The ancients believed by intuition that the sky was round and the earth was square. In order to find out the shape of the earth, people do not know how much effort has been spent since ancient times. Now, just take a picture of the earth taken on a man-made satellite or the moon, and you can see the spherical earth clearly without words.
Due to the rotation of the earth, the centrifugal force on the equator is much greater than that in other parts of the earth. At the poles, the centrifugal force becomes zero. Therefore, the material must be concentrated to the equator, so that the equator bulges relative to the poles, that is, the equatorial radius is longer than the radius of the poles, forming a nearly circular flat sphere. Today, there is no need to elaborate on this knowledge, and everyone knows it. But why is the earth spherical, not square, not triangular, not grotesque? This has to start with the origin, gravity and rotation of the earth.
Whether it is the theory of cataclysmic genesis or evolutionary genesis, in the process of radioactive material decay and heat generation, the earth is still in a state of high-temperature melting from below the mantle, except for the solidification of the thin crust. The core may be a hot metal solid due to high pressure. Under the action of gravity, heavy elements sink and light elements float. So it extends from the center of the earth to the surface in all directions, showing a concentric ring structure. In other words, it is basically composed of the same material at the same distance from the earth's center. Therefore, their density is roughly the same, and their gravity is also the same, which can be called the gravity self balance mechanism. What shape can ensure the balance of gravity in all directions? Of course, it is only spherical.
At the same time, the continuous high-speed rotation of the earth since its formation is also conducive to the uniform distribution of molten materials in the earth.
However, when a celestial body is too small, the gravity is weak, or the interior of the solid has never been able to melt, or the original spherical large body is accidentally smashed into a solidified small body, the gravity self balance mechanism will lose its function. Such celestial bodies are rarely round, but strangely shaped. For example, Mars satellites and many asteroids are like this. This also proves from the opposite side that only a celestial body with enough gravity and once or now molten inside can be spherical. Such a celestial body is the earth.