Magnetic Declination: Definition & Angles

Instructor: Josh Corbat

Josh has taught Earth Science and Physical Science at the High School level and holds a Master of Education degree from UNC-Chapel Hill.

Some cars have a compass that tells you in which direction you're traveling. Did you know that this compass can be wrong depending on where you are? This lesson will discuss this discrepancy by explaining magnetic declination and giving a few examples.

What Is Magnetic Declination?

Everything's packed! You're ready to go! You and your friends jump in the car and start your cross-country road trip. You have your routes planned, the maps are all marked (you're going for the nostalgic feel, so you're trying to avoid using your phone's GPS), and the gas tank is full. As you make your way across the country, your navigator starts to notice something strange. Even though you're heading west, your car's compass insists you're traveling southwest. What is going on?

Finding the northernmost point on Earth is easy; just locate the North Pole and you're all set. However, our compasses are not high-tech enough to point directly to the North Pole at all times. Instead, we rely on the Earth's magnetosphere, a field of magnetism generated by our spinning iron core, to locate north. The problem is that Earth's North Pole and the point called magnetic north (north according to the magnetosphere and compasses) are not the same. They're close to each other, but slightly off. This doesn't cause a huge problem when using a compass, though, because the Earth is so big that the difference is usually very small.

The reason your car's compass may become off is explained by magnetic declination. Magnetic declination is the difference between true north (the North Pole) and magnetic north, and is measured as an angle. If magnetic north is east of true north, the angle is recorded as a positive number. If magnetic north is west of true north, the angle is negative. Magnetic declination changes over time, and is entirely dependent on where you are on Earth. The Earth's magnetic field is caused by its spinning iron core, which is by no means stable. Over time, the core shifts and tilts, causing the magnetic field to change. The amount of magnetic declination also changes with these shifts of the core.

The Earth and its magnetic field lines. As you can see, they do not run directly north to south. Depending on where you are on Earth, magnetic north will be a slightly different angle relative to the North Pole, even though your compass will always point to the same location.
Earth and its magnetic field lines

Examples of Magnetic Declination

Due to magnetic declination, navigators must either calibrate their compass - especially if they're electronic - or be aware of the angle of declination in order to make accurate readings of direction. There are parts of Earth with no magnetic declination (the angle is zero), and other areas have relatively high angles of declination.

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