The American Physical Society's This Month in Physics History; June, ca. 240 B.C. Eratosthenes Measures the Earth says:

Eratosthenes had heard from travelers about a well in Syene (now Aswan, Egypt) with an interesting property: at noon on the summer solstice, which occurs about June 21 every year, the sun illuminated the entire bottom of this well, without casting any shadows, indicating that the sun was directly overhead. Eratosthenes then measured the angle of a shadow cast by a stick at noon on the summer solstice in Alexandria, and found it made an angle of about 7.2 degrees, or about 1/50 of a complete circle. (emphasis added)

However in this small video clip titled Carl Sagan - How We Know The Earth Is Round, Prof. Sagan describes it differently. Here is a transcription of a small bit after the 01:00

He was also the chief librarian of the great library of Alexandria, and one day, while reading a papyrus book in the library, he came upon a curious account: Far to the south, he read, at the military outpost of Syene, something notable could be seen, on the longest day of the year. (emphasis added)

I understand there might not be certainty, but is there some general agreement if it was the words of travelers, or a book in the library where Eratosthenes first heard of the "well of Syene"? Of course, it's also possible that the book was written by travelers, and so the two descriptions are not necessarily contradictory.

Carl Sagan discusses Eratosthenes


1 Answer 1


The earliest extant description of Erathostenes' method is found in a book by Cleomedes written in the 1st century BC, and no wells are mentioned at all, if it's true that it's stated that whenever the Sun, 'being in Crab at the summer solstice, the pointers of the sundials in Syene throw no shadows...'

  • $\begingroup$ Thanks for your answer! The well does not seem to be part of Erathostenes' method at all, so it would not be surprising that a description of the method would not mention it. It seems to be more of human interest or anecdotal value - just something that triggered Erathostenes to decide to plan and initiate the measurement. Dr. Sagan doesn't seem to be the type to just completely make up stories, so if he says that Erathostenes read about the well in a book, then he's probably getting that suggestion from somewhere. I don't have any of his books, but he may have footnoted it somewhere. $\endgroup$
    – uhoh
    Aug 29, 2017 at 10:51

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