The earth's orbit is getting shorter

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.

What are asteroids, meteorites and comets?

On some nights you can observe a special moment in the sky: it looks like a star is falling from the sky. Superstitious people even think that whoever sees such a shooting star could wish for something. But what is really behind it and where do the shooting stars come from?

In our solar system there are not only the sun, planets and moons. Many small pieces of rock and metal have also been discovered. They are much smaller and not as nicely round as planets, hence they are called minor planets or Asteroids. Like their big siblings, they circle the sun in regular orbits. Most asteroids can be found in the "asteroid belt" between the orbits of Mars and Jupiter.

Every now and then two of these asteroids collide. A crash like this creates a lot of debris and splinters. These fly away from the previous orbit, across the solar system. Some of them get close to the earth, are attracted to it and fall to the earth. These falling chunks are also called meteorite.

On earth they would literally fall like a stone from the sky - if it weren't for the atmosphere. Because the meteorites are so fast that the air cannot move to the side quickly enough. The air in front of the falling rock is compressed and therefore extremely hot. The air begins to glow and the meteorite begins to evaporate. We can then see that as a shining streak that moves across the sky - a shooting star.

Most meteorites are so small that they burn up completely as they travel through the air. The trail then simply ends in the sky. Larger debris also lose mass on the way, but does not completely evaporate. They reach the ground and strike there.

What these meteorites do to Earth depends on how big they are. Small meteorites a few centimeters in diameter, for example, just leave a dent in a car roof.

The largest known meteorite hit about 65 million years ago. It was several kilometers in diameter and tore a crater 180 kilometers in diameter. The impact threw so much dust into the air that the sun was eclipsed for hundreds of years. As a result, plants and animals all over the world died out - this was the end of the dinosaurs.

Fortunately, such large meteorites are very rare so we don't have to worry. In addition, unlike the dinosaurs, we can observe the sky with telescopes and discover such large asteroids long before the impact.

While a shooting star burns up in a few seconds, another phenomenon remains visible longer: Comets with its tail there are days or weeks in the sky. In the past, people also attributed many properties to them - as divine signs, heralds of calamity or harbingers of happy events. But the truth is a little less spectacular.

Astronomers also call comets "dirty snowballs". They come from the outer solar system, far from the warming power of the sun. It's so cold there that water immediately freezes to ice. This is how lumps of ice and dust form - dirty snowballs.

Even a comet initially travels far away from the sun - until it is deflected by a collision and flies in the direction of the inner solar system. It gets closer to the sun and over time receives more and more light and warmth. This will cause the frozen surface to begin to thaw and even to evaporate. This creates an envelope of water vapor and dust around the comet.

At the same time, the comet gets to feel the “solar wind” - tiny particles that fly out of the sun at high speed. They hit the comet's vapor envelope. This will blow away the comet's vapor envelope, creating an elongated cloud that points away from the sun. When this cloud is then hit by sunlight, it appears as a glowing streak - the comet's tail.

The comet makes an arc around the sun and then moves away again. When it is far enough away from the sun, thawing and evaporation will also stop. The tail disappears and the comet moves like a dirty snowball through the vastness of the outer solar system. Depending on the comet's orbit, it will take many decades or even centuries before it comes close to the sun again.

What is the moon

It is the brightest celestial body in the night sky: the moon. It shines so brightly on full moon nights that some people find it difficult to sleep. It appears as big as the sun and the stars look like tiny points of light next to it.

But the impression is deceptive: In reality, the moon (diameter: 3474 km) is only about a quarter the size of the earth (12742 km) - and the sun (1.39 million km) is even four hundred times larger. The moon only seems the same size to us because it is so close to us - the sun (distance to the earth about 150 million km) is also about four hundred times further away than the moon. (384,400 km, an airplane needs 18 days for this distance!)

The bright light is also deceptive: unlike the sun, the moon does not shine by itself, but is illuminated by the sun. Some of this light is then reflected back from the surface of the moon and hits the earth. Just because the moon is so close to us, enough light arrives on earth to light up the night - at least if the moon doesn't just seem to have disappeared without a trace ...

Why do planets have moons?

Earth has one, Mars has two, Jupiter and Saturn even over sixty each! Only two planets in the solar system have to do without moons: Mercury and Venus, all other planets have at least one moon. But why do most planets have moons? And what is a moon anyway?

For us, the moon is first and foremost the bright circle that stands in the sky at night. It looks small, but in reality it is a large rock ball 3475 km in diameter that circles the earth. And it is exactly the same with the other planets: They are also orbited by smaller or larger celestial bodies on regular orbits. Astronomers also call these celestial bodies “moons”.

To get to a moon, a planet usually has two options: Either the moon is created together with its planet, or the planet is created first and later captures a smaller celestial body.

These smaller celestial bodies are asteroids that fly ownerless through the solar system. When they get near a much larger planet, they are drawn to its gravity. This forces the asteroid into an orbit around the planet - the planet has got a moon. This “catching” of a moon works better, the heavier the planet is. This is why the large and heavy planets Jupiter and Saturn also have most of the moons in the solar system.

Other moons formed from debris left over when their planets formed: In the beginning, the solar system was nothing but a large disk of dust, gas, and ice. In the middle, the matter agglomerated particularly strongly - here the sun was created, surrounded by the remaining disk of dust, ice and gas. The same thing was repeated on a small scale in this disk: compact lumps formed again - the planets - and the remaining dust collected in a disk. And if there was enough matter in this disk, smaller lumps were formed there: moons. (Only when the gravitational pull of the planet was very strong were the lumps immediately torn apart. This was the case, for example, close to Saturn, which is still surrounded by rings of dust to this day.)

Both moons that emerged from the dust debris and the captured moons are much smaller than their planets.

The earth is the big exception: its moon is much larger than it should be compared to the earth. That is why it can neither have originated from leftover dust nor simply been captured. Instead, the earth owes its moon to a cosmic catastrophe that almost destroyed the planet:

Shortly after the earth was formed, it collided with a celestial body about half the size of itself. The force of this impact cannot be imagined: The explosion was so strong that most of the young earth melted again - and the other celestial body as well. Part of the molten mass was thrown away and gathered in an orbit to form a second ball. Over time, these two spheres cooled and solidified again. Today the larger sphere orbits the sun as the earth - and the smaller one orbits the earth as the moon.

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 sun differently high in the sky?

On hot summer days you can look forward to a cool shade, but in winter 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 actually differently high 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”, it is exactly the opposite: the sun seems to come from the north, 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.

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 continuously bright there for half a year. The other half year the North Pole tilted backwards. A six-month “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.

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 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. This distributes the sunlight over a larger area, so that each individual spot 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 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.