Compass in the body

Robins can perceive a magnetic field and use it for their orientation. Wolfgang Wiltschko, an ornithologist from Frankfurt, has proven this in experiments. Exactly how the migratory birds’ magnetic sense works is still unexplored.

In autumn, many robins regularly migrate to warmer latitudes – to the southwest towards Spain and northern Africa. To find their way there, the migratory birds use the sun during the day and the stars at night. But what happens when fog and clouds block the view of the stars? Biologist Wolfgang Wiltschko asked himself the same question.

To get to the bottom of the mystery, he constructed a special cage in the basement of Frankfurt Zoo. On the one hand, it can be shielded from the earth’s magnetic field, but it can also create a weak artificial magnetic field. He placed the robins in this windowless cage. The result: if the magnetic field was virtually switched off, the birds fluttered wildly. If, on the other hand, an artificial magnetic field was created in the cage, the birds wanted to fly in only one direction: And that was in the direction that corresponded to the south-west in the artificial system.

With this experiment, Wiltschko was the first researcher to succeed in proving that animals have a magnetic sense. But where is this compass located in the body and how exactly does it work? The newly discovered sense organ still poses many questions for science.

Where does the robin go?

With its orange-red front, the robin is well known to us. It is trusting, eats insects and small worms and sings from early morning until late at night. But does it actually stay with us in winter? Or is it a migratory bird? The answer is: it depends. Because the robin is widespread and lives in large parts of Europe and Asia Minor. In the warmer regions, the robin also spends the winter, where it is a resident bird. Here in Central Europe, only some robins migrate to the western Mediterranean, and some spend the winter there. So here it is a typical partial migrant. And only the robins that inhabit Eastern and Northern Europe are true migratory birds: they leave the field in October and do not return to their breeding grounds until March.

Compass and orientation

Without it, Christopher Columbus would have got lost on the Atlantic, Ferdinand Magellan’s ships would never have sailed around the world. For over a thousand years, sailors have used the compass for orientation. But how could this little thing show them the long way across the oceans?

The secret of the compass was only revealed very late: In 1600, the English scientist William Gilbert was the first to give a reasonable explanation for the compass needle’s property of aligning itself in a north-south direction: The earth itself is magnetic. The compass needle simply aligns itself with the earth’s magnet. And thus helps people to orient themselves.

So with compass and map in your luggage, no one can get lost – provided you know how their interaction works. The compass consists of a magnetised steel needle that sits freely movable on a point in its centre. Similar to the hands of a watch on the dial, this tip is placed on a scale with all four cardinal directions. When the compass is horizontal and still, its magnetic needle aligns itself with the field lines of the earth’s magnetic field: It points in a north-south direction. If you place a map under the compass and turn it until its north direction points in the same direction as the compass needle, the map is aligned. The direction of the destination is now easy to see.

But there is one small difficulty: because the magnetic poles of the earth are constantly moving, the magnetic pole and the geographical pole do not coincide exactly. This leads to an angular difference between the two poles. This so-called declination must be taken into account when using a compass, and it is not the same everywhere on earth. Here in Germany, the declination is relatively small and is approximately between 1° and 3° to the east.

Not only humans, but also animals orient themselves to the earth’s magnetic field. Many animal species undertake long migrations during the year, for example migratory birds. To find their way south, they orientate themselves by the landscape, the position of the sun and, at night, by the stars. In addition, they have their own sense of magnetism that gives them direction. With this “compass in their body” they can find the right way even in poor visibility.

Earth magnet

We don’t notice it, but the compass needle shows us clearly: the earth is a huge magnet. It has two magnetic poles, a north pole and a south pole. And like all magnets, the earth is surrounded by a magnetic field: the earth’s magnetic field.

In the area of its magnetic field, a magnet exerts force on other magnets, for example on a compass needle. The effect of a magnet can also be made visible by fine iron filings: they arrange themselves around the magnet and point in the direction of its two poles. A line-like pattern is created that indicates the magnetic forces. The lines of this magnetic field are the so-called field lines.

The earth’s magnetic field also has such field lines. They emerge from the earth near the south pole, run outside the earth to the north pole and disappear into the earth again there. They are thus arranged as if a huge bar magnet were running through the middle of the earth.

The south pole of this imaginary bar magnet points approximately to the geographic north pole, its north pole to the geographic south pole. What sounds confusing at first has a simple explanation: the north and south poles attract each other. That is why the north pole of the compass needle points to the magnetic south pole of the earth, and the south pole on the needle points to the magnetic north pole.

However, the Earth’s magnetic field is not only used for orientation on this planet. Together with the atmosphere, it also protects us from threats from space. One of these threats is a charged particle stream that the sun constantly emits in all directions. This so-called solar wind is deflected by the Earth’s magnetic field. Like a capsule, the Earth’s magnetic field deflects the charged particles so that they fly past the Earth and can no longer be dangerous to us.

Why are there seasons?

We enjoy the first warm rays of sunshine in spring, look forward to visits to the swimming pool in summer and trudge through colourful leaves in autumn. In December at the latest, we take our thick jumpers out of the cupboard, because it can get quite cold in the winter months – and it usually snows too. The seasons influence our lives, but also those of plants and animals. But how do the seasons change?

The most striking difference between the seasons: It’s warm in summer, cold in winter. The heat comes mainly from the sun, so the difference between summer and winter must have something to do with the sun.

In fact, there are several reasons: In summer, the days are long and the nights short. In summer, the air and soil have plenty of time to warm up during the day and cool down only slightly during the short night. In winter it is the other way round: the sun only brings a little warmth for a short time, and the air and soil cool down during the long nights.

In addition, the warming rays of the sun are weaker in winter. Compared to summer, the sun is lower in the sky. The sun’s rays therefore hit the ground more flatly. This spreads the sunlight over a larger area, so that each individual spot on the ground receives less light and heat. In addition, the sun’s flat rays have to travel a longer distance through the atmosphere, and more energy is lost in the process.

In summer, on the other hand, the sun is high in the sky. The light rays hit the ground steeply and bring a lot of warmth with them.

But while we in the northern hemisphere enjoy the warm summer, in the southern hemisphere it is winter. Because whether the sun is high or low in the sky and whether the days are long or short depends on whether it is the northern or southern hemisphere that is tilted towards the sun.

Near the equator, the length of the day and the position of the sun change very little during the year, so that it is tropically hot there all year round.