The Edge of the Earth

In 240 BCE Eratosthenes calculated the the size of our dear planet Earth. That's right, nearly twenty-three centuries in the past someone not only provided proof of a round Earth, but calculated the size and set about putting collected maps of the known world in terms of latitude and longitude. His figures were in stadia, a distance measurement of the time, and according to which type of stadia were used his calculation for the circumference of the Earth is either off by 20% or just 1%.

Honestly speaking, it doesn't matter which type of stadia it was. For his time, with the accuracy of the measurements he had, either result is just a result of smaller errors either adding up or canceling out. As one of the first to set about a way of measuring such a thing and proving it viable it would be up to later measurements to get more accurate figures.

The issue here is the science of how he did it, why he did it, and what he did right.

Basically he used shadows cast by the sun down wells on a particular day to determine the angle between two cities. Then by taking the distance between the two cities and using geometry he calculated for the circumference. The Chinese did a similar measurement, but by assuming the Earth was flat, they instead calculated for an incorrect distance to the Sun. Eratosthenes on the other hand assumed the Sun was at an effectively infinite distance and thus the light arriving from it was effectively parallel. This is technically wrong, but the Sun is not a point source either. The Sun is instead larger than the Earth, which is why Eratosthene's assumption worked.

Both are on their own, with the knowledge they have, valid calculations. There is nothing within the calculation itself that points to the Earth being either round or flat. That information comes from some other source of knowledge. This is why general research and knowledge is so valuable in science. Eratosthenes had the advantage of being the chief librarian of the Library of Alexandria which was the greatest library in antiquity, the one that romantics of knowledge refer to with the story about the royal decree that all ships entering port would have their books confiscated with copies returned in their place. What is known however is that the library sent people out to various cities to buy books from locales such as Rhodes and Athens.

From records of the time however, the likely proof for a round Earth calculation as opposed to flat Earth was based on observation of the Lighthouse of Alexandra, one of the wonders of the ancient world. Why? One of the easiest ways to observe a round Earth is to observe the curvature of the Earth affecting the horizon on a large body of water. Of all the natural landscapes, water is effectively the flattest of them all over large distances, not to mention naturally at the lowest visible elevation. As a boat travels away from the Lighthouse of Alexandria, the bottom of the structure appears to descend below the horizon until finally the peak vanishes. Indeed, with a known height for the lighthouse and a known distance from it over water a rough estimate for curvature can be calculated.

There is also the matter of independent observations leading to verification, which also happened with the Greeks. Since the Earth rotates in such a way as to make the heavens appear to move across the sky, the same observation can be performed for various celestial bodies, not just the Sun. Indeed, a century later Posidonius made such an observation of the star Canopus above the horizon from two different locales and came up with independent numbers for the circumference of the Earth.

The Dark Ages might have set things back, but we've come a long way since then.

This is for you Eratosthenes.

 ~ Welcome to Earth, enjoy the ride. ~ VP