Why is the Earth a sphere? Not all celestial bodies are spheres!

Since ancient times, there have been countless discussions on the question of "What shape is the earth?" The ancient Chinese book "Zhou Bi" records that "the sky is round like a canopy, and the earth is square like a chessboard", which is the origin of "the sky is round and the earth is square".

The ancient Babylonians and ancient Indians believed that the earth was a mountain carried on the back of a sea turtle. However, because the ancient Indians worshipped elephants, the mountain they imagined was not carried directly by the sea turtle, but by three sacred elephants standing on the back of the sea turtle.

The ancient Greeks were closer to the right answer (although their reasons were also sloppy). They believed that the earth was round, not because they had any substantial evidence to prove this, but because they believed that the sphere was the most perfect shape among geometric figures.

It was not until Magellan completed the first circumnavigation of the world in human history, which took three years, that the idea of ​​"the earth is round" was truly confirmed.

Why is the Earth not a cube or a cone, but a nearly sphere? Is there really some mysterious force outside controlling its shape? Let's take a look with Pang Ke!

The indispensable electromagnetic force

In physics class, we all learned that the existence of all things in the universe can be roughly divided into three states: solid , liquid and gas . Due to the interaction between the electromagnetic force of molecules, the solid has a definite shape. The distance between liquid molecules is much farther than that of solid, so the electromagnetic attraction between molecules is weaker, so the liquid has no fixed shape.

Fat Ke: Let’s learn about “tear drop shape”.

Aren't liquids not fixed in shape? So what's going on? In fact, the main factor that causes a droplet to become a ball is the surface tension of the liquid, which is essentially the electromagnetic force between molecules mentioned above.

Do you remember a picture shown in the "Sky Classroom"? Without the influence of the earth's gravity, let the drop of liquid fall freely, and the surface tension will make the drop of liquid present a perfect spherical shape. This is because the surface potential energy of the droplet is the smallest when the shape is spherical, and when the potential energy is the smallest, the system is the most stable. This is the " minimum potential energy principle ".

Gravity is everywhere

In addition to the electromagnetic force between molecules, " gravity " is certainly not absent. As long as there is mass, there will be gravity, so each molecule in the object will have an attraction on other molecules. If you want to destroy the integrity of the object, you must overcome the gravity between the molecules.

In addition to their own gravity, objects also have a gravitational relationship with each other, which is proportional to the mass of the object and inversely proportional to the distance between the two. In other words, the magnitude of the gravitational force between objects depends on their mass and the distance between them, and has nothing to do with the shape of the object.

Electromagnetic force and gravitation complement each other

Gravitation is related to mass, while electromagnetic force is not related to mass but only to electric charge. For small objects, the self-gravity of each molecule is much smaller than the electromagnetic force, so when the electromagnetic force is greater than the gravitational force , the object can maintain its shape .

However , when the mass of an object increases to a certain extent , exceeding the critical point , gravity will overcome electromagnetic force and become dominant . According to the " principle of minimum potential energy ", the object will eventually become a ball . At this scale, even the hardest object will be "kneaded" into a shape with the least gravitational potential energy - a sphere - under the action of gravity. Gas planets will also follow this principle under the premise of gravity dominating.

It is under the influence of these two factors and its rotation that the earth has become an imperfect "sphere". Although there are still mountains on the surface of the earth, these mountains are created by the movement of the earth's plates. Compared with the diameter of the mountain, these can be ignored.

Not all celestial bodies are spheres

Not all celestial bodies are spheres. The shapes of asteroids, comets, and comet nuclei that are only tens of kilometers in size vary greatly. The alien visitor "Oumuamua" that visited the solar system in 2017 was shaped like a long cigar.

In addition to the two forces mentioned above, gas giants such as Saturn and Jupiter are also affected by rotation. The inertial centrifugal force generated by rotation is largest near the equator and smallest near the poles, and is equal to zero at the poles. Therefore, their shape will be closer to an ellipsoid. In addition, their own rotation period is shorter, so the flat shape is quite obvious.

Some people also suggest that if a celestial body rotates fast enough, it will eventually break through the ellipsoid and become a ring, because the ring has lower potential energy and is more stable than the ellipsoid. Although this statement is still only in the theoretical stage, it also indirectly proves the diversity and unpredictability of the shapes of celestial bodies.

But one thing is clear: gravity will not allow the appearance of large-mass irregular celestial bodies . In other words, whether it is a celestial body with a molten interior like the Earth, or a rocky celestial body with a solid interior like the Moon and Mercury, as long as its mass is large enough, the final shape will be a sphere. And these celestial bodies of various shapes that have appeared now are still in the stage of being "tamed", but the final "end" is also obvious - they will all be reduced to spheres.

Do you feel enlightened after reading this? The universe is vast and boundless, and there are more unknowns waiting for us to explore~