Can you explain me the hypothesis made by erathostenes?
No.
The calculations of a 2,000 year old philosopher might be of academic interest but that has nothing at all to do with how we know the size and shape of our planet.
We have tools to measure it with astonishing accuracy. I am probably using satellites right now to send data instantly around the earth. We navigate by a network of GPS satellites that depends on precise calculations of the earth's curvature.