How Does a Compass Work?

Demystifying the Mystery of how compasses work

Brian Connelly
Last Updated: October 28, 2020

Ever asked yourself “how does a compass work?”

You’re in the right place! In this guide, we will be covering the following:

    • A short history of compasses
    • Why the Earth has two “Norths”
    • Discover the factors that make a compass function
    • Learn the fundamentals of how to use a compass

One of the highlights in the hit film series The Pirates of the Caribbean is Captain Jack Sparrow’s magic compass. For the benefit of those who’ve never seen the movie, Captain Jack’s compass doesn’t point towards magnetic north, but the direction of whatever the holder wants most at the moment.

Sadly, the regular compasses you and I are used to using aren’t quite imbued with the same kinda magic, meaning we have to put in a few study hours and a bit of practice to be able to use them effectively. One of the most important stages of this journey is learning how compasses work.

In this article, we aim to help you with this by providing a simple, easy-to-follow guide to the basics of compass functioning.

The Origins of the Compass

The first magnetic compasses were invented by the Chinese during the Han Dynasty or Tang Dynasty and were used for divination.

One of the first forms of compasses came in the shape of spoons or ladles that were magnetized using lodestone, a type of naturally occurring magnet, and pointed south instead of north. These were known as “South Pointing Spoons” and were used, together with divination, to orient military maneuvers.

Numerous further developments, of course, were made before compasses finally came to be the navigational tools we know today.

Fun Fact: The compass rose used to indicate direction once varied according to its place of origin. Compasses from the West and from the Arab regions both have roses with 32 points. Meanwhile, compasses from the East either have 24 or 48 compass points. Today, compasses usually give direction in degrees or bearings instead of or in addition to cardinal points.

So, How Do Compasses Help Us Navigate?

The Earth is a Magnet

The Earth’s magnetic field is formed thanks to a principle called the “Dynamo Effect”.

This phenomenon can be observed in a bike’s dynamo light. When a bike turns its wheels, the magnets inside the dynamo start to spin simultaneously. The magnets’ motion generates an electric current that is then used to power the light.

This phenomenon also works in reverse: if you rotate electric currents, the currents will transform into a magnet.

And this is why our planet provides the giant, ever-present magnetic force that allows us to navigate with the aid of a compass.

Beneath the Earth’s surface are millions of tons of hot, liquified materials mostly made up of dense iron. As the Earth rotates, these liquid metals are sloshed around, producing a huge amount of electric currents.

In turn, these electric currents generate the Earth’s magnetic field, which is otherwise known as the Geomagnetic Field.

Why a Compass Needle Points Toward North

As with all magnets, the Earth also has a north pole and a south pole. These maybe aren’t, however, exactly where you might think they are.

Try to imagine a giant, hypothetical magnet buried beneath the Earth, placed there in such a way that the north end of the magnet is pointed towards the geographic South Pole, while the south end of the magnet is pointed towards the geographic North Pole.

The ends of this imaginary giant magnet are called the Geomagnetic Poles. In short, the Geomagnetic North Pole is down in the South Pole, while the Geomagnetic South Pole is up towards the North Pole.

The reason why a compass always points north is that the compass’s needle itself is a magnet. Since magnets work on the principle that opposites attract and similarities repel, the Geomagnetic South Pole attracts the compass’ magnetic needle’s North end.

Caution: The Geomagnetic North Pole is different from Magnetic North. The former refers to the imaginary magnet (magnetic fields) buried beneath the Earth while the latter refers to the direction indicated by a magnetic compass needle.

A Tale of Two Norths

The North that your compass indicates isn’t True North. This is because the Earth actually has two Norths: the magnetic kind and the geographic kind.

The Earth rotates on a fixed point called the Axis. This is a straight line connecting the Geographic North Pole and the Geographic South PoleThese, however, are not the same as Magnetic North and Magnetic South, i.e. the north and south indicated by your compass.

The north that your compass indicates is known as the Magnetic North Pole (again, different from the Geomagnetic North Pole, which is located at the geographic South…stay with us!).

This degree of separation between the North indicated by your compass needle and True North exists because Earth’s magnetic poles don’t line up with the geographic poles. This difference is called Magnetic DeclinationMagnetic north, moreover, is not a fixed point, but in continuous migration. This is because the Earth’s liquid outer core is in constant movement, a phenomenon referred to as “secular variation”.

The take-home for backcountry navigators? The north indicated by your compass is different from the north you’ll use to navigate (True North). As such, you must always remember to adjust your readings and bearings to account for the divergence, i.e. for magnetic declination.

Know Thy Compass, Know Thy Way! 

Knowing how our compass works serves us in two important ways…

First, it allows us to understand the different forces and principles behind the workings of what is arguably our most important piece of backcountry kit. Secondly, with your newly acquired navigational smarts, you can seriously impress and school your hiking friends by explaining to them one of our planet’s more complex phenomenons on your upcoming trips in the wild!

So, what did you think of our article? Are there any points that still need clearing up or anything that we have missed? If so, feel free to share them with us in the comments box below.

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