What is declination?
At most places on the Earth's surface, the compass doesn't point exactly toward geographic north. The deviation of the compass from true north is an angle called "declination" (or "magnetic declination"). It is a quantity that has been a nuisance to navigators for centuries, especially since it varies with both geographic location and time. It might surprise you to know that at very high latitudes, the compass can even point south!
The collar of USGS topographic maps shows the magnetic declination at the center of the map the year that the map was made. That's important information for anyone who is using the map and a compass to navigate. NOAA has an online calculator for estimating the declination at any longitude/latitude on a specific date.
Declination is simply a manifestation of the complexity of the geomagnetic field. The field is not perfectly symmetrical; it has non-dipolar "ingredients," and the dipole itself is not perfectly aligned with the rotational axis of the Earth. If you were to stand at the north geomagnetic pole, your compass, held horizontally as usual, would not have a preference to point in any particular direction, and the same would be true if you were standing at the south geomagnetic pole. If you were to hold your compass on its side, the north-pointing end of the compass would point down at the north geomagnetic pole, and it would point up at the south geomagnetic pole.
The USGS Geomagnetism Program operates magnetic observatories in more than a dozen locations around the United States.
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In a sense, yes. The Earth is composed of layers having different chemical compositions and different physical properties. The crust of the Earth has some permanent magnetization, and the Earth’s core generates its own magnetic field, sustaining the main part of the field we measure at the surface. So we could say that the Earth is, therefore, a "magnet." But permanent magnetization cannot occur...
Is it true that Earth's magnetic field occasionally reverses its polarity? Is it true that Earth's magnetic field occasionally reverses its polarity?
Yes. We can see evidence of magnetic polarity reversals by examining the geologic record. When lavas or sediments solidify, they often preserve a signature of the ambient magnetic field at the time of deposition. Incredible as it may seem, the magnetic field occasionally flips over! The geomagnetic poles are currently roughly coincident with the geographic poles, but occasionally the magnetic...
Why measure the magnetic field at the Earth's surface? Wouldn't satellites be better suited for space-weather studies? Why measure the magnetic field at the Earth's surface? Wouldn't satellites be better suited for space-weather studies?
Satellites and ground-based magnetometers are both important for making measurements of the Earth’s magnetic field. They are not redundant but are instead complementary: Satellites provide good geographical coverage for data collection. Ground-based magnetometers are much less expensive and much easier to install than satellites. An array of magnetometers provides coverage from numerous locations...
What do the different north arrows on a USGS topographic map mean? What do the different north arrows on a USGS topographic map mean?
A diagram at the bottom of most USGS topographic maps shows three north arrows--true north, grid north, and magnetic north--and the angles between them. Some maps, especially very old maps, do not have this diagram. True north, also called geodetic north or geographic north, is the direction of the line of longitude that bisects the quadrangle. All longitude lines converge to points at the north...
Related
Is the Earth a magnet? Is the Earth a magnet?
In a sense, yes. The Earth is composed of layers having different chemical compositions and different physical properties. The crust of the Earth has some permanent magnetization, and the Earth’s core generates its own magnetic field, sustaining the main part of the field we measure at the surface. So we could say that the Earth is, therefore, a "magnet." But permanent magnetization cannot occur...
Is it true that Earth's magnetic field occasionally reverses its polarity? Is it true that Earth's magnetic field occasionally reverses its polarity?
Yes. We can see evidence of magnetic polarity reversals by examining the geologic record. When lavas or sediments solidify, they often preserve a signature of the ambient magnetic field at the time of deposition. Incredible as it may seem, the magnetic field occasionally flips over! The geomagnetic poles are currently roughly coincident with the geographic poles, but occasionally the magnetic...
Why measure the magnetic field at the Earth's surface? Wouldn't satellites be better suited for space-weather studies? Why measure the magnetic field at the Earth's surface? Wouldn't satellites be better suited for space-weather studies?
Satellites and ground-based magnetometers are both important for making measurements of the Earth’s magnetic field. They are not redundant but are instead complementary: Satellites provide good geographical coverage for data collection. Ground-based magnetometers are much less expensive and much easier to install than satellites. An array of magnetometers provides coverage from numerous locations...
What do the different north arrows on a USGS topographic map mean? What do the different north arrows on a USGS topographic map mean?
A diagram at the bottom of most USGS topographic maps shows three north arrows--true north, grid north, and magnetic north--and the angles between them. Some maps, especially very old maps, do not have this diagram. True north, also called geodetic north or geographic north, is the direction of the line of longitude that bisects the quadrangle. All longitude lines converge to points at the north...