Читать книгу Geography For Dummies - Jerry T. Mitchell - Страница 17

In the third century B.C., the Greek scholar Eratosthenes made a remarkably accurate measurement of Earth’s circumference. At Syene (near Aswan, Egypt), the sun illuminated the bottom of a well only one day every year. Eratosthenes inferred correctly this could only happen if the sun were directly overhead the well — that is, 90° above the horizon. By comparing that sun angle with another one measured in Alexandria, Egypt, on the same day the sun was directly overhead at Syene, Eratosthenes deduced that the distance between the two locations was one-fiftieth (1/50th) of Earth’s circumference. Thus, if he could measure the distance from Syene to Alexandria and multiply that number times 50, the answer would be the distance around the entire Earth.

There are diverse accounts of the method of measurement. Some say Eratosthenes had his assistants count camel strides (yes, camel strides) that they measured in stade, the Greek unit of measurement. In any event, he came up with a distance of 500 miles between Syene and Alexandria. That meant Earth was about [500 x 50 =] 25,000 miles around (“about” because the relationship between stade and miles is not exactly known). The actual circumference is 24,901 miles at the equator, so Eratosthenes was very close.

Interesting fact: The circumference is 41 miles less pole to pole; more on that in Chapter 4!

About a century-and-a-half later, another Greek named Posidonius calculated Earth’s circumference and due to differences in the lengths of Roman versus Greek stadia, others reported his measurement as 18,000 miles. Posidonius’ measurement became the generally accepted distance mainly thanks to Strabo, the great Roman chronicler, who simply did not believe that Earth could be as big as Eratosthenes said it was. About 18 A.D. Strabo wrote his Geography, which became the most influential treatise on the subject for more than a millennium. Geography credited the calculations of Posidonius and rejected those of Eratosthenes. And that leads to an interesting bit of speculation. Columbus was familiar with Geography, so he was aware of the official calculation of Earth’s circumference — 18,000 miles. Had he known the true circumference was 25,000 miles, like Eratosthenes said, Columbus would have known that China was thousands of miles farther to the west than Strabo suggested. And if he had known the true distance to China, would Columbus ever have set sail?

As a result, modern geographers are into all kinds of stuff. Some specialize in patterns of climate and climate change. Others investigate the distribution of diseases, or the location of health care facilities. Still others specialize in urban and regional planning, or resource conservation, or issues of social justice and equality, or patterns of crime, or optimal locations for businesses — the list goes on and on. Certainly, the ancient ge and graphe still apply, but geography is much more than it used to be.