For most of human history it was not known how big earth was or the shape of the planet. Through observations of the skies and earth-sun geometry it became more logical that the earth is round. Once satellites and space craft could see earth from space there was no doubt that the earth is round.

In examining the sun and moon in the sky they take on a disk or ball shape. Stars and planets appear more as points of light but when examined by telescope they also have the disk or ball shape. The moon is the easiest object to analyze since it is close to earth, can easily be looked at without hurting eyes and it has topography. From the shadow casting on the moon by the sun it can be concluded it must be a ball shape and not a disk.

Gravity is the attraction that holds mass together. Gravity increases as the mass of an object increases. To get a significant amount of gravity it takes a very large object such as a moon or planet but even the smallest objects have some gravity. Since gravity acts from all directions the shape of least resistance is a ball. From space earth appears as a rather smooth ball. On the surface though we notice significant topographic and elevation changes. Gravity over time tries to even out these changes but other forces are able to counter gravity such as the uplift produced when tectonic plates collide.

If a theoretical planet could be a square or triangle shape the edges would be eroded by gravity over time since they are weak points in the shape. Over time the object will become more and more circular.

Earth is not a perfect ball. The main reason for this is because of the earth's spin. When laundry is in the spin cycle you notice the clothes go out to the edges of the washing machine. The spin of the earth puts extra force on the equatorial regions thus the shape of earth slightly bulges outward at the equator. The shape of earth is described as an oblate spheroid.