# Why is the sun a sphere

### Why is the sun differently high in the sky?

On hot summer days you can look forward to a cool shade, in winter, on the other hand, you don't want to stand in the shade and freeze. But the world is unfair: In summer, of all places, the shadows are short, because the sun is high in the sky. And in winter the sun is so low that even small hills cast long shadows. But why is the sun at different heights in the sky?

In reality, the sun is always in the same place, at the center of the solar system. Only from our point of view does it look like the sun is coming from different directions. That's because we live on a sphere.

How the light from the sun arrives on the globe depends on where you stand on this globe. If you stand exactly on the “belly”, ie the point that is directed exactly towards the sun, the rays of light hit the surface of the sphere at exactly right angles. So the sun is exactly over you in the sky.

If you go north from there, the surface of the earth curves away from the sun. Therefore, the rays of the sun no longer hit at a right angle, but at an angle from the south. From the earth, the sun is no longer exactly above you, but something in the south.

And the further north you go, the flatter the rays of light hit, that is, the lower the sun is above the horizon. If, on the other hand, you go south from the “belly”, the opposite is true: the sun seems to come from the north, and the flatter the further south you go.

But that's not all: Since the earth's axis is crooked, our position in relation to the sun changes over the course of a year. In summer, when the northern hemisphere is tilted towards the sun, we are closer to our “belly”. The sun's rays therefore hit the earth at a steeper angle and the sun is higher in the sky. In winter, on the other hand, the northern hemisphere has tilted away from the sun and we are further away from the “belly”. The light then hits the earth flatter and the sun is lower in the sky.

In addition, the earth also rotates, and so there is a second movement every day: During the day, the sun moves from east to west across the sky - more or less high above the horizon, depending on the season.

17.2.2014

For a long time the people of Qaanaaq endured in complete darkness. Now, in mid-February, the moment has come that they have been looking forward to for months. Despite the freezing cold of minus 35 degrees Celsius, they gathered at lunchtime. When the first rays of sunshine shine on their faces, people sing a song in the old tradition and throw their hats in the air.

Qaanaaq is one of the northernmost settlements in the world. It is located at the extreme point of Greenland, just south of the 78th parallel. Around 600 people, most of them Inuit, live here - almost four months a year in complete darkness. In winter it is the polar night, the sun stays behind the horizon around the clock. In the summer it does not go under for four months. It seems only flat, but at least the temperatures climb above freezing during these months. In between there are months in the twilight when it is neither day nor night. The seasons in Qaanaaq cannot be compared to ours.

### The mirror sun

Thomas Schuler has a problem similar to that of the Inuit: For four months there was no sun on his farm. However, he does not live far in the north, but deep in the Black Forest, in Simonswald. There the sun rises every day even in winter, but it only drags a flat path across the sky. Too flat for the Schuler's farm, which is surrounded by the ridges of the Black Forest. In winter these cast shadows for so long that no ray of sunshine reaches the courtyard all day. But the resourceful inventor knew what to do: he installed a large mirror on the opposite mountain slope. Now at least a little sunlight shines through the window even in winter.

### Why are there seasons?

We enjoy the first warm rays of sunshine in spring, look forward to swimming pool visits in summer and trudge through colorful foliage in autumn. In December at the latest we get our thick sweaters out of the closet, because in the winter months it can get really cold - and most of the time it also snows. The seasons influence our life, but also that of plants and animals. But how does this change of seasons come about?

The most noticeable difference between the seasons: it's warm in summer and cold in winter. Most of the heat comes 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. The air and the ground therefore have a lot of time to warm up during the day in summer and only cool down a little during the short night. In winter it is the other way round: The sun only brings a little warmth for a short time, while the long nights cool the air and soil.

In addition, the warming rays of the sun are weaker in winter. Compared to summer, the sun is lower in the sky. The rays of the sun hit the ground more flat. As a result, the sunlight is distributed over a larger area, so that each individual point on the ground receives less light and heat. In addition, the flat rays of the sun 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 look forward to the warm summer in the northern hemisphere, it is winter in the southern hemisphere. Because whether the sun is high or low in the sky and whether the days are long or short depends on whether the northern or southern hemisphere is inclined towards the sun.

In the vicinity of the equator, the length of the day and the position of the sun change little over the course of the year, so it is tropical hot all year round.

### Why is the earth round?

“What happens if you keep going in the same direction? Will one come to the edge of the world at some point or is the world infinitely large? ”More than 2300 years ago, the famous Greek scientist Aristotle was certain: Neither one nor the other. Because the earth is not flat like a disk, but a sphere - but why?

To understand this, one has to go back to the time when the earth was created. The force that was responsible for this is gravity - all massive objects attract each other. This force made chunks of rock collide and combine to form a planet. And it gave shape to the planet. Because gravity acts equally strong in all directions.

