1. Gravity:

* The Earth's gravity pulls equally in all directions towards the center. This force is strongest at the center and weakens with distance.

* Due to this, the Earth tends to be spherical, the shape that minimizes the average distance between points on the surface and the center.

2. Rotation:

* The Earth's rotation creates a centrifugal force that acts outward, perpendicular to the axis of rotation.

* This force is strongest at the equator, where the rotational velocity is highest, and weaker at the poles.

* The centrifugal force counteracts gravity slightly, causing the Earth to bulge at the equator and flatten at the poles.

3. Internal Structure:

* The Earth's interior is not homogeneous. The dense core and mantle, combined with the less dense crust, contribute to the overall distribution of mass and influence the shape.

4. Flexibility:

* The Earth's surface is not rigid. The lithosphere, the rigid outermost layer, is flexible enough to deform under the influence of gravity and centrifugal force.

5. Historical Factors:

* The Earth's shape has evolved over billions of years. Early in its formation, the Earth was likely a molten sphere. As it cooled, its rotation and internal structure resulted in the present shape.

In Summary:

The Earth's oblate spheroid shape is a dynamic equilibrium between the forces of gravity, centrifugal force, and the internal structure and flexibility of the planet. This shape is not static and is constantly evolving, although the changes are very slow.