How did the Greeks know the size of the earth over 1,700 years before anyone sailed around it?

In the third century B.C., Eratosthenes needed no more than a gnomon, or shadow stick, to measure the whole earth, and his answer missed our contemporary, sophisticated measurements by only 5 percent.

On the summer solstice, Eratosthenes stood at Alexandria with his gnomon. When the sun reached its zenith over Syene, a city 5,000 stades, or 527 miles, due south of Alexandria, he measured the length of the shadow cast by his stick.

At Alexandria the angle of the sun was 7°12′ away from its zenith. From this observation and knowledge of the distance to Syene, he calculated the earth’s circumference as 250,000 stades, or 26,350 miles.

The actual polar circumference, according to scientists today, is 24,859 miles.