June 19, 240 BCE

On June 19, 240 BCE Eratosthenes estimated the circumference of the Earth using a well in Syene and a stick at Alexandria and some simple geometry. He’d previously noted that on the summer solstice, in a well at Syene, you could see the reflection of the sun at noon. The well was sufficiently deep enough for the full reflection to be only seen on this day. On June 19, 240 BCE, which was the summer solstice, which was almost due north of Syene, at precisely noon, he had a person measure the shadow from the stick at Alexandria, because he hypothesized that the earth was round and that the sun’s rays all struck the earth at parallel lines (because even the scientists in 240 BCE had come to the conclusion that it was many, many, many times bigger than earth), and found that that the stick cast a shadow at 7 degrees. Eratosthenes had already had a person measure the distance from Synene to Alexandria and they got a value that was approximated 920 km (570 miles). Since he now knew that 920 km equaled 7 degrees, 1 degree would equal 134.42 km (83 miles). Since a circle is equal to 360 degrees, then the world’s circumference was approximately 47,300 km (29,400 miles). Current measurements with much more accurate ways to measure distance currently measure the earth’s circumference at 40,800 km (25,400 miles). For calculating the circumference using just a stick, a well, and a man measuring the distance by ‘walking it’ in 240 BCE, that was damn close. If the earth was truly flat, as some idiots today are saying, the stick in Alexandria at middle-day wouldn’t have cast a shadow.

About Joel Byers

Born in North Georgia and educated at some very fine public institutions. Real education started after graduating from college and then getting married and raising two boys. Has the ability to see the funny and absurd in most things and will always remark on it, even if it means getting the stink-eye from his victims.
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