The distance d to the horizon from a point h feet above the surface of the earth can be calculated if you know Pythagoras' Theorem and the Radius of the Earth. Eratosthenes lived about 200 BC and Pythagoras about 500 BC. So the distance could be calculated any time after 200 BC.

It's an important number to know. Mariners especially find it useful, as do explores.

But when was it first calculated? Or rather, when was the formula first reported? I recently learned that Galisteo used it in the early 1600's when he found the height of a mountain on the moon knowing the moon's radius and the distance of the mountain from the 'horizon'.

Does anyone know of an earlier reference?