A Compass Pointer Is Aligned by Earth S Magnetism to Point North

INTRODUCTION

Have you ever used a compass like the car compass in the figure below? The pointer in a compass always points north because Earth is a giant magnet.

A compass pointer is aligned by Earth’s magnetism to point north.

MAGNET EARTH

Imagine a huge bar magnet passing through Earth’s axis, as illustrated in the figure below. This is a good representation of Earth as a magnet. Like a bar magnet, Earth has north and south magnetic poles and a magnetic field.

Earth is like a giant bar magnet.

EARTH’S MAGNETIC POLES

Although a compass always points north, it doesn’t point to Earth’s geographic north pole, which is located at 90° north latitude (see the figure below). Instead, it points to Earth’s magnetic north pole, which is located at about 80° north latitude. Earth’s magnetic south pole is also located several degrees of latitude away from the geographic south pole.

Earth’s magnetic north pole is close to the geographic north pole.

A compass pointer has north and south poles, and its north pole points to Earth’s magnetic north pole. Why does this happen if opposite poles attract? Why doesn’t the compass needle point south instead? The answer may surprise you. Earth’s magnetic north pole is actually the south pole of magnet Earth! It’s called the magnetic north pole to avoid confusion. Because it’s close to the geographic north pole, it would be confusing to call it the magnetic south pole.

EARTH’S MAGNETIC FIELD

Like all magnets, Earth has a magnetic field. Earth’s magnetic field is called the magnetosphere. It is a huge region that extends outward from Earth for several thousand kilometers but is strongest at the poles. You can see the extent of the magnetosphere in the figure below.

The magnetosphere extends outward from Earth in all directions.

MAGNETIC FIELD REVERSALS

Do you like to read science fiction? Science fiction writers are really creative. For example, an author might write about a time in the distant past when compasses pointed south instead of north. Actually, this idea isn’t fiction—it’s a fact! Earth’s magnetic poles have switched places repeatedly over the past hundreds of millions of years, each time reversing Earth’s magnetic field. This is illustrated in the figure below.

We think of today’s magnetic pole orientation as “normal” only because that’s what we’re used to.

Scientists don’t know for certain why magnetic reversals occur, but there is hard evidence showing that they have occurred. The evidence comes from rocks on the ocean floor. Look at the figure below which shows a ridge on the ocean floor. At the center of the ridge, hot magma pushes up through the crust and hardens into rock. Once the magma hardens, the alignment of magnetic domains in the rock is frozen in place forever. The newly hardened rock is then gradually pushed away from the ridge in both directions as more magma erupts and newer rock forms. Rock samples from many places on the ocean floor reveal that magnetic domains of rocks from different time periods are aligned in opposite directions. The evidence shows that Earth’s magnetic field reversed hundreds of times over the last 330 million years. The last reversal was less than a million years ago. What might happen if a magnetic reversal occurred in your lifetime? How might it affect you?

The alignment of magnetic domains in rocks on the ocean floor provide evidence for Earth’s magnetic reversals.

WHY IS EARTH A MAGNET?

The idea that Earth is a magnet is far from new. It was first proposed in 1600 by a British physician named William Gilbert. However,explaining why Earth acts like a magnet is a relatively recent discovery. It had to wait until the development of technologies such as seismographs, which detect and measure earthquake waves. Then scientists could learn about Earth’s inner structure (see the figure below). They discovered that Earth has an inner and outer core and that the outer core consists of liquid metals, mainly iron and nickel. Scientists think that Earth’s magnetic field is generated by the movement of charged particles through the molten metals in the outer core. The particles move as Earth spins on its axis.

Charged particles flow through Earth’s liquid outer core, making Earth a giant magnet.

BENEFITS OF EARTH’S MAGNETIC FIELD

Earth’s magnetic field helps protect Earth and its organisms from harmful particles given off by the sun. Most of the particles are attracted to the north and south magnetic poles, where Earth’s magnetic field is strongest. This is also where relatively few organisms live.

Another benefit of Earth’s magnetic field is its use for navigation. People use compasses to detect Earth’s magnetic north pole and tell direction. Many animals have natural "compasses" that work just as well. Birds like the garden warbler in the figure below use Earth’s magnetic field to guide their annual migrations. Recent research suggests that warblers and other migrating birds have structures in their eyes that let them see Earth’s magnetic field as a visual pattern.

The garden warbler flies from Europe to central Africa in the fall and returns to Europe in the spring. Its internal “compass” helps it find the way.