Understanding how the earth’s magnetism works helps sailors understand how the compass works.
In this guide I will talk about the difference between magnetic north, declination and magnetic heading.
External forces in navigation
Remember that every boat is subjected to external forces – other than propellers and thrusters. If these external forces did not exist, to navigate in the correct direction it would be sufficient to orient the true heading, toward the value indicated below the line on the wind rose.
We should calculate the effect produced by the Earth’s magnetic field on the compass needle, and add it to other external agents, such as declination.
It is essential to calculate the declination value in order to obtain the magnetic heading value, which should be read below the compass rose at the line of belief.
Now, let’s go see how magnetism works.
How magnetism works in a compass
In the compass there are two magnets that are attracted by the earth’s magnetic field.
As for the Earth’s magnetic axis, it is made up of the different metals found within the Earth’s core. At the north end of the Earth we have the south pole of the magnet, or Magnetic North, which has negative polarity and is very close to the Geographic North Pole.
At the South end we have the positive pole, representing the north pole of the magnet, which is near the Geographic South Pole.
This may seem like a complex concept, but suffice it to say that opposite poles attract.
The compass needle also has the same polarity and points, under optimal conditions that we will see later, toward magnetic north.
Magnetic North vs. geographical North
As we have said, magnetic North and geographic North do not coincide. In fact, the Earth’s axis is not in a constant position, so the magnet within it continues to rotate slowly.
This continuous rotation generates a change in the position of polarity, which affect the magnetics of the compass.
By magnetic north we mean the union of magnetic meridians-which differ from geographic meridians.
Magnetic North then is North is indicated by any compass on land or on fiberglass boats, free of metal elements or live electrical equipment, including cell phones.
Magnetic North keeps on shifting towards the geographic North Pole (also known as True North).
What is declination
The angle between True North and Magnetic North is known as “declination” and is critical to calculate for mariners who are very close to True North and wish to reach it.
To find geographic North, you should simply perform a subtraction: subtract from Magnetic North the angular value of declination, which varies depending not only on the weather, but also on where you are.
The result will be the magnetic heading. The captain of the boat will then be able to reach exactly True North.
Declination can have a positive sign, if the compass needles are moved relative to the position of True North toward the East, while if they are toward the West the declination has a minus sign, with a maximum value of 180 degrees.
Where is the declination read?
In the center of the Wind Roses of each chart is marked the declination.
Be careful, however: the value given for declination refers to the date the chart was issued.
To calculate the current year’s declination, one must multiply the annual increase of about 7 primes by the years since the edition of the nautical chart, and then add them to the values read.
For Italian ministerial charts, for example, you should know the definition of declination as the difference between True North and Magnetic North.