Here's why:
* Sphere: While the Earth is often referred to as a sphere, it's not perfectly round. It's slightly squashed at the poles and bulging at the equator.
* Oblate Spheroid: This term accurately describes the Earth's shape. "Oblate" means flattened at the poles, and "spheroid" means an object that is almost spherical.
Imagine a ball that has been slightly flattened at the top and bottom. That's a good visual representation of the Earth's shape.