How compass apps can tell direction

Credit: Kyung Min Lee/ Credit: Kyung Min Lee/

If you were to go back in history and meet with the explorers and navigators of yesteryear, they would probably be wielding — at least one — magnetic compass. Whip out your compass app on your smartphone, and they’d probably be flabbergasted — well, with that and your time machine. But how has the compass worked to help everyone from ancient Chinese seafarers to today’s Boy Scouts of America? And how exactly does that timeworn technology work in your iPhone?

First, we’ll go over the magnetic compass. This trustworthy piece of equipment has been around since 200 BCE, according to William Lowrie, a professor emeritus at ETH Zurich. Navigators started using this compass regularly on land and sea closer to 1000 CE, in present-day China. The standard magnetic compass of the 20th century is made up of a magnetized needle in its heart with a face showing cardinal directions — north, south, east, and west. The needle is mounted on a surface with low friction so that it can easily turn; if held flat, one end of the needle will point toward magnetic north and one to magnetic south.

The compass is able to determine north and south due to the magnet’s interaction with the Earth’s magnetic field. The cause of the magnetic field is not completely known, but geologists have made hypotheses regarding the phenomenon by analyzing the layers of the Earth. The Earth is made up of an outer crust, followed by the upper mantle, the inner mantle, the outer core, and then finally the inner core at the very center. The inner core is made up mostly of molten iron, but the very center of the inner core is under so much pressure that the iron becomes solid, according to It is believed that the rotation of the Earth and the immense heat from the core cause the iron to move in a rotational pattern. This rotational pattern may be the source of the magnetic field that we see on Earth. The field produced is very weak, however, which is why the needle on the compass needs to be very lightweight and on a surface with minimal friction.

As expected with any technology created over two millenniums ago, the compass has its issues. First, it has to be completely level to work — making it rather difficult to use on something like an airplane. Also, a magnetic compass can take a long time to correct itself. Another confusing thing is that the magnetic north pole is actually the geographical south pole (and vice-versa).

So if the traditional needle compass works because of a small magnet, how do the compass apps in phones work? As it turns out, the smartphones do have a small magnetometer, which can measure the Earth’s magnetic field. This information is combined with an accelerator inside the phone. The accelerator gets information regarding the phone’s position in space. It is able to pinpoint the phone’s position from solid-state sensors within the phone that can measure their tilt and movement. The information provided by these devices means that the compass app can display cardinal directions no matter which orientation the phone is in, according to the algorithmic software development company Sensor Platforms.

Whether using a 200 BCE compass or the compass app on the iPhone 5S, the Earth’s magnetic field is the key to having reliable directions. Now, if only Apple could develop a moral compass app.