The question of how the earth’s geographic and magnetic axes are oriented relative to each other is a hotly debated topic. There is also no consensus among scientists on how the Earth’s axis is oriented, as there are two dominant theories:
1. The first theory states that the Earth’s axis always points towards Polaris, which is termed “the pole star”.
2. The second theory states that the Earth has two poles and that some ancient people believed they were at the North and South Poles respectively. The Earth’s axis would then change position according to these ancient views.
The Earth’s axis should be thought of as an imaginary line through the center of the planet from pole to pole and not a point at either end.
How are earth’s geographic and magnetic axes oriented relative to each other?
1. There are no two poles.
The same pole is always pointing towards the same direction of the celestial north pole (either Polaris or the declination of the North Star), with a small wobble over a 40,000 year cycle.
2. The poles are not as close to North and South as you might think.
Earth’s geographic axis is not parallel to its orbital plane. At any given time, one of the poles is on a different side of the planet from the other and the direction of this difference changes over a period of about 40,000 years.
This is caused by a combination of Earth’s rotation and its wobble: Earth’s axis does not point at 90 degrees, but at an angle which varies between 22.1 and 24.5 degrees with respect to true north (see diagram).
The mechanism that causes this change in orientation is called “wobble theory. It is caused by the centripetal force of gravity; the force that is responsible for a rock spinning on Earth’s surface. The gravitational attraction of the moon (and other planets) creates a torque on the solid Earth, which in turn causes it to wobble on its axis.
3. The Earth’s axis is tilted at a 23.5 degree angle.
This means that over a period of about 40,000 years, the axis changes from pointing to a star in the north celestial hemisphere and then to a star in the south celestial hemisphere. At present, Polaris is not exactly aligned with the equator, but it is close enough for serious astronomers to use it for their precision instruments (astro-navigation).
However, by 14,000 years from now when Polaris will be directly above the North Pole, true north and magnetic north will be separated by about 18 degrees. At present the difference between magnetic and true north is about 3 degrees.
4. A hard force of nature cannot cause a 180° change in orientation.
Although a force field such as gravity can change the orientation of an object, it cannot do so in all directions at once. The Earth’s axis changes direction because the planet’s spin changes direction; from being parallel to the orbital plane to being perpendicular to it, for a period of about 40,000 years (see diagram).
It is not possible for this to happen on something like a needle or rod because the rotation axis would then be stuck pointing in exactly one direction: perpendicular to the other two forces. In the Earth’s case, the axis can be deflected in many different ways because it originates on a sphere.
5. The Earth’s axis is not parallel to its equator.
This was known as early as 2nd century BC by Eratosthenes, who measured the circumference of the earth and compared it with his knowledge about the distance between Alexandria and Aswan and deduced that: “If you set out from Syene (Aswan) to sail northward, you would everywhere come to places where the Northern Pole was on your left hand.
To confirm the fact, [you could] observe that the shadow thrown by a pillar or a tree will be longer, the farther south you go.” The deviation between Earth’s axis and its equator is about 12 degrees.
6. The ancient Greeks knew that one pole was in the North and one in the South.
The Greeks knew of these deviations from true north because they had circumnavigated Africa and determined that compass north did not point to true north at either end of their trip. Eratosthenes knew that the Earth was round, the Greeks knew it was a sphere and had calculated its circumference with some precision.
Their calculations were based on the assumption that the Earth was perfectly spherical and that its surface distance equaled its center distance. Eratosthenes found out this wasn’t true when he found out by observation (with a measuring stick) that the angle between Syene and Alexandria was about 7.2 degrees (the actual value is 6.1 degrees).
He also determined by measurement that 1 degree of latitude at Alexandria is equivalent to 69 miles of land. Multiplying 7.2 degrees by 69 miles gives about 515 miles: the distance from Alexandria to Aswan.
