Why do compasses point north?

A simple but effective navigational instrument, the compass points north. Hence the question posed by Roland Daheron on our Facebook page on the occasion of the immutable Question of the week: why does the compass always indicate north, and never south? First of all, it should be noted that the magnetic needle of a compass does not point just any north: it shows what is conventionally called magnetic north. Here are some explanations.

The earth core allows the use of compasses

The Earth’s magnetic field originates at a depth of 2,900 kilometers. There is the earth’s core, composed essentially of constantly stirred liquid iron. Within it lies a solid seed composed of crystallized iron and nickel. This structure – and this convective motion in the liquid core – gives rise to the Earth’s magnetic field. The latter is of capital importance: it deflects solar wind particles and cosmic particles, thus protecting our planet.

The Magnetic North Pole should not be confused with the Geographic North Pole. Thus, the magnetic North Pole (like the South Pole) is defined as a point on the Earth’s surface where the magnetic field is exactly vertical (see diagram below). It is towards this place that a compass tirelessly tends.

Description of the characteristics of the Earth’s magnetic field during the Swarm mission. Credits: Sophie Ramis, abm / AFP

The North and South magnetic poles are not exactly antipodal. “The North Magnetic Pole is in the Canadian Far North, while the South Magnetic Pole is off the French base of Dumont d’Urville in Antarctica, explains on its website the Institut de Physique du Globe in Paris. The secular variation of the Earth’s magnetic field results in a slow drift of the magnetic poles. Thus the North Magnetic Pole is currently moving at the speed of 55 km per year towards Siberia“. And he dresses with him the magnetic needles of the compasses.

When the magnetic field tries to reverse

The Earth is not even immune to a polarity inversion! Such an event was last near-stabilized 42,000 years ago, causing the magnetic north pole to shift south. It was relatively short and took the name “Laschamps excursion”. The term “excursion” indeed translates an event during which “the field tries to reverse without succeeding and therefore returns to its initial state”, relevant during a conference the geophysicist Jean-Pierre Valet. Conversely, an “inversion” usually results in a complete and stable process. The last one took place more than 770,000 years ago.

42,000 years ago, the magnetic north pole moved south. This process did not happen overnight: it took 500 years. The poles stayed that way for 500 years… before turning back for another 250 years.

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