What makes the Earth round?

Have you ever stopped to wonder why the Earth is round? It's a fundamental fact we learn early in life, but the science behind this seemingly simple observation is quite fascinating. Our planet isn't a perfect sphere, but it's close, and the forces that shaped it into this form are powerful and universal. From the relentless pull of gravity to the effects of Earth's own rotation, several key factors contribute to our planet's unique shape.
In this article by thedailyECO, we’ll explore the question of what makes the Earth round, by examining gravity, centrifugal force, and the scientific term "oblate spheroid" to understand why our planet appears as it does from space.
Why are planets and stars spherical?
The answer lies in gravity. Gravity is a fundamental force of attraction between all matter. In a large celestial body, every particle of matter exerts a gravitational pull on every other particle. This results in a net force directed towards the center of the object.
This inward gravitational force causes the object to assume the shape with the lowest potential energy: a sphere. A sphere is the most efficient shape for minimizing gravitational potential energy because it brings all the mass as close as possible to the center of gravity.
For smaller objects, the forces holding them together, such as electromagnetic forces within rocks or everyday objects, are significantly stronger than the force of gravity. Consequently, gravity has a negligible effect on their overall shape. However, as an object's mass increases, the force of gravity becomes increasingly dominant.
There isn't a strict size threshold where an object instantly becomes perfectly spherical. Instead, there's a gradual transition. Objects with sufficient mass, generally hundreds of kilometers in diameter, possess enough gravitational force to overcome their inherent material strength and pull themselves into a roughly spherical shape. The precise shape can also be influenced by the object's composition and its rate of rotation. Rapidly rotating objects, for example, tend to bulge at their equators due to centrifugal forces.
Curious to know more about how does Earth move through space? Read about it in this other article.

Is the Earth a perfect sphere?
The Earth isn't a perfect sphere, it's a bit squashed and bulgy. It bulges out around the middle (the equator) and is a bit flattened at the top and bottom (the poles). This shape is mostly because the Earth spins.
As the Earth spins, it creates a force that pushes things outwards. This force is strongest at the equator and weakest at the poles.
At the same time, gravity pulls everything towards the Earth's center. So, you have two forces: gravity pulling inwards and the spinning force pushing outwards. These two forces working together give the Earth its bulgy shape. The outward push is strongest at the equator, so that's where the bulge is.
The Earth has mountains and valleys, but compared to the whole Earth, they're tiny. If you shrunk the Earth down to the size of a billiard ball, it would actually be smoother than the billiard ball. So, while the Earth isn't perfectly smooth, its overall shape is very close to a smooth, bulgy sphere.
The Moon's gravity pulls on the Earth and causes tides in the oceans, but it doesn't really change the Earth's overall solid shape. The main reason for the Earth's bulgy shape is its spinning motion.
What is the true shape of the Earth?
The Earth's shape is best described as an oblate spheroid, a sphere that bulges at the equator and is flattened at the poles. For very precise measurements, scientists use the geoid, a model of global mean sea level that accounts for local variations in gravity. However, for most purposes, thinking of the Earth as a sphere is a good enough approximation.
Interested in knowing how strong is Earth's gravity? Read about it in this article.

How do we know the Earth is spherical?
We have a wealth of evidence demonstrating that the Earth is spherical. Here are some of the most compelling reasons:
- Ships disappearing hull first over the horizon: this was observed by the ancient Greeks. As a ship sails away from an observer, it doesn't simply shrink into a tiny dot. Instead, the hull disappears first, followed by the masts. This is because the ship is sailing over the curve of the Earth.
- The Earth's shadow during lunar eclipses: during a lunar eclipse, the Earth passes between the Sun and the Moon, casting a shadow on the Moon. This shadow is always round, regardless of the Earth's orientation. The only shape that consistently casts a round shadow is a sphere.
- Changing constellations: as you travel north or south, you see different stars in the night sky. This wouldn't happen on a flat Earth. On a sphere, different parts of the sky are visible from different locations.
- Varying time zones: it's daytime in one part of the world while it's nighttime in another. This is because the Earth is rotating, and different parts of it are facing the Sun at different times. This wouldn't be possible on a flat Earth.
- Observing the Earth from space: satellites and astronauts have taken countless photos and videos of the Earth from space, clearly showing its spherical shape. This is perhaps the most direct and undeniable evidence.
- Air travel: planes can fly in a straight line and eventually return to their starting point, which is only possible on a sphere.
- Gravity: gravity pulls everything towards the center of the Earth. On a sphere, "down" is always towards the center, no matter where you are on the surface. This is why people on opposite sides of the Earth don't fall off.
- The behavior of water: large bodies of water curve along the Earth's surface. This is why we have horizons at sea. If the Earth were flat, water would simply spread out indefinitely.
Interested in why the Sun rises in the east and sets in the west? Do not miss this other article.

How can you prove that the Earth is a spherical by looking at the horizon?
Think about watching a ship sail away. You don't just see it get smaller and smaller until it vanishes into a tiny dot. Instead, the bottom part of the ship, the hull, disappears first, then the middle, and finally the top of the masts. It's like it's sinking below the horizon, which wouldn't happen if the Earth were flat. This shows us we're looking at something curving away from us.
Similarly, if you climb a hill or a tall building, you can suddenly see much further. That's because you're seeing over more of the Earth's curve from a higher vantage point. If the Earth were flat, your view wouldn't change much, no matter how high you climbed.
Finally, consider looking at a tall building far out at sea. You'll often find that you can't see the bottom of it, as if it's partly submerged. This isn't because it's actually underwater, but because the curve of the Earth is blocking your view. These simple observations of the horizon provide clear evidence that the Earth's surface is curved.
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