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Handout #1

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Several centuries before Christ, Greek astronomers had made a number of observations that suggested to them the earth was not flat. They had observed, for example, that the stars visible in the night sky were not all the same at widely separated places. They had noticed that the shape of the earth's shadow on the moon during an eclipse was rounded and that ships putting out to sea gradually disappeared over the horizon.

About B.C. 250 a Greek astronomer named Eratosthenes made a set of observations that demonstrated that the carth's surface was indeed not flat. He knew that at noon on June 21st of any year the sun was directly over the city of Syene, in Egypt. He had deduced this fact by observing that the buildings at that time cast no shadows and that the sun's reflection could be observed in the water at the bottom of a deep well. Eratosthene himself lived in Alexandria, a city 800 kilometers (km) north of Syene. He demonstrated that at noon, the 21st of June in any year, the sun was not overhead in Alexandria but slightly to the south (Figure 1-1). By measuring the length of noontime shadows cast by buildings, Eratosthenes was able to determine that the sun was approximately 7.2 degrees to the south at its meridian passage.

Eratosthenes had proved that the Earth was not flat. Assuming it was spherical, he calculated the circumference of the Earth by multiplying the distance between the two cities by 50 (7.2° = 1/50 of a full circle, 360°). His result, 40,000 km, was surprisingly close to the true distance. This experiment stands as a major scientific accomplishment.

Eratosthene's measurement of the Earth's circumference. What assumptions were made in this method?

1. Are rays from the sun parallel? If not would this method be valid?

2. Is Alexandria ~800 km north of Syene?

3. Do buildings and wells lie along lines that point to the center of the carth?