That the Earth is equipped with a magnetic field is known to most people who go to sea, that this field allows the operation of the Magnetic Compass also.
However, many do not know the causes of his presence, how and why it varies from year to year and what role it plays for our survival.
Let us start from the causes, that is, the composition of Our planet and in particular from the one hidden under the earth’s crust; there are various theses and theories but the one that is more valid and accredited to us is connected to the studies of theNational Institute of Geophysics and Volcanology:the theory of the dynamics of fluid iron present in the outer core.
At the center of the Earth there is in fact a molten core, the legacy of the formation of our planet. The interaction of this with the outer one formed mostly by iron, represents the real engine of the Earth’s magnetic field but is not the only one; the total magnetic field that will affect the direction of our compass is also given by other factors:
- Crust field, generated by the magnetized rocks of the earth’s crust.
- External field, generated by the electrical currents that flow into the ionosphere and magnetosphere as a result of the interaction between the solar wind and the geomagnetic field.
- Electromagnetic inductionfield , generated by induced currents in the Earth’s crust and mantle from the outer field with temporal variability.
On the Earth’s surface, the geomagnetic field represents the contribution of the Earth’s Magnetic field for about 99, rising in altitude instead the contributions of the other fields rise in percentage.
The geomagnetic field also plays an important task for our survival: it does not extend freely but is confined by the solar wind in a well-defined space that takes the name of magnetosphere; This interaction allows the most energy-laden sunlight to be deflected and shielded from severe solar storms.
With an approsimation of 95 it can be said that the Earth’s magnetic field (given by the result of those seen above) is similar to what would generate a large dipole (a large magnet) placed in the center of the Earth and oriented with respect to the axis Earth’s rotation of about 11.5 degrees to the west; This dipole would show its negative pole (S) towards the North Geographic, the Positive Pole (N) towards the direction of the geographical south.
The first approximation introduced is related to the fact that the direction of the geomagnetic field is not identical in all directions being connected to the dynamics of the Earth’s nucleus.
Now let’s see how the difference between North Magnetic and Geographic needs to be addressed for the purposes of Maritime Navigation.
Each geographical area, in fact, compared to the intensity of the three fields in play, will have a different difference between geographic north and magnetic.
This difference is known as Magnetic Decline, which varies from zone to zone and year by year.
The value of declination must be taken from the nautical charts of the area in which you navigate and appropriately updated with the annual variation value (the annual change may change in the years for which it is important to have the nautical charts always updated ).
The values are observed and elaborated by the National Institute of Geology and Volcanology.
The topic of magnetic declination will be further explored in another article.