Eratosthenes measures the Earth - simplify in simple English, please!?

It is an article about how Eratosthenes measures our Earth and I don't understand a single word. Please help me: In a remarkable feat, the Greek astronomer and geographer Eratosthenes estimated the size of Earth in about 240 B.C. He did it by comparing the altitude of the Sun passed directly overhead in... show more It is an article about how Eratosthenes measures our Earth and I don't understand a single word. Please help me:

In a remarkable feat, the Greek astronomer and geographer Eratosthenes estimated the size of Earth in about 240 B.C. He did it by comparing the altitude of the Sun passed directly overhead in Syene on the summer solstice. He also knew that in the city of Alexandria to the north the Sun came within only 7 degrees of the zenith on the summer solstice.

He therefore reasoned that Alexandria must be 7 degrees of latitude to the north of Syene. Because 7 degrees is 7/360 of a circle, he concluded that the north-south distance between Alexandria and Syene must be 7/360 of the circumference of Earth.

Eratosthenes estimated the north-south distance between Syene and Alexandria to be 5000 stadia.
Thus he concluded that:

7/360 x circumference of Earth = 5000 stadia

From this he found Earth's circumference to be about 250000 stadia. Today, we don't know exactly what distance a stadium meant to Eratosthenes. Based on the actual sizes of Greeks stadiums, it must have been about 1/6 kilometers. Thus, Eratosthenes estimated the circumference of the Earth to be about 250000/6 = 42000 kilometers - remarkably close to the modern value of just over 40000 kilometers.

Your help would be much appreciated it. Thanks for your time.
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