Early human travelers used a variety of methods to prevent them from becoming lost on their journeys. For terrestrial travel they often used plants or other landmarks. That didn’t work at sea, however, where mariners instead depended on objects in the sky, including the sun, moon and North Star to determine their path. Of course, when clouds masked these bodies, boats could easily lose their way. The invention of the magnetic compass, thought to have been independently developed in China and Europe during the 11th or 12th century, made navigation both by sea and land safer and much more accurate.
Experiment with the compass in this tutorial to see how it responds to magnetic fields. To move the horseshoe magnet to a new location, click and drag it. To see how a compass behaves when there is no magnet nearby to affect it, click the Hide Magnet button. You can make it reappear by clicking the Show Magnet button. Notice that when the magnet is absent, the compass needle points north, but when the magnet is present, the needle points toward the magnet. This is because the compass needle is magnetized and mounted in a way that allows it to move in response to magnetic fields. When the horseshoe magnet is present, the north end of the needle (colored red) is attracted to its magnetic field and aligns itself so that it is pointing toward the object. The closer the magnet is to the compass, the more powerful the effect. Even when the magnet is removed, the compass is still being affected by magnetic forces – those associated with the Earth. These forces normally cause the compass needle to orient itself toward the north (unless another magnet interferes) and make the device useful for navigation.
No one knows for certain what generates the magnetic field of the Earth, but one of the most widely accepted explanations involves turbulent activity and currents in the molten iron core of the planet. It is known that the field varies over time and even reverses periodically. So compasses, if they had been around at various times in the distant past, would not have pointed north, but south, as will compasses in future ages.
In fact, even the compasses now in use generally do not point to true geographic north, because the magnetic field of the Earth is not exactly parallel to the globe’s north-south axis (though it’s pretty close). The difference between true north and the magnetic north is called declination, and this value varies depending on your location on the planet. Some modern compasses are designed to take declination into account, providing a significantly better indication of direction.