The earth is round but why do we only see it as flat?

In ancient times, people believed that the Earth was shaped like a flat, round or square disk, and to this day some people still believe in the Flat Earth theory.

Ever since the ancient Greeks observed the sky and the full Moon, scientists have known that the Earth is spherical.

This image of a tropical storm seen from the International Space Station clearly shows the curvature of the Earth's surface (Photo: NOAA/Getty Images).

Today, we see many pictures of the Earth from space, taken by astronauts or sent back by satellites in orbit. But why can't we see a round Earth when we stand in a park or look out the window?

The answer is a matter of perspective. Humans are very small creatures living on an extremely large sphere.

Imagine you are a circus performer, standing on a ball about 1 meter in diameter. As you look down at the ball, you will see it bend in all directions.

Now imagine a bee sitting on that ball, because it is so much smaller than the ball it cannot see the whole ball.

The Earth is about 12.8 million meters in diameter. Even an adult standing on the ground would only have his or her eyes about 2 meters above the ground, so we cannot see the entire sphere of the Earth when standing on its surface. Even if you climbed to the top of Mount Everest, which is 8,850 meters high, you would not be able to see the Earth as a sphere.

Only when you reach an altitude of 10km can you see that scene , because the length of the horizon we can see depends on our altitude above the ground.

When standing on the ground with nothing blocking the view, we can see the horizon about 3 miles away. This distance is not enough to cover a fraction of the Earth's circumference to see the horizon begin to curve.

To see the entire Earth as a sphere, you have to fly high up in satellites or spaceships.

Some large commercial aircraft can also fly high enough to see a slight curvature of the Earth's surface, but this is more apparent from the pilot's cockpit than from the passenger's seat.

In this photo taken from an airplane flying over Colorado, USA, the Earth's surface is seen to be curved rather than flat (Photo: Paul Comstock/ Flickr, CC BY).

Even from space, you can't detect one important thing about the shape of the Earth. Our planet is not perfectly round.

In fact, the Earth is a slightly flattened sphere, or ellipsoid. Its diameter at its largest point, the equator, is slightly larger than its height.

This is due to the centrifugal force created by the Earth's rotation . This force causes the Earth to bulge slightly at the "waist". In addition, topographic features on the surface, such as mountains and deep sea trenches, also change the shape of this sphere due to changes in the Earth's gravitational field.

Earth science has a branch called geodesy , which studies the shape of the Earth and our planet's place in the universe.

Geodesy provides a wealth of information, from building drainage systems and mapping sea level rise to launching and tracking spacecraft. It is an important area of ​​scientific research today.