Ancient Wisdoms

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Eratosthenes Measures the Earth

Eratosthenes' method for determining the size of the Earth was an elegant application of simple geometry to an otherwise very difficult problem. By using