Poles, Magnetic and Geographiac

Poles, Magnetic and Geographic

Many people do not realize that there are several different "north poles." The most familiar is the north geographic pole. The north magnetic pole is not the same as the north geographic pole. It is the point on Earth's surface that a compass needle points to. Other interesting north poles include the instantaneous north pole, the north pole of balance, and the geomagnetic north pole. The mathematics of measuring angles and degrees, as well as distances, are essential tools in locating these various poles.

Polar Exploration

The north geographic pole lies in the Arctic Ocean. It is the north pole important in map making, since it is the point where all of Earth's lines of longitude come together. The geographic south pole is in Antarctica. The expedition led by American explorer Robert E. Peary is generally considered to be the first to reach the north pole. The expedition included Peary, his assistant Matthew Henson and four Inuit: Ootah, Egingwah, Ooqueah, and Seegloo. They made the trip by dogsled in 1909.

In 1926, American explorers Richard E. Byrd and Floyd Bennett became the first persons to fly over the north pole. The submarine U.S.S. Nautilus became the first ship to reach the pole when it passed under the ice in 1958. Since then, many expeditions have traveled over the Arctic ice and various research stations have been established on the ice.

The position of the north magnetic pole was determined about the same time. Magnetic observations made by explorers showed that the magnetic north pole and the geographic pole were not in the same place. By the early nineteenth century, the accumulated observations proved that the magnetic north pole must be somewhere in Arctic Canada.

The first expedition to the area of the north magnetic pole was on a different mission. In 1829, the explorer Sir John Ross set out on a voyage to discover the fabled (and non-existent) Northwest Passage. His ship became locked in the ice off the northwest coast of Boothia Peninsula (in far northern Canada). During the four years the ship was trapped, James Clark Ross (Sir John's nephew) explored the Boothia coast and made a series of magnetic observations. These observations convinced him that the north magnetic pole was close by. So in the spring of 1831 he attempted to locate it. On June 1, 1831, at Cape Adelaide on the west coast of Boothia Peninsula, he measured a dip of 89° 59′. A dip of 90° would occur at the north magnetic pole, so he was within a few kilometers of the spot at the most.

In 1903, the Norwegian explorer Roald Amundsen made another attempt to reach the north magnetic pole. While the expedition had a secondary goal of trying (yet again) to find a northwest passage, his primary goal was to set up a temporary magnetic observatory in the Arctic and to make a second determination of the position of the north magnetic pole. According to Amundsen's observations, the pole was several kilometers north of where the younger Ross had located it.

Shortly after World War I, Canadian scientists measured a compass dip of 89° 56′ at Allen Lake on Prince of Wales Island. This, in conjunction with other observations made in the vicinity, confirmed that the pole was moving. It had moved 250 km (kilometers) northwest since the time of Amundsen's initial observations. Additional observations by Canadian scientists were made in 1962, 1973, 1984, and 1994. These observations have shown that the average position of the north magnetic pole is moving in a general northwestward direction at around 10 km per year. It also seems to be accelerating its northward motion.

Suppose you followed a compass north until you reached the north magnetic pole. How would you know you were there? At some point the compass would begin to move erratically. If you measured the vertical direction of the magnetic field it would be pointing almost straight down. The point where the magnetic field is exactly straight down (90°) is magnetic north. This pole can change position more quickly than any of the other poles. It can move many kilometers in a few years. It even has a daily motion following a roughly elliptical path around its average position. At times, it wanders as far as 80 km from its average position.

Today, the north magnetic pole is located near Ellef Ringnes Island in northern Canada. The magnetic south pole lies in Antarctica, near Vostok. It also appears to be moving, but, curiously, not in the same direction or at the same rate as the north magnetic pole.

Other Poles

Earth rotates on its axis (an imaginary line through the Earth) once every day. However, Earth wobbles a bit as it rotates, so this axis is not fixed. The instantaneous north pole lies at the point where Earth's axis passes through the surface at any moment. This point is close to the north geographic pole, but it is not exactly the same. The path followed by the instantaneous north pole is an irregular circle called the Chandler Circle whose diameter varies from a few centimeters to a few meters. It takes about 14 months for the instantaneous north pole to complete one circle.

