Earth's magnetic field

Our planet is a giant magnet. Current theory suggests that electric currents circulating in its molten core form an electromagnet. This is consistent with evidence suggesting that the north-south direction of the field has reversed a few times in Earth's history. The average strength of the field is strong enough to cause a lightweight magnet like a compass needle to align with the field lines, which are illustrated below.

Image: Xaktly. (CC BY-NC-SA)

Earth’s liquid iron core forms a convection current because it is heated from beneath by the inner core. Because iron is a metal and conducts electricity (even when molten), its motion generates a magnetic field.

Image: Wikipedia Commons (CC BY)

Earth’s magnetic field is defined by north and south poles representing lines of magnetic force flowing into Earth in the northern hemisphere and out of Earth in the southern hemisphere. 

Because of the shape of the field lines, the magnetic force is oriented at different angles to the surface in different locations. The tilt, or inclination of magnetic field lines is represented by the tilt of compass needles in the diagram below. At the north and south poles, the force is vertical. The force is horizontal at the equator. Everywhere in between, the magnetic force is at an intermediate angle to the surface.

Image: University of Saskatchewan. Panchuk, K. (CC BY-NC-SA)

Earth’s magnetic field depicted as the field of a bar magnet coinciding with the core. The south pole of the magnet points to Earth’s magnetic north pole. The red and white compass needles represent the orientation of the magnetic field at various locations on Earth’s surface. 

Earth’s magnetic field is generated within the outer core by the convective movement of liquid iron, but although convection is continuous, the magnetic field is not stable. Periodically, the magnetic field decays and then becomes re-established. 

When it does re-establish, the polarity may have reversed (i.e., your compass would point south rather than north). Over the past 250 million years, there have been hundreds of magnetic field reversals, and their timing has been anything but regular. The shortest ones that geologists have been able to identify lasted only a few thousand years, and the longest one was more than 30 million years.

Magnetic declination

The difference between the rotational and magnetic poles has a consequence for us when we're using a compass for navigation. In order to align a map properly along the north-south line, we need to account for the fact that our compass will not be pointing exactly to true north, but to magnetic north.

There are only a few locations on Earth where it points exactly to the True (geographic) North. The direction in which the compass needle points is known as Magnetic North, and the angle between Magnetic North and the True North direction is called magnetic declination.

Magnetic declination varies both from place to place, and with the passage of time. As a traveler cruises the east coast of the United States, for example, the declination varies from 20 degrees west (in Maine) to zero (in Florida), to 10 degrees east (in Texas), meaning a compass adjusted at the beginning of the journey would have a true north error of over 30 degrees if not adjusted for the changing declination. The magnetic declination in a given area will change slowly over time, possibly as much as 2-25 degrees every hundred years or so, depending upon how far from the magnetic poles it is. 

Complex fluid motion in the outer core of the Earth causes the magnetic field to change slowly with time. This change is known as secular variation. Because of secular variation, declination values shown on old topographic, marine and aeronautical charts need to be updated if they are to be used without large errors. 

True North is north, according to the earth’s axis. True north points to the North Pole.

Grid North refers to the direction northwards along the map projection grid lines. This comes from a 3-D object (the earth) being depicted on a 2D object (a map).

Magnetic North is the direction that a compass needle points to. Magnetic north is the direction of the Earth’s magnetic field lines.

Image: Xaktly. (CC BY-NC-SA)

The blue map of the United States shows isogonic lines, along which the angle between magnetic and true north is constant. East of a line passing through Illinois and Alabama (the agonic line), the compass will point to the west of true north, producing what we call a west declination. True north lies 20˚ to the east of where the compass is pointing. To the west of the agonic line, we find east declinations up to 20˚ in the 48 contiguous US states, meaning that true north lies a number of degrees to the west of what the compass says.

Click on the link below to use the interactive map showing declination all over the globe, including Zanzibar:

https://www.magnetic-declination.com/