![]() He worked at the famous Great Library that was located there. The Original Eratosthenes ExperimentĮratosthenes lived in Alexandria, Egypt, 22 centuries ago. ![]() ![]() But if the earth is flat, the Eratosthenes experiment can be used to measure how high above the flat earth the sun is. If the earth is a globe, then the Eratosthenes experiment can be used to measure the earth’s size. Flat-earthers also frequently say that Eratosthenes’ experiment can be explained on a flat earth. Since Eratosthenes already knew the earth was a globe, flat-earthers are correct when they claim Eratosthenes didn’t prove the earth is a globe. While there is some uncertainty about the unit of measure that Eratosthenes used, his measurement of the earth’s size is close to the modern accepted value. Therefore, if on the same day the altitude (or zenith distance) of the noonday sun is measured at two locations separated in the north-south direction, one can calculate the earth’s size, provided that the distance between the two locations is known. If the earth is a globe, then the altitude of the sun at noon on a given date will depend upon how far north or south one is. Alternately, we could express this concept as an astronomical body’s zenith angle, the complement of its altitude. Altitude varies between 0° (on the horizon) and 90° (at the zenith, the point directly overhead). How did Eratosthenes do this? The angle that any object in the sky makes with the horizontal is called the altitude.
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