certain compass systematic error (eg a not exactly along the needle extending magnetic axis, a slightly twisted Inappropriate graduation, etc.). Do not be corrected by changing local disturbances, such as the influence of a ballpoint pen, which is held from one measurement to another in the hand. The precision of the data in this way declination corresponds approximately to the accuracy of the compass used.
Trivia
The runway identifiers of airports are usually derived from the related magnetic north path directions. The corresponding number of degrees is divided by ten and rounded to an integer. A sheet having the identifier 04/ 22, for example running in the direction of 40 ° ( and 220 ° in the opposite direction). Because of the secular change in declination, the reference direction in the longer term changes magnetic north and thus the related magnetic north direction of the web. It comes Occasionally, runway identifiers must be changed. For example, the designation of the runway of the airport of Salzburg on 23 August 2012 from 16/34 to 15/33 has been updated. ( The local declination increases, and the reference direction magnetic north so drifts to the east, the magnetically north through east counted number of degrees of path direction is therefore smaller. )
Related terms
Meridian convergence is the angle between true north and grid north. The grid lines of a map are parallel to the main meridian and only in this fall true north and grid north together.
Needle deviation (English grivation ) is the angle between grid north and magnetic north of a card. The following applies: needle deviation = magnetic declination - Meridian convergence. The sign must be observed, that the directions of the deviations.
Deviation or distraction is the angle between magnetic north and deflected by local interference real magnetic needle.
Is the inclination angle at which the flux lines pass through the earth's surface.