Geographic, magnetic, of inaccessibility… But how many North Pole are there?

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When they say: show me the North, or where exactly is the North Pole, exactly where on Earth do we turn? The question is not a wandering question, for there is more than one north, and therefore of the north pole. If we limit ourselves to the most common ones, we can find at least five of them: ithe geographic North Pole, also called terrestrial North Pole or true North; the Magnetic North Pole; the Geomagnetic North Pole; the North Pole of Inaccessibility and the Celestial North Pole. Let’s see them in order.

Geographical north pole

The first and best known is the geographic North Pole, also known as True North. It is the point at which the axis around which the earth rotates passes. Alternatively, it can be said to be the point with latitude 90° north. It is curious to note that at the North Pole, as well as at the South Pole, there is no latitude. Here the meridians meet and so if from the geographic North Pole we move in a straight line, we go any way towards the geographic South Pole, and vice versa.

Thanks to extremely precise measurements of the position of some stars, at the end of the 19th century the American astronomer Seth Carlo Chandler discovered that the geographic North Pole does not really standstill. Instead, he describes an approximate circle of three meters radius with the so-called “average rotation pole” at the center. Chandler’s oscillation, as it was named in his honor, is this circular motion that the earth makes in about a year, two months and 10 days.

The cause is the irregular shape of the planet and the imperfect alignment between the Earth’s axis of rotation and the axis of inertia. This misalignment causes the Earth, in addition to the rotation on its own axis, to oscillate slightly, just as it does with a spinning top that is not perfectly balanced. A study published in 2007 by Richard Gross of the Jet Propulsion Laboratory Nasa would show that the main cause of this phenomenon is due to the percentage of salts dissolved in seawater. Such variations in pressure on the ocean floor would periodically unbalance the Earth.

I wonder where the pole is…

A last resort, however, it is impossible to find a measurement or reference system (including geographical coordinates with latitude and longitude indications) where all the founding points are fixed. Clump tectonics, with the consequent continental drift, volcanism, erosion and other factors make it necessary to arrive at conventions. Thus the International Earth Rotation and Reference Systems Service (IERS) and the International Astronomical Union (UAI) have developed a mathematical model: the International Earth Reference System (ITRS), according to which the geographic North Pole is a conventional point that is not on the Earth’s axis of rotation.

 

Magnetic North Pole

The definition is more like four centuries old. It was given by William Gilbert, a dignitary at the court of Elizabeth I of England. He formulated it more or less like this: the magnetic north pole is the boreal point of the Earth’s surface where the flow lines of the geomagnetic field are perpendicular to the ground and directed towards the ground. At this point on Earth a compass working with a horizontal axis of rotation would point towards the ground. In essence, the definition is still valid today. Although it is based on a basic contradiction. It would actually be the magnetic south pole. The Magnetic North Pole of the Earth in fact attracts the magnetic north pole of magnets, i.e. it is actually a magnetic south pole. Given, however, its proximity to the True North was called the Magnetic North Pole.

And yet it moves

The Magnetic North Pole is currently located at 86.50°N and 164.04°E. It is off the coast of Canada and is moving (at a speed that has varied from 11 km per year at the beginning of the century to about 60 in recent years) towards Siberia where it could arrive around 2050. In 2019 the magnetic North Pole crossed the Greenwich Meridian twice in one year. This forced to update the World Magnetic Model (VMM), a tool developed by the US National Oceanic and Atmospheric Administration and the British Geological Survey that provides data to satellites of modern navigation systems.

Where to point my compass

According to the WMM model, the 2020 position of the north magnetic pole is 86.50°N and 164.04°E and the south magnetic pole is 64.07°S and 135.88°E.

But it is not a datum to the millimetre, on the contrary, it is not precise even to the km, because because of the disturbance caused by the solar magnetism the magnetic north of the Earth describes an ellipse, with a maximum displacement from its average position of 80 km. However, wherever it is, the magnetic north pole is more than a point, it is an area and also of good extension that, depending on the periods, varies its size.

