Why Does Our Environment Appear Flat Despite the Earth Being Round?

[Un article de The Conversation écrit par Kelly R. MacGregor – Professor of Geology, Macalester College]

The answer is a question of perspective. Human beings are tiny creatures living on a very large sphere.

Imagine that you are a circus acrobat standing on a ball about 1 meter wide. From the top of the ball, you see it moving away from your feet in all directions.

Now imagine a little fly on this circus ball. Its vantage point would probably be a millimeter or less above the surface. Because the fly is much smaller than the ball and its viewpoint is close to the surface, it cannot see the entire ball.

The Earth is about 12.8 million meters across, and even a tall adult's gaze only reaches about 2 meters above its surface. It is impossible for our eyes to grasp the size of the spherical Earth when we are standing on it. You wouldn't be able to visualize that the Earth is a sphere even by climbing to the summit of Mount Everest, which is 8,850 meters above sea level.

The only way to see the curve of the Earth is to fly more than 10 kilometers above its surface. This is because the length of the horizon we see depends on how high we are above the Earth's surface.

Standing on the ground, with nothing blocking our vision, our eyes can see about 3 miles of the horizon. This is not enough for the horizon line to begin to show its curve. Like a fly on a circus ball, we don't see enough of the edge where the Earth meets the sky.

To see the entire planetary sphere, you would have to travel with an astronaut or aboard a satellite. This would give you a full view of the Earth at a much greater distance.

Large airliners can also fly high enough to provide a glimpse of the curvature of the Earth, although pilots have a much better view from the front of the plane than passengers do from side windows.

Not quite a sphere

Even from space, you wouldn't be able to detect something important about the shape of the Earth: it's not perfectly round. It is actually a slightly flattened spheroid or ellipsoid. This means that around the equator, it is a little wider than it is tall, like a sphere that you have sat on and squashed a little.

This phenomenon is due to the rotation of the Earth, which creates a centrifugal force. This force produces a slight bulge in the size of the planet.

Topographic features of the Earth's surface, such as mountains and underwater trenches, also slightly distort its shape. They cause slight variations in the intensity of the Earth's gravitational field.

Earth science, the field I study, has a branch called geodesy which is devoted to the study of the shape of the Earth and its position in space. Geodesy is used for everything from building sewers to accurately mapping sea level rise to launching and tracking spacecraft.