# What is special about the leap year

### What are leap years for?

When a year comes to an end, 365 days have passed on our calendar. But there are also years that have an additional day: February 29th. These years are called leap years, but why do they exist?

A year is the time in which the earth orbits the sun once. But there is a problem with this: the earth needs exactly 365 days, 5 hours, 48 ​​minutes and 46 seconds to circumnavigate it. However, a year can only have whole days in the calendar. The calendar year starts almost 6 hours too early!

In a year, the few hours are hardly noticeable. But if you just keep counting, you add six hours every year. After four years the calendar would be a whole day too early, after a hundred years it would be almost a month - and after six hundred years, July would be the dead of winter!

A simple solution has been found to prevent this from happening: you extend the year by one day every four years. In these “leap years” February has 29 days instead of 28. Then the calendar will match the earth's movement again.

(Unfortunately that is not quite enough. The year is not exactly six hours too long. Every year this method is off by eleven minutes and fourteen seconds. To even out this difference, there are two additional rules: Years that go through 100 are divisible, are no Leap years - but if the year is divisible by 400, then but again.)

### 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 simple: 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 when 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 from your Thailand vacation, 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 opposite: we experience day and night because the earth is turning. And the earth is neither still nor in the center, but circles 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.)

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 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. So the air and the ground 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 the ground.

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 summer, on the other hand, the sun is high in the sky. The rays of light 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.