What assumption must be made about the sun's rays if you use Eratosthenes' method for determining the circumference of a large sphere?

In order to use Eratosthenes' method for determining the circumference of a large sphere, one must assume that the sun's rays are parallel when they reach the Earth. This assumption allows for the creation of similar triangles formed by the sun, the Earth, and the two locations where the shadows are measured. By measuring the angle of the shadows at each location, the distance between the two locations, and the Earth's radius, one can calculate the circumference of the Earth using trigonometry and geometry.