What Would Happen if the Earth Were Flat?

Nearly every child today knows for a fact that the Earth is round, but in the past the prevailing opinion was that the Earth is flat. In fact, there is still a small but active community of people who strongly argue that we live on a flat disc, establishing organizations like the Flat Earth Society, which acts to promote this idea. In the realm of fantasy literature, Terry Pratchett’s Discworld, which is borne on the backs of four elephants, who in turn stand on the back of a giant tortoise, is well-known and brings the flat Earth concept to life. 

Gravity is the force that pulls two bodies of mass toward each other, as demonstrated for the first time in the Cavendish experiment. The Earth, for instance, being massive and approximately spherical, pulls every object with mass towards its center of mass, making us perceive a downward pull. 

However, outside of fantasy literature, the spheroidal nature of the Earth’s surface was apparent to many in ancient times, based on astronomical and geographical phenomena, such as the way ships disappear over the horizon on their way to the open ocean, or the changing appearance of the moon. What people lacked in the past was the understanding of physics necessary to explain these observations, including the concept of gravity and its properties.

Even if the world were flat, gravity would still pull us towards its center. When found in proximity to the disc's center, gravity would feel to us similar to our current experience, pulling us downwards. But as we moved closer towards the edges of the disc, the direction of gravity would become diagonal towards the disc’s center, and we would feel as if we’re climbing up an ever steeper mountain. Finally, if we managed to reach the edge of the disc, the gravitational force pulling us to the center would act parallel to the surface, allowing us  to stand upright on the world’s edge. This concept is illustrated in a video by Tech Insider.

In such a flat world, sports like basketball would differ regionally, as the disc’s gravitational pull would affect the trajectory of the ball. Estimating the force required to successfully throw the ball into the basket would be a considerable challenge, as the ball would move differently in each location, under the influence of gravity pulling it towards the disc’s center. Moreover, trees and other plants, which typically grow counter to gravity, would grow at different angles across the flat world. But the strangest thing of all would be the behavior of water bodies. In a disc-shaped world, gravity would draw water towards the disc’s center, creating one giant extremely deep ocean at the center of the world, while the far edges would remain completely dry, forming barren and desolate deserts.

Today, thanks to our accumulated knowledge of physics, we understand that any celestial body with a radius of more than a few hundred kilometers, including the moon and the planets, will collapse under its own gravity and assume a more or less spherical shape. Smaller celestial bodies, such as asteroids and moons like Phobos and Deimos, orbiting Mars, do not collapse under their own gravity and thus do not become spherical.

How else do we ascertain the roundness of the Earth? It becomes readily apparent through the examination of satellite images and photographs captured by spacecraft launched into space. These visual records offer compelling evidence of our planet's spherical shape. Furthermore, terrestrial engineering projects often have to account for the Earth's curvature. A notable example is the planning of the Verrazano-Narrows Bridge in New York, which connects Staten Island to Brooklyn. During the bridge’s design process, the curvature of the Earth was a significant factor that had to be taken into account. The two towers of the bridge, positioned at a distance of 1,300 meters apart and and completely perpendicular to the Earth’s surface, illustrate this effect. The distance between their bases is 41 millimeters shorter than the distance between their tops due to the Earth’s curvature.

Further evidence was provided by the Greek mathematician and astronomer, Eratosthenes, in the third century BCE, who measured the shadows of two identical-height pillars in Egypt, one in Aswan and the other in Alexandria. By employing geometric calculations, he accurately calculated the Earth’s circumference based on the observed curvature of the Earth’s surface. Lunar phenomena also contribute to this knowledge. If the Earth were flat, different parts of the moon would be visible from different locations on Earth. In reality, we know the moon appears identical from any location on Earth on any given night.

The Flat Earth Society claims that the gravity we feel is the result of an “upwards” acceleration of the entire celestial system, comprising the sun, the planets and the moon. While this may explain the downward pull we feel, it fails to account for other observed phenomena, such as the behavior of satellites orbiting the Earth.

When would the world be considered flat? That's a matter of perspective. Interestingly, in the context of special relativity, a flat Earth perspective is possible. According to  the theory of special relativity, when a body moves at a speed approaching the speed of light, objects found in the direction of its motion appear shorter than those behind it. Consequently, particles such as neutrinos, that move towards us at speeds close to that of light, indeed may perceive the Earth as an extremely flat disc.