# How do we know the Earth is round and what makes it possible?

How do we know the Earth is round and what makes it possible?

In: Physics

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Today? We don’t need sticks, shadows, and ships disappearing beyond the horizon. We actually have a bunch of satellites up there, circling around it. You are using a subset of them whenever you turn on the GPS feature in your phone. Another subset of them was involved in taking pictures and measurements thanks to which we don’t just know “it’s round”, we already have a pretty decent 3D model of its surface.

There are multiple ways we currently KNOW how the Earth is round but if you’re asking about olden days then it’s a different story. Basically people saw that the moon was round and it makes sense that the earth would be too. Then there is the observation that you can’t see past the horizon. Out on the sea you see the tops of things first. Like mountains, you see the top first then the bottoms when moving towards them. This gave them some confirmation that the Earth was spherical.

We have various ways of knowing that the Earth is round – many of which involve some degree of geometry. For one, we have had to account for the curvature of the Earth while navigating and mapmaking for centuries. Going into the modern day, you can literally fly a plane in a straight line and end up right back where you started. The ancient greeks measured the roundness of the Earth by comparing the angle of the sun in the sky at different places at the same time on the same day. They used these angles to calculate the amount that the Earth curved between those two places. Another way we know that the Earth is round and spinning is foucault’s pendulum, or gyroscopes. Someone holding a gyroscope will see it rotate over the course of a day, and this rotation will tell them exactly how far and in what direction the Earth has rotated over the course of the gyroscope’s spinning. Focault’s pendulum is a pendulum which is used in such a way that it behaves somewhat like a gyroscope, and the rotation of the Earth causes it to rotate. This rotation varies based upon latitude, once again allowing us to extrapolate the curvature of the Earth. Last, but not least, we can literally send people to space who can look at the Earth and say “Huh. Looks like a ball from here.” There are a million other ways to demonstrate the Earth’s flatness, but hopefully that gives you some insight into easy ways to do it at home.

If you observe objects in the horizon you observe phenomena that is only possible if the Earth is round. With a little bit of trigenometry it is also possible to calculate the size of the Earth to very high precision. This is even written in some of the earliest records we have and have never been disputed until recently. The reason why the Earth is round is that gravity pulls everything to each other. So if there is a feature that makes the Earth less then round such as a mountain gravity will be able to pull the matter downhill to the side making it smoothe again.

The greek mathematician Eratosthenes knew that in the city of Syene in Egypt, there was no shadow at noon. So he planted a stick in Alexandria and at Noon there was a shadow, the angle of that shadow was 7.2 degrees. Since sunlight is parallel (not exactly parallel, but the Sun is so far away that it’s almost exactly parallel), this mean that the surface of the earth is curved, otherwise at Noon there would be no shadow in Syene and Alexandria. So he paid someone to measure the distance between Syene and Alexandria, which was 500 Stadia (a measurement at the time).

So if 7.2 degrees is equal to 500 Stadia (800 km), then the 360 degrees of the earth would have a circumference of the Earth. 360 X 800km / 7.2 = 40 000 km. The real circumference of the Earth is 40 075 km at the equator and 40 007 km at the meridian. So he was pretty close for someone using ancient technology.

We can also look at other planet and see that they are indeed spherical and that we have no reason to think that our planet is different. And finally today we can go into space and look at the Earth and see that it’s a sphere.

Why is that possible, well gravity exist. The mass of the earth pull on each other. So you have a force pulling the matter toward the centre, which compact the matter into a sphere. If the mass of an object isn’t big enough the force of gravity won’t be strong enough to compress the material, basically the material is too strong and will keep it’s current form. That’s is why asteroid have irregular shape and are not round. But the more mass the object is, the stronger the gravity is and the more it can compact the material into a sphere.

Here a good video explain it. Look at 2min where he start to talk about how gravity shaped the earth.