The Shadow That Measured Earth
Written by Studio AM.
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More than two thousand years ago, the Greek scholar Eratosthenes directed the Library of Alexandria in Egypt. He learned that at Syene, farther south, the midday Sun near the summer solstice could shine into a deep well and leave upright objects with almost no shadow. At the same time in Alexandria, an upright object did cast a small shadow. Eratosthenes treated that difference as a geometric clue.
Sunlight reaching Earth is effectively parallel at this scale. If Earth were curved, vertical lines at the two cities would point in slightly different directions. With the cities treated as sharing a north-south line, the Alexandrian shadow angle was the same fraction of a full circle as their separation was of Earth’s circumference. Traditional accounts say the angle was about one fiftieth of a circle. Eratosthenes multiplied the estimated city-to-city distance by fifty.
His exact result is difficult to translate into modern units because ancient measures varied, and details of the account survive through later writers. Even so, the method was a landmark. It joined a local shadow, an estimated distance, and a geometric model to estimate a planetary size no traveler could measure directly.
The achievement did not depend on seeing Earth from afar. It depended on asking what two different shadows would mean if the same sunlight met a curved surface.
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Question 1 of 4
What is the main idea of the passage?
The answer is B: Eratosthenes combined shadows, distance, and geometry to estimate Earth’s circumference without measuring it directly.
The historical account follows the full method from two shadow observations through a geometric ratio to an estimate of planetary size.
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Question 2 of 4
Why does the passage describe sunlight as effectively parallel?
The answer is C: To connect the difference in vertical shadows to Earth’s curved surface
With incoming rays treated as parallel, the different shadow angles reflect the changing direction of vertical lines on a curved Earth.
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Question 3 of 4
What is the “circumference” of Earth?
The answer is D: The distance around Earth
Eratosthenes scales the city-to-city distance up to a full circle, so circumference is the distance around Earth.
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Question 4 of 4
About what fraction of a full circle was the Alexandrian shadow angle?
The answer is A: One fiftieth
The second paragraph reports the traditional account that the angle was about one fiftieth of a circle.
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