Since the earth was hot and liquid at the beginning, the material was able to flow into the shape dictated by gravity. If a piece of earth protruded somewhere further, it was attracted by the rest until the surface was smooth and the same force of gravity was acting in all places. And since gravity is the same in all directions, the shape of a sphere was automatically created - because only with a sphere are all points on its surface the same distance from the center of gravity.

However, if you look closely at the shape of the earth, you will see that the earth is not a perfect sphere: it is slightly flattened at its poles and, on the other hand, somewhat bulbous at the equator.

This is due to the rotation of the earth: the earth rotates once around its axis in the course of 24 hours. The rotary movement creates a force, the centrifugal force. We know this from the chain carousel when we fly outwards on the swings. In the case of the earth, the centrifugal force causes the rock masses to slide outwards a little from the axis of rotation, i.e. from the poles towards the equator. There, the diameter of the earth is around 41 kilometers larger than between the north and south poles.

### How does the earth move?

Every morning we see the sun rise, move across the sky and set again in the evening. To us it looks like the sun is moving around the earth. Up until the late Middle Ages, many people actually believed that the earth stood still in the middle of the universe and that everything revolved around it.

Today we know that it is exactly the opposite: we experience day and night because the earth is turning. And the earth is neither still nor in the center, but revolves around the sun.

The pull of the sun holds the earth tight, like on a long leash. More precisely: an almost 150 million kilometers long line. This is the distance at which the earth orbits the sun.

The time it takes the earth to orbit is called a year. During this time, the earth covers a distance of around 940 million kilometers. That means it races through space at a speed of over 100,000 km / h! (That's nearly thirty kilometers per second.)

Incidentally, the earth's orbit is not exactly circular, but rather elongated: at the beginning of January, the earth is closest to the sun. Half a year later, at the beginning of July, the gap is greatest. The earth is then a few million kilometers further from the sun than it was in January. But this has nothing to do with the change of the seasons: the difference is so small that the amount of sunlight hardly changes. (And besides, when the earth is closer to the sun in January, it is winter here in the northern hemisphere.)

### Why is the earth crooked - and what does that mean for us?

The earth hangs crooked in space! If you look at the earth's orbit from the side, you see: The earth's axis is not pointing straight up, but the earth has tilted to the side by about 23 degrees - but why?

When the earth was freshly formed, the axis was still straight. But scientists suspect that it was hit by a large asteroid in the early days of the solar system. It hit the earth a little sideways, so that it tilted a little - this 23.5 degrees. In addition, the impact tore out part of the still liquid earth and threw it into orbit, which became the moon.

We can still feel the consequences of this crooked earth axis: During one year, the earth orbits the sun. The oblique axis of the earth always points in the same direction. Sometimes the northern hemisphere is inclined towards the sun, sometimes the southern hemisphere - depending on where the earth is on its orbit. From the earth it looks as if the sun is higher or lower in the sky.

As a result, the sun rises and sets at different times over the course of the year. The days are of different lengths, and depending on the position of the sun and the length of the day, our place of residence on earth receives different amounts of heat. We feel this change in solar radiation as the seasons. They make life on earth more varied - seen from this point of view, this crash in the earth's childhood had its good side too.

### Why is there day and night?

We spend our life to the rhythm of day and night: in the morning it gets light, we get up. During the day we go to school or work, meet up with friends, do sports. In the evening it gets dark, we go to bed, and at night we sleep. The next morning the same process starts all over again, day after day, throughout our lives. The change between day and night is so natural for us that the question sounds almost surprising: Why is there actually day and night?

At first glance, the answer is very easy: day is coming because the sun is rising. Then it curves across the sky, finally disappears behind the horizon and night falls. So you could think that day and night alternate because the sun is moving.

But this impression is deceptive: in reality we humans live on a sphere that rotates: the earth. The sun stands still and illuminates the globe - but only ever one side. It is then light there, and if our place of residence is on this side, it is day for us right now.

But because the earth rotates, this place moves on. To us it looks like the sun is moving across the sky. And when our place turns over the edge of the light side, we can no longer see the sun: It goes down and it becomes dark night. Fortunately, the earth continues to rotate, and so we come back to the sunny side, it gets light again and a new day begins. Once the earth has rotated on its own axis, a day - i.e. 24 hours - has passed for us.

And in which direction is the earth turning? From a spaceship you could immediately see that the earth is turning to the east. On the surface of the earth you have to think about something: to us it looks like the sun is in the morning out the east comes. But the reality is that in the morning we turn towards the sun, so to East.

That also means: the sun is already shining to the east of us. So it rises earlier in the east - and the earlier the further east you go: In Dresden, for example, the sun rises almost half an hour earlier than in Cologne. And if you call Germany in the morning while on vacation in Thailand, your conversation partner rings from deep sleep: The day starts six hours earlier there. Finally, in New Zealand, almost exactly on the other side of the world, it is always day when it is night here - and vice versa.