# How smooth is the earth

### 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 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.

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, 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. The diameter of the earth there is around 41 kilometers larger than between the north and south poles.

6.9.1522

Big reception in the Spanish port of Sanlúcar: after almost three years, the “Victoria” is returning from her expedition. But the joy is clouded. Of the five ships in the fleet, only this one comes back, and of the 237 crew only 18 survived. The leader of the expedition, the Portuguese Ferdinand Magellan (42), was also killed on the way.

So he could no longer experience the great triumph himself: his expedition succeeded for the first time in the history of mankind to completely circumnavigate the world! Ferdinand Magellan set out from Seville with his five ships on August 10, 1519. He was on the way on behalf of the Spanish Crown to find the western route to the lucrative Spice Islands in the Pacific Ocean. This was necessary after the rival sea power Portugal had taken the sea route in an easterly direction around the African Cape of Good Hope.

After an arduous search, Magellan and his team discovered the long-awaited “Gateway to the West” on the southern tip of South America. At the end of November 1520, they sailed through a narrow 600-kilometer strait into the Pacific Ocean.

But the setbacks were inevitable. In an attempt to take islands of the newly discovered Philippines for the Spanish crown, Captain Magellan fell victim to the lances and poison arrows of the natives. Numerous crew members died with him in the fighting, others had died in the months before from the catastrophic supply situation on the ships. One crew member reported that they had to eat leather and rats cooked in salt water. Many died of scurvy.

But the survivors can be proud of themselves. You are the first to travel around the world. So they provided the final proof: the earth is a sphere. And in addition to fame, honor and knowledge, they brought home over 26 tons of valuable spices.

### Globe larger than expected

The logbook entries of the "Victoria" will now give the scientists some homework. It seems that up to now the extent of the earth has been underestimated.

The most prominent victim of this misjudgment was Christopher Columbus. He also wanted to find the western route to India, but assumed about a quarter less circumference of the earth. At that time, nothing at all was known about the new continent in the west, Asia was considered too big and the Pacific Ocean too small. When he came across the Bahamas in 1492, he was therefore firmly convinced that he had discovered "India".

Now it looks like the Greek scientist Eratosthenes was right. As early as 240 BC he had taken measurements using the simplest of methods and calculated a circumference of around forty thousand kilometers from them.

24.12.1968

Upside down world - the earth rises above the moon horizon. The American astronaut William Anders took this famous photo on Christmas Eve 1968.

Together with Frank Borman and James Lovell, he circled the moon several times on the Apollo 8 mission. When their space capsule came out from behind the moon during one of these orbits, they saw the globe emerge behind the lunar horizon. They were deeply impressed by the sight and took several photos - although Anders jokingly remarked that this was not provided for in the mission plan.

A real "earth rise" - like the way we see the moon rise while standing on earth - cannot be experienced on the surface of the moon. Because the moon always turns the same side to the earth. If you stay on this side, you can see the earth all the time - and always in the same place in the sky. And from the back of the moon, the earth can never be seen.

### The second face

The moon shows itself from a completely new side: On October 7, 1959, the Russian probe "Luna 3" took the first photo of the back of the moon. However, the world had to wait eleven days for this historic photo: it was only when the probe flew back towards earth that the radio link was good enough to send the image.

At first glance, the picture doesn't look very spectacular. The resolution is poor, and since the sun shines almost perpendicularly on the surface of the moon, no shadows from mountains and craters can be seen.

But there was a surprise: The moon has far fewer dark spots on the back than on the front. Astronomers are still puzzling over the reason!

Until this photo was taken, mankind had no idea what it looked like there. Because from the earth you only ever see the same side of the moon.

### The beginnings of the earth

We would not recognize the earth immediately after its formation. It was an extremely uncomfortable planet: there were neither continents nor oceans, but a seething surface of glowing hot, viscous magma. Why couldn't the earth's crust form for a long time?

