How did erasthostenes measured the earths circumference?

Eratosthenes measured the Earth's circumference by comparing the angles of the sun's rays at two different locations in Egypt: Alexandria and Syene (now Aswan). He noted that at noon during the summer solstice, the sun was directly overhead in Syene, while in Alexandria, it cast a shadow at an angle. By measuring the angle of the shadow and knowing the distance between the two cities, he used simple geometry to calculate the Earth's circumference, estimating it to be about 39,375 kilometers (24,662 miles), which is remarkably close to the actual value.