What led Eratosthenes to the conclusion that Syene was in adiract line between the sun and the center of the earth?

Eratosthenes concluded that Syene was in direct line with the sun and the center of the Earth based on observations of shadows cast by objects at noon during the summer solstice. In Syene, vertical objects cast no shadow, while in Alexandria, vertical objects cast a measurable shadow. By measuring the angle of the shadow in Alexandria, he inferred the curvature of the Earth and calculated the distance between the two cities, leading to his estimate of the Earth's circumference. This demonstrated that at one location, the sun was directly overhead while at another, it was at an angle, confirming the Earth's roundness.