A good 4.5 billion years ago comets, asteroids, gas and dust condensed to form our planet. Its own gravity pressed these individual parts together so that they were subjected to strong pressure. This pressure was naturally highest in the core of the earth, on which the weight of the entire outer layers weighed. As a result of the high pressure, the rock was heated up and melted. Outwardly, the pressure and thus also the temperature became less. Even so, the surface of the earth remained very hot for several hundred million years and could not cool down and solidify.

In order to understand the reason for this, the scientists had to look at the moon: Ancient lunar craters from the time the solar system was formed tell us that the moon was hit by numerous meteorites when it was young. It is therefore assumed that the earth was also exposed to a real rock bombardment from space at the same time. The lumps fell to the earth at high speed - and the impacts were correspondingly violent: Even lumps of a few hundred tons could easily cause an explosion the strength of an atomic bomb!

So the earth's surface continued to heat up for a long time, stirred up again and again and remained so fluid. Only when the impacts gradually subsided after a few hundred million years did the temperatures on the earth's surface drop. The rock could slowly solidify and form an earth crust that became thicker and thicker over the course of millions of years. But to this day it is only a very thin layer that floats on a viscous, hot interior of the earth.

### What is our solar system and how did it come about?

The earth is not alone in space: people have been observing the sun, moon and stars in the sky for a long time. They discovered early on that some stars are moving. These wandering stars were observed and their paths followed. For a long time, however, their movements were not understood - until about five hundred years ago a man by the name of Nicolaus Copernicus solved the riddle: The earth and the "wandering stars" are actually planets, all of which orbit the sun at different distances.

Today we know eight planets. To remember their names in the correct order, the first letters of the sentence "M.a V.ater eclarifies mir jEden S.monday uurens Nachthimmel. “- or in short: M-V-E-M-J-S-U-N.

M.Erkur is the planet that orbits closest to the sun. Then come V.enus, E.rde and M.ars. These four inner planets have a solid surface made of rock and are still relatively close to the sun - only a few hundred million kilometers.

They are circling further out, at a distance of about one to 4.5 billion kilometers from the sun outer planets: Jupiter, S.aturn with his rings, Uranus and all the way outside Neptun. They are made of gas (mostly hydrogen and helium) and are much larger than the inner planets. Jupiter and Saturn are about ten times the size of the earth, that's why they are also called that Gas giants.

And finally, there are asteroids, comets and clouds of dust that also orbit the sun. The gravitational pull of the sun holds all these heavenly bodies together and forces them to fly in a circle like on a long line. Everything together is called that Solar system. The moons are one of them - but they are held in place by the gravitational pull of the planets.

But why does the sun even have planets? This has to do with how the sun came into being: a cloud of gas and dust contracted by its own gravity and became a star. But not all of the material in this cloud was "built into" the star - around one percent was left over. And when the sun began to shine, the radiation pushed the remaining matter back outwards.

The light gases were pushed far outwards, the heavier dust and rocks remained close to the sun. From these clouds of dust and gas, the planets emerged over time. Therefore there are the gas planets outside in the solar system, further inside the rock planets - including our earth - and in the very center the sun. It contains 99% of the mass of the solar system and holds everything together with its gravity.

### Why is the earth warm inside?

The liquid interior of the earth bubbles under our feet. Volcanic eruptions and geysers show the heat there - over 6000 degrees Celsius in the earth's core. But why is it so hot in the earth?

Much of the heat comes from Earth's childhood days when dust and rocks condensed into a planet. The word “condense” sounds a little too harmless, however: In reality, you have to imagine how many large meteorite impacts - each impact a gigantic explosion that heated up the young planet and melted the material.

Since then it has become a little quieter and the earth is cooling down again. However, it does this extremely slowly, the heat in the interior of the earth can only very slowly escape into space. Hot magma flows in the tough earth mantle transport the heat upwards. There it remains enclosed under the rigid earth's crust as if under a lid. The crustal rock only slowly releases its heat into space.

In addition, heat is still being produced inside the earth. This is because the core of the earth contains a lot of radioactive substances such as uranium. Since the formation of our planet, they have been disintegrating and giving off heat over a very long period of time. This “fuel” will last for billions of years.