How the Ancient Greeks Proved that the Earth was Round

knowledge of the earth's spherical shape is nothing new well at least for most of us that is you might even be surprised to learn that the earth shape was discovered over 2 000 years ago well before any advanced satellites were able to make the exact measurements that we have today to be fair many of the ancient civilizations likely assumed that the earth was round even before having the evidence the first credited to propose such an idea was philosopher and mathematician pythagoras back in 500 bc but when it came to proving that the earth is round it wasn't until aristotle came around that hard evidence was first laid out when aristotle wrote his book on the heavens in 350 bc he laid out several pieces of evidence for why he believed the earth assumed a spherical shape one thing he noticed and pointed out was that each time there was a lunar eclipse the shadow of the earth upon the moon could be seen and since that shadow is always round regardless of where the earth is in its rotation the earth must be round as well another point he made was that the stars are constantly in different positions depending on where you are on the earth aristotle noticed that there were stars that he could see in egypt that could not be seen from cyprus some 1 000 kilometers away and that as you move further and further away from the equator the visible constellations in the sky would dramatically change that proved as he wrote that not only that the earth is circular in shape but also that it is a sphere of no great size for otherwise the effect of so slight a change of place would not be quickly apparent several years later another greek scholar eratosthenes would go a step further and not only prove that the earth was round but also measure its circumference with only the power of the sun and a stick known as one of the most prominent scholars of his time eratosthenes studied in athens and produced impressive works in astronomy mathematics geography and philosophy in 240 bc he was even appointed as the chief librarian of the great library of alexandria one of the largest and most significant libraries of the ancient world one of eratosthenes many ambitions was to create a world map and to do that he knew that he first needed to determine the size of the earth eratosthenes lived in the city of alexandria near the mouth of the nile river in present-day modern egypt he had first heard about a well located in the ancient city of cyene which is now present-day modern aswan he realized that at noon on the summer solstice when the sun was directly overhead sunlight illuminated the entire bottom portion of the well without casting any shadows on the sides like during other days of the year he then checked to see if the same phenomenon was happening in alexandria by placing a stick upright in the ground at noon on the day of the summer solstice here he noticed that the sun did cast a shadow at an angle of 7. 2 degrees this is because the city of alexandria is north of the tropic of cancer or also known as the northernmost point where the sun will be directly overhead eratosthenes realized that he could then perform some simple calculations with this measurement and easily determine the circumference of the earth however one important factor in this calculation was determining the distance between alexandria and cyene with modern technology this would be no big deal but back in 240 bc this was much more difficult to perform and especially perform accurately eratosthenes hired what was known as a bemetist who essentially was a professional walker trained to measure distances by counting steps although not much is known about their measurement methods they were known to have a high degree of precision and were thought to have accompanied alexander the great on his campaign throughout asia these bemidjis which measured distances and length of stadia an ancient greek unit of length based on the circumference of a typical sports stadium determined that the distance between alexandria and saini was roughly 5 000 stadia or 800 kilometers in today's terms since eratosthenes knew that the sun hit straight on in saini and at a 7. 2 degree angle in alexandria the distance between them should be a 7.

2 degree piece of the 360 degree sphere that is the earth conveniently 7. 2 degrees is 1 50th of a full circle meaning that the distance from alexandria to saini equates to roughly 2 percent of the total earth's surface by taking this estimated distance of 800 kilometers and multiplying it by 50 he was able to determine that the earth has a total polar circumference of roughly 40 000 kilometers so was he right well he was really really close today we know the earth has a circumference of forty thousand seventy-five kilometers around its equator and forty thousand and eight kilometers when measuring the polar circumference this is due to the slight bulge that the earth has around its middle due to the spin of the earth it is important to remember as well that he had to make many important assumptions which at the time were not so accurate number one that the distance between alexandria and saini was exactly 5 000 stadia or 800 kilometers we know today that this distance is measured at 841 kilometers number two he assumed that alexandria is due north of saini which although it is north it is not directly due north as he originally assumed number three he assumed that saini lies on the tropic of cancer today we know that present-day aswan is actually 73 kilometers away from the tropic of cancer number four that light rays emanating from the sun are parallel and finally that the earth is perfectly spherical in shape with proper measurement tools and accurate distance measurements between alexandria and cyeni eratosthenes would have likely been able to get a much closer result in fact in 2012 a similar experiment was conducted with more accurate measuring tools and it was calculated that the distance was 40 074 kilometers which is only 0.