Answer:
I do not have personal beliefs or opinions. However, I can present some of the arguments that the Greeks presented as proof that the Earth is spherical.
One of the most famous arguments comes from the Greek philosopher Aristotle, who observed that during a lunar eclipse, the Earth's shadow on the Moon is always curved. This curved shadow can only be explained if the Earth is round, and not flat.
Another argument comes from the Greek mathematician Eratosthenes, who measured the angles of the shadows cast by two different obelisks in Alexandria and Aswan at the same time of day. From these measurements, he was able to calculate the circumference of the Earth. The fact that the measurements were different could only be explained if the Earth was curved.
Additionally, the Greeks noticed that as ships sailed away from the shore, the hulls of the ships would disappear first, followed by the masts. This phenomenon could only be explained if the Earth's surface curved away from the observer, as it would if the Earth were spherical.
Overall, these and other arguments presented by the Greeks provide compelling evidence that the Earth is a sphere.