# Get shorter as the day progresses

### Why are our days different in length?

In summer we look forward to long days and short nights, in winter, on the other hand, it gets dark in the afternoon. And around the North and South Poles there are even areas where the sun does not rise or set for months. So day and night can be of different lengths - but why?

We experience day and night because the earth is a ball that rotates: When our place of residence rotates into the illuminated area, it becomes day; when it turns out again, night.

In addition, the earth's axis is crooked: for half a year the northern hemisphere tilted towards the sun, while the other half the southern hemisphere.

If you look at how the tilted globe is illuminated by the sun, you can see that the northern and southern hemispheres are not evenly illuminated. When our northern hemisphere is inclined towards the sun, the illuminated area there is larger than in the southern hemisphere. As a result, the place in which we live turns into sunlight earlier and out again later. So our day is longer than in the southern hemisphere.

The longest day is when the northern hemisphere has tilted the most towards the sun. That is always the case on June 21st. In Stuttgart, for example, there are around sixteen hours between sunrise and sunset. Then the days get shorter again, which is why one speaks of the summer solstice.

It is the other way around when the northern hemisphere is tilted furthest away from the sun. This winter solstice happens exactly half an orbit (i.e. half a year) later, on December 21st. In Stuttgart, the sun can only be seen for about eight hours.

March 21st and September 22nd are exactly in the middle between the solstices. On these days, day and night last exactly the same length (namely twelve hours), which is why they are called equinoxes.

The closer you get to the equator, the smaller the differences become. And exactly at the equator, day and night always last twelve hours.

The situation around the North Pole is completely different: it is inclined towards the sun for half a year, so that it is uninterruptedly light there for half a year. The other half year the North Pole tilted backwards. A six month long “polar day” is followed by an equally long “polar night”. The area around the North Pole, where there are days when the sun does not rise or set, is called the Arctic Circle. The same thing happens around the South Pole, only with the seasons reversed: it is day at the North Pole, night at the South Pole, and vice versa.

21.6.2009

It is June 21st, three in the morning. The parking lots in Stonehenge are hopelessly overcrowded. Food stands and hundreds of toilets have been set up. Thousands of Stone Age enthusiasts celebrate their party of the year around the gigantic boulders - the summer solstice. There is drumming and dancing, self-appointed druids blow their horns. At exactly 5:58 a.m. the time has come: The sun rises. Despite the overcast sky, great jubilation breaks out. The longest day of the year can begin. Everyone is looking forward to the moment when the first rays of sunshine fall through the northeast entrance onto the "altar stone" in the middle of the stone circles.

Stonehenge, the world famous "hanging stones", eight miles north of Salisbury in southern England, attracts millions of tourists from around the world every year. The solstice celebrations, to which tens of thousands of pilgrims arrive every year, are particularly popular. In 1985 there were riots, as a result of which the celebrations were banned. It has been possible to celebrate again since 1998, but for safety reasons with a large contingent of paramedics, police officers and monument preservers.

### The enigmatic stone circle

Stonehenge has been thoroughly researched - and yet remains incomprehensible. Almost one hundred stones, some up to seven meters high, are arranged in several circles here. The oldest parts of Stonehenge were built almost 5000 years ago, in the following one and a half thousand years the complex was expanded, rebuilt and finally abandoned several times.

But who put the stones there? And for what purpose? Was it a ritual place of worship? Or rather a kind of observatory and an astronomical calendar that druids could use to predict the summer and winter solstices?

We will probably never be able to definitively answer these questions. And that is exactly where part of the fascination emanates from this mysterious place.

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 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.

### 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.