 

Pole Inversion

The
North Pole and South Pole magnetic (which are not at the respective antipodes, given the slight asymmetry of the Earth’s magnetic field: a line between the two would pass about 530 km from the center of the planet) have also been reversed several times in the history of the Earth. In one study published in 2018 in the journal Science Advanced based on ice cores, volcanic deposits and rocks of the ocean floor it was seen that in the last magnetic inversion occurred 770 thousand years ago. But it was not a phenomenon that in a second “flipped” the Earth’s magnetic field. The operation lasted a total of 22,000 years to complete. The actual inversion took four thousand years, preceded by a period of instability and two partial and temporary inversions lasting 18 thousand years.

Magnetic declination

As the maps refer to the geographic north pole, while compasses indicate the magnetic north pole that does not coincide navigation had to take this difference into account. In other words, geographical meridians and magnetic meridians (i.e. magnetic field flux lines) are not parallel. The angle formed by the two different meridians at each point on the Earth’s surface is called magnetic declination. In other words, it is the angle at that point between the compass needle and the position of the geographic north (according to the map).

As can be seen a little above from the animated gif that shows the declination trend from 1590 to 1990, this magnitude varies not only according to the place on the Earth’s surface where it is measured but also according to the time in which the measurement was made, because the magnetic and geographical poles move over the years. By convention, the eastern declination, i.e. where the compass needle points further east than the true North is considered positive (to find true North to the degrees read on the compass, the degrees of declination must be added); conversely, the western declination, i.e. when the compass needle points further west than the geographical North, is negative, as it must be subtracted from the compass reading to indicate the geographical North. Each nautical chart shows the magnetic declination of that area at a certain date (which is indicated) and its annual variation.

Geomagnetic North Pole

Everyway magnetism has something to do with it but we are talking about another pole, the North Geomagnetic pole, defined as the point to the north where the axis of the Earth’s magnetosphere ends. As for the Magnetic North Pole, it is magnetically a South Pole. It is not a real point, but a convention based on the physical model of the dipole (a bar magnet), which in 90% of cases explains the real behavior of the Earth’s magnetic field. According to this theory, a magnetic field acting as it is measured experimentally could be generated by a huge magnetic bar that crosses the Earth from side to side at an angle of about 11.5° to the Earth’s axis of rotation. The point at which it intersects the Earth’s surface is called the geomagnetic north pole. Geomagnetic north pole and magnetic north pole do not coincide, currently there is an average distance of about 30° longitude. Also this pole moves in time, but with a lower speed and distances than its magnetic relative.

Northern Lights

The area around the geomagnetic north pole is the one most affected by the northern lights. Here in fact Van Allen’s Fasces disappear, that plasma ring that protects the Earth from the solar wind, the flow of ions, waves and charged particles emitted by the sun and very harmful to all forms of life. Where the bands do not block their arrival, as basically happens at latitudes lower than the polar circles, placed at about 23° of latitude, when the solar wind hits the ionosphere, these phantasmagorical plays of light in the sky are created.

North Pole of Inaccessibility

north pole inaccessibility

There is then the North Pole of Inaccessibility. Unlike the others it is not characterized by physical peculiarities, but, like the other poles of inaccessibility, it has the characteristic of being complex to reach for some reason. In this case, it indicates the point of the Arctic Ocean furthest from any coastline. It is located about 1 100 km from the nearest coast at the following geographical coordinates: 84°03′N – 174°51′W.

The Celestial North Pole

north celestial pole
The polar star is almost perfectly in the center of the circle apparently made by the other stars during the night.

According to the UAI, the north pole of an object of the solar system and therefore also of the Earth, is the one that faces the north pole of the plane that forms an angle of 90° with respect to the vector of the total angular momentum of the solar system. Since the orbits of the planets are little inclined with respect to the ecliptic (the apparent motion of the sun against the background of the fixed stars), the invariant plane and the plane of the ecliptic are almost coincident and makes the north pole of a planet the one that faces the north pole of the ecliptic plane.

Goddess polar star

The Celestial North Pole is the projection of the Geographic North Pole of a planet on the celestial sphere. At this moment the celestial North Pole is in the immediate vicinity of the North Star or Polaris. The Earth’s axis of rotation, however, is not fixed. Like a spinning top, it moves in a circular motion. It is as if it were drawing two opposing cones with the vertices in the center of the Earth. This phenomenon gives rise to the so-called precession of the equinoxes. In the short period of time, this does not have consequences, but in more prolonged epochs things change. Just to say, in about 13 thousand years our Earth’s axis will no longer point towards the polar star but in the direction of Vega.

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