In the book "Cosmos", Carl Sagan describes the methods used by Eratosthenes in determining the radius of earth (Chapter 1). Going through the steps of the method, he makes the statement "The Sun is so far away that its rays are parallel when they reach the Earth", which is crucial for Eratosthenes' conclusion. Sagan does not explain how Alexandrian scholars knew this quoted fact.
Is it true that the Alexandrian scholars already knew this fact? If yes then how did they determine this or how might they have determined this? Did they know the distance of Sun from Earth? If yes then how?