Eratosthenes Measures Earth's Circumference
Eratosthenes, a Greek scholar and the head librarian of the Library of Alexandria, made significant contributions to our understanding of the Earth's size in 240 B.C. He is best known for his remarkable calculation of the Earth's circumference, which he estimated to be approximately 25,000 miles. Contrary to the myth that people universally believed in a flat Earth before Columbus, educated circles in ancient times already recognized the planet's spherical shape. On the summer solstice, Eratosthenes observed that the Sun was directly overhead in Syene, while in Alexandria, it was slightly off by just over 7 degrees. By measuring this angle and knowing the distance between the two cities, he multiplied this difference to estimate the Earth's circumference. His close approximation showcased the advanced knowledge of geography and mathematics of the time, and he later authored works on geography and star charts. This early scientific endeavor laid groundwork for future explorations and calculations of the Earth's dimensions.
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Eratosthenes Measures Earth's Circumference
Eratosthenes Measures Earth's Circumference
On June 19, 240 b.c., the Greek scholar Eratosthenes completed his calculations of Earth's circumference, arriving at a measurement that was fairly close to the true figure of approximately 25,000 miles.
Contrary to popular belief, the world was not universally considered to be flat prior to Christopher Columbus's voyage of 1492. It had been known since ancient times that the world was round, but for the most part only elites with access to books and a higher education were aware of this. Uneducated commoners, who made up the great mass of people, were ignorant of geography and thought the world was flat. When Columbus decided to reach Asia by sailing west from Europe, he was unique not for believing that the world was round but for believing that such a long ocean voyage was feasible. Nearly 2,000 years before him, the Greeks were aware of the spherical shape of the world as well. Eratosthenes, born sometime around 276 b.c., wanted to translate this knowledge into a precise calculation of Earth's size.
Eratosthenes lived in Egypt, where he was the head librarian of the famous classical library at the city of Alexandria. He had noticed that at noon on the summer solstice, which is the longest day of the year, the Sun was directly overhead in the city of Syene near modern-day Aswan far to the south of Alexandria. However, on the same day at the same hour, it was not directly overhead in Alexandria. Therefore, Eratosthenes measured the difference between the location of the Sun in the two cities at that point in time and determined that in Alexandria it was just over 7° shy of being directly overhead. Since seven degrees is roughly one-fiftieth of a circle, which has 360°, he took the known distance from Syene to Alexandria and simply multiplied it by 50. Eratosthenes came very close to the correct figure of 25,000 miles; significantly more precise calculations were not made until several centuries later. He went on to compose a book on geography and a star chart, among other projects, before he died sometime around 196 b.c.