Working out where we are in the world is made easy with GPS systems using satellites, but before we had satellites orbiting the Earth we used compasses to find our way.
Figuring out where the poles of the Earth are can be done by using the movement of the Sun and stars, but we can do this more conveniently with electromagnetism. Compasses use a magnet that aligns itself to the Earth’s magnetic field which runs from the North Magnetic Pole to the South Magnetic Pole of the Earth. All magnetic fields run from north to south, and this is how we differentiate between the two poles. Here’s a science fact to throw at your class today: the actual North Magnetic Pole of the Earth is located in Antarctica!
The north end on a compass is attracted to an area near the geographic North Pole. But the first rule of magnetism is that opposites attract, so that means the geographic North Pole actually sits near the Magnetic South Pole of the Earth’s magnetic field. It was named the North Magnetic Pole because it is located near the geographic North Pole. This was meant to make things less confusing somehow…
So when it is said that all compasses point north, what is actually true is that all compasses point south. Compasses will direct you to the geographic North Pole, but what is actually happening is the compass is pointing to the South Magnetic Pole of the Earth’s magnetic field. This has been true since the compass was first invented during the Han Dynasty in China, but has not always been the case. The locations of the magnetic poles flip every few hundred thousand years, and it has been over 780,000 years since the last flip in the Earth’s magnetic field! So our magnetic field is overdue for an inversion.
Geologists can show when the Earth undergoes a flip in its magnetic field by studying the polarity of magnetized volcanic rock. Lava that has magnetic properties will align with the direction of the Earth’s magnetic field, and remain that way after it has cooled into solid rock. So by digging deep, geologists can see when different layers of lava rock were cooled with a change in magnetic field alignment.
But what would a flip mean for life on Earth as we know it? The previous field reversals have not been linked with any major events in the Earth’s natural history records. However, during the last field reversal we did not have the technology that we’ve developed since before the invention of writing. With a weaker or reversed field, much of our current technology would be affected. But don’t worry, it can take anywhere from 1,000 to 10,000 for a flip to complete, giving us lots of time to adapt our technology.