How the Greeks Knew that the Earth is Spherical?

The Greeks believed that the Earth is spherical based on the following:
- Every object on Earth is compressed and converged toward the center forming a sphere;
- The North Star was believed to be at a fixed position in the sky. however, when the Greeks traveled to places nearer the equator, they noticed that the North Star is closer to the horizon;
- During a lunar eclipse, the shape of Earth's shadow reflected on the Moon's surface is circular.
Eratosthenes estimated the Earth's circumference by observing the shadow casts by a pillar and correlating it with the information that while an object in Alexandria during noontime cast a shadow, in Seyene the light rays that hit the water well is reflected back, thus, no shadow is formed. This is another proof presented to support the idea that the Earth is indeed round.