calculating the circumference of the Earth

Calculating the circumference of the Earth is easy to do, and was done already in antiquity. Learned men since antiquity have known not only that the Earth was round, but how far around. The men who advised Columbus not to make his voyage were not afraid that he would fall off the edge of the Earth, but that his store of supplies would not be sufficient for the long voyage.
The circumference of the Earth was calculated in antiquity using the geometry of two parallel lines cut by a transversal (colloquially referred to as ‘sticking a stick in the ground’). It was done by Eratosthenes.
Here is the link to the Wikipedia article explaining how Eratosthenes did it.
And here is my own write-up on it:
Circumference of the Earth
keywords: shadow, measurement, proportionality, 25000 miles