The Earth: Round like a ball

​[laughter] What are you doing? ​ ​It was supposed to be a surprise. It's a space time ship. ​ ​Hmm. What is it for? ​ ​It can travel through space and through time. ​ ​Amazing. Where's my seat? Wow!

From here, you can really see that the earth is round, like a ball. ​ ​Yeah. You know, the first people who figured that out did it without the help of a spaceship, more than 2,000 years ago. Let's go there and have a look. ​ ​Here in the city of Alexandria lived a great mathematician, Erastothenes. He noticed that when he enjoyed his lunch at noon, the obelisk at the square cast a very short shadow. At noon, when the sun is at its highest, the rays of sunlight come from almost straight above, but not completely.

As they have a slight angle, there's a tiny bit of shadow. ​ ​One day, Eratosthenes received a letter from a friend who lived in the city of Syene, located 800 kilometres south of Alexandria. The friend told Eratosthenes that where he lived, there were no shadows at all at noontime. In Syene, the sun is shining from directly overhead. This is odd, thought Eratosthenes. The earth is flat.

Anyone can see that. Then the sun should shine at the same angle in all places, shouldn't it? The rays of the sun should be parallel, but they're not. Hmm. Aha!

If the earth was not flat at all but curved, then the shadows could be at different angles. And if the earth is curved, its ends must eventually meet, forming a sphere. That's how Eratosthenes figured out that the world is round like a ball. ​ ​That's not all he figured out. Eratosthenes even calculated the size of the ball. Once he knew the difference in the angle between the shadow in Alexandria and in Syene, all he needed was the distance between the two cities.

To measure that, he sent a camel to Syene. It's an 800 kilometres march. ​ ​The difference in angles was 7.2 degrees. 360, a full circle, over 7.2 times 800 kilometers is 40,000 kilometres. That way, Eratosthenes figured out not only that the world was round like a ball, he also estimated its circumference with a staggering precision. ​Even the meter comes from this estimate. One meter was originally defined as one forty-millionth of the earth's circumference. ​ ​Hey!

Since the earth is round like a ball, we could dig right through and get out at the other side. Buckle up and let's go! ​ ​Here is what the earth would look like if you cut it in half. It consists of layers stacked on top of one another, like an onion. First, the ship has to dig through a hard layer of rock, the crust. Although it can be as thick as 50 kilometres in some places, it's just a thin sheet compared to the size of the earth. ​ ​Having passed through the crust, Kim's and Philip's ship heads further down into the heaviest of the earth's layers, the mantle.

It's almost 3,000 kilometres thick, and makes up the vast majority of the earth's volume - more then 80%. Just like the crust, the mantle is mostly solid, although it is much hotter and it's slowly moving like a river of rock. In a cartoon like this one, a ship can travel through the earth, but no real person has ever been down even to the mantle. So how do we even know what there is in the earth's depths? ​ ​When there is an earthquake or a very large explosion, scientists study how the vibrations pass through the earth. Then they make tiny calculations to find out what materials the inner layers of the earth are made of. ​ ​After traveling through the mantle, the ship has reached the core of the earth.

The core is different from the two outer layers. While the crust and mantle are made up of rock, which mostly consist of oxygen, magnesium, silicon, and aluminum, the core consists mostly of iron. The outer core is fluid. It is a thick sheet of constantly moving molten iron, and this is something that matters to us a lot, as the spinning sphere of melted iron is what causes the earth's magnetic field. We have good reason to be grateful for that magnetic field.

Thanks to the earth's magnetic field, we can navigate with a compass, birds can find their way when migrating, and our planet is protected from cosmic rays that would otherwise hurt life on earth. ​ ​The innermost part of the core is as hot as-- it's as hot as the surface of the sun, 6,000 degrees celsius or so. But even though it is that hot, it is still in solid form because it is under such immense pressure. The inner core is way too hot for this ship, so it's best Kim and Philip return to the surface. They leave the solid inner core and move on through the liquid outer core. Then they pass into the slow moving, semi-solid mantle, and finally break through the solid crust.

They have covered a total distance of 12,700 kilometres - the diameter of the earth.