Because in the 3rd Century B.C. Eratosthenes; a Greek mathematician and geographer who famously calculated the Earth's circumference with remarkable accuracy using basic tools and observations of the sun's angle at different locations, essentially "walking" the distance between two points to determine the Earth's size.

Method:
He compared the angle of the sun at noon in Syene (now Aswan, Egypt) with the angle in Alexandria, Egypt, on the same day, utilizing the fact that the sun's rays were directly overhead in Syene at the summer solstice.

Calculation:
By measuring the distance between the two cities and using geometry, Eratosthenes
calculated the Earth's circumference to be remarkably close to the modern value.

https://youtu.be/G8cbIWMv0rI?si=Yj9uP0XANsIfudBs