The average of the positions of the instantaneous north pole is called the north pole of balance. It lies at the center of the Chandler Circle. The pole of balance marks Earth's actual geographic north pole, which is not exactly the same as the map makers' geographic north pole. Even this point is gradually moving toward North America at around 15 centimeters per year. Consequently, the latitude and longitude of every point on Earth is constantly changing.

The geomagnetic north pole is defined by the magnetic lines of force that loop into space outside of Earth. The geomagnetic field is sort of the average of the local magnetic field. The geomagnetic north pole lies near Etah, Greenland, north of the town of Thule. In the upper atmosphere, Earth's magnetic field points down toward Earth at this point.

Magnetic Declination

Since the geographic north pole and the magnetic north pole are different, a magnetic compass usually does not point to geographic north. The difference in degrees between magnetic north and geographic north is called the magnetic declination. Since Earth does not have a uniform magnetic field, the magnetic declination at any point on Earth's surface is the result of a complex interaction between Earth's various internal and external magnetic fields. The magnetic declination also changes over time as the magnetic north pole moves around and as local variations in Earth's magnetic field change.

Navigating with a magnetic compass requires adjusting the compass by adding or subtracting the magnetic declination from the compass reading. In the western United States, the compass needle will point a few degrees east of geographic north, and in the eastern United States the needle will point a few degrees west of geographic north. So to get the correct compass reading, the magnetic declination must be added or subtracted from the compass reading. Many modern compasses can be adjusted for magnetic declination by turning a small screw. Then the compass may be read directly, without having to add or subtract the declination.

Magnetic Anomalies

In addition to corrections that must be made for the differing positions of the north geographic and magnetic poles, corrections must also be made for local magnetic anomalies. These are local variations in the magnetic field that cause a compass to be off as much as three or four degrees from what would be expected from the position of the north magnetic pole.

Causes of the Magnetic Field

Seismic data have shown us that Earth has a molten core. Because of the overall high density of Earth, much higher than surface rocks, the core is most likely nickel and iron. Shortly after Earth first formed, it got hot enough to melt all the way through. The heavy materials, such as nickel and iron, sank to the center. Since then, Earth has been gradually cooling off. However, the core is still molten. The molten core is due to leftover heat from Earth's formation. At the center of the molten outer core, there is a solid inner core of nickel and iron. Earth's solid inner core actually rotates a tiny bit faster than the rest of the planet, making one additional rotation every 400 years or so.

Although Earth's magnetic field resembles the magnetic field that would be generated by a huge bar magnet, a bar magnet is not the source. It is not even a permanent feature of the planet. The circulation of electrical currents in the hot liquid metal in the outer core generates the magnetic field. Earth's rotation contributes to this. Venus probably has a core similar to Earth, but Venus probably does not rotate very rapidly and, consequently, it does not have a strong magnetic field. The theory that explains planetary magnetic fields in terms of rotating, conducting material in the core is known as dynamo theory. Both rapid rotation and a conducting liquid core are necessary.

Moving Poles

If the Earth acts as a large permanent bar magnet, the magnetic pole would not move, at least as rapidly as it does. In nature, processes are seldom simple. The flow of electric currents in the core is continually changing, so the magnetic field produced by those currents also changes. This means that at the surface of the Earth, both the strength and direction of the magnetic field will vary over the years. As the strength and direction of the electrical currents in the outer core changes the magnetic field of Earth changes. So the position of the north magnetic pole slowly moves across the Arctic.

Magnetic Reversals

Earth's magnetic field has regularly reversed its orientation many times over the last few million years. Such reversals seem to be part of the way in which planetary magnetic fields are generated. The current rapid and accelerating movement toward the northwest of the north magnetic pole may be in some way related to an impending magnetic reversal in Earth's core.

see also Flight, Measurements of; Navigation.

Elliot Richmond

Bibliography

Asimov, Isaac. The Ends of the Earth. New York: Weybright and Talley, 1975.

Chaisson, Eric and Steve McMillan. Astronomy Today, 3rd ed. Upper Saddle River, NJ: Prentice Hall, 1993.

Epstein, Lewis Carroll. Thinking Physics. San Francisco: Insight Press, 1990.

Giancoli, Douglas C. Physics, 3rd edition. Englewood Cliffs, NJ: Prentice Hall, 1991.