When did people discover the earth is a sphere?

The idea that the Earth is a sphere dates back to ancient Greece, with philosophers such as Pythagoras and Plato suggesting a spherical Earth as early as the 6th century BCE. However, it was Aristotle in the 4th century BCE who provided observational evidence, such as the shape of the Earth’s shadow on the moon during lunar eclipses. By the 3rd century BCE, the mathematician Eratosthenes calculated the Earth's circumference, further solidifying the understanding of its spherical shape. This knowledge gradually spread and became widely accepted in the centuries that followed.