Science

Why do compasses point north? – Science and the future

A simple yet effective navigational tool, the compass points north. Hence the question asked by Roland Daeron on our Facebook page about the unchanging Question of the Week: why does the compass always point north and not south? First of all, it should be noted that the magnetic needle of the compass does not point to some kind of north: it shows what is conventionally called magnetic north. Here are some explanations.

The Earth’s core allows the use of compasses

The Earth’s magnetic field originates at a depth of 2900 kilometers. There is an earth’s core, consisting mainly of constantly stirred liquid iron. Inside it is a hard grain consisting 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, thereby protecting our planet.

The magnetic north pole should not be confused with the geographic north pole. Thus, the magnetic North Pole (as well as the South Pole) is defined as a point on the surface of the Earth where the magnetic field is strictly vertical (see diagram below). It is to this place that the compass relentlessly strives.

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 opposite. “The magnetic north pole is located in the far north of Canada, and the magnetic south pole is at the French base Dumont d’Urville in Antarctica,” explains the Institute of Physics of the Earth in Paris on its website. the magnetic field leads to a slow drift of the magnetic poles. Thus, the North Magnetic Pole is currently moving at a speed of 55 km per year towards Siberia.” And attracts magnetic compass needles.

When the magnetic field tries to change direction

The earth is not even immune to polarity reversal! The last time such an event almost stabilized was 42,000 years ago, causing the north magnetic pole to move south. It was relatively short and was called the Lachamp Excursion. The term “excursion” really means an event during which “the field tries to turn around, but fails, and therefore returns to its original state,” geophysicist Jean-Pierre Vale noted during the conference. Conversely, “inversion” usually reflects a completed and stable process. The last one happened over 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 another 250 years.

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