When did people first figure out how large Earth is?
JB: This is Earth and Sky for Saturday, June 22. A listener writes, “When did people first figure out how large Earth is?”
DB: The first fairly accurate measurement of the size of the Earth was made many centuries ago. It relied on observations of the sun around this time of year. We’ve just passed the June solstice – the day on which the sun takes its northernmost path across the sky for the year . . .
JB: On the day of the solstice, in the ancient city of Syene in Egypt, the sun was said to shine directly down into water wells exactly at local noon. Syene was located along the Tropic of Cancer – the imaginary line on a map marking the northernmost point on Earth at which the sun can be seen directly overhead. Thus, at Syene, the sun passed overhead at noon on the day of the June solstice.
DB: Meanwhile, the sun never appeared exactly overhead as seen from Alexandria – about 800 kilometers – or 500 miles – to the north. Early Greek scientists measured the position of the sun at noon on the day of the solstice in Alexandria – and found it to be about 7 degrees south of overhead. Over two thousand years ago, they used this information – and some simple mathematics – to estimate Earth’s circumference at about 38,000 kilometers, or 24,000 miles.
JB: And that’s remarkably close to today’s satellite measurements. With thanks to the National Science Foundation, we’re Block and Byrd for Earth and Sky.
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