On October 12, 1492 Columbus landed somewhere in the Bahamas, thus becoming the first person to see the Americas, apart, that is, from those who’d lived there for more than 10,000 years and Leif Erikson and his crew.
In the USA this event is celebrated on the 2nd Monday of October, thus today most states are celebrating the day that Christopher Columbus got fairly close to almost discovering America.
What better day, therefore to acknowledge Eratosthenes of Cyrene, born around 276 BC, died around 195 BC. He rose to prominence as the third chief librarian of the Great Library of Alexandria, the centre of scholarship of the world at that time. An edited brief biography filched shamelessly from Wikipedia has it thus …
He was a mathematician, geographer, poet, athlete, astronomer and music theorist. He invented the discipline of geography as we understand it. He invented a system of latitude and longitude.
He was the first person to calculate the circumference of the earth (with remarkable accuracy), the first to calculate the tilt of the earth’s axis (also with remarkable accuracy). He may also have accurately calculated the distance from the earth to the sun and invented the leap day In addition, Eratosthenes was the founder of scientific chronology; he endeavored to fix the dates of the chief literary and political events from the conquest of Troy.
He calculated the circumference of the earth over 2,200 years ago and this is how he did it.
Eratosthenes had been told that the shadow of someone looking down a deep well would block the reflection of the Sun at noon on the summer solstice in the Ancient Egyptian city of Swenet (which happens to be on the Tropic of Cancer). This put the sun directly overhead. He also knew, from measurement, that in his hometown of Alexandria, the angle of elevation of the sun was 1/50th of a circle (7°12′) south of directly overhead at noon on the same day. Assuming that the Earth was spherical (360°), and that Alexandria was due north of Swenet, he concluded that the distance from Alexandria to Swenet must therefore be 1/50th of the total circumference of the Earth. Egypt had been sufficiently well surveyed for the distance between the two cities to be known to be 5000 stadia. He rounded the result to a final value of 700 stadia per degree giving a circumference of 252,000 stadia.
The stadion, like the British Standard Handful, is not a precisely fixed quantity but if we assume that Eratosthenes used the “Egyptian stadion” of about 157.5 m, his measurement turns out to be 39,690 km, an error of less than 2%.
By comparison Columbus set sail 1,700 years later believing the earth to be about 25% smaller than it really is.
bobmcgee 