Why is there an earth

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 coming to the east. 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.

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. 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 other way round: 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 gravitational pull of the sun holds the earth tight, like on a long leash. More precisely: an almost 150 million kilometers long line. That 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. This means that it races through space at a speed of over 100,000 km / h! (That's nearly thirty kilometers per second.)

By the way, 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 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 further out, it was attracted by the rest until the surface was smooth and the same force of gravity was acting in all places. And since the force of gravity is the same in all directions, the shape of a sphere was created automatically - because only with a sphere are all points on its surface equidistant from the center of gravity.

But if you take a closer look 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 somewhat bulbous at the equator.

The earth's rotation is to blame for this: 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, 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.

Why can we see the moon during the day too?

The tasks are clearly distributed: the sun shines during the day and the moon shines at night. But that's not true at all: The moon can sometimes be seen during the day - what is it doing there?

Day and night have a simple cause: the earth rotates. If our location on earth is pointing towards the sun, it is light, i.e. day. Later, when the earth continues to rotate, our location moves to the side facing away from the sun. We watch the sun go down and it gets dark.

The moon rises and sets too - for exactly the same reason: because the earth rotates. But the moon also moves: in the course of four weeks it circles the earth once. Half of this time its orbit is on the side of the earth facing away from the sun. From there you can always see it when your position has just turned away from the sun - or in short: when it is night. But two weeks later the moon is on the side facing the sun. Then it is exactly the other way round: You can see it together with the sun during the day when your own location is facing the sun.

So the moon can sometimes be seen during the day and sometimes at night, even if for us it actually belongs to night. But that's simply because the moon is the brightest light in the sky at night and is therefore much more noticeable.