Prior to the 6th century BC, the Earth was generally understood to be flat, like a disk, based on what scholars and intellectuals could observe and conclude at the time. Slowly Greek philosophers like Pythagoras in the 6th century BC, Parmenides in the 5th century, and Plato in the 4th century began alluding to the idea of a round earth, although not much evidence was provided to back the theory up.
It was 200 years later that Greek astronomers and scientists made greater strides in accumulating physical and observational arguments supporting the idea of a spherical Earth. Aristotle, in particular, was adept at accumulating data to support an argument, and observed in 330 BC that stars seen in Egypt and Cyprus were not viewed the same way farther north. He also promoted his theory of a spherical Earth with observations of a round shadow of the Earth on the moon during a lunar eclipse. Aristotle and his colleagues, through their data collections and application of logic, were able to change the minds of many of their contemporaries, but not all.
It wasn’t until more than 2,000 years later when Ferdinand Magellan successfully circumnavigated the Earth in 1522 that the flat Earth paradigm was finally put to rest.