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LETTER OF THE MONTH
In the shadows
In ‘The Counterintuitive Cosmos’ (February, p73) you mentioned Eratosthenes measuring the size of the Earth. He would have needed an accurate measurement of the distance from Alexandria to Syene, about 1,000km (620 miles) and a very accurate way of measuring time. How could he have done this?
Anthony Fenwick-Wilson, Shropshire
Marcus Chown replies:
Ancient people were smart! While working at the library in Alexandria, Eratosthenes learned that on the first day of summer there were no shadows cast in the Egyptian town of Syene. So he needed only to observe the shadow of a vertical pillar at Alexandria on the same day. He made his estimate of the distance between Alexandria and Syene from the time it took walkers, who were trained to measure distances by taking regular strides, to trek between the cities.
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