Why did Greek scientist study the circumference of earth

It was part of astronomy and geography. Greek scientists used that measurement to calculate the size of the sun and the moon and the distance from Earth to the Sun and the Moon.

Greek philosophers believed that the Earth was a sphere because they believed that the sphere was the perfect shape. Eratosthenes was the first to calculate the circumference of the Earth because he was interested in geography and wanted to create a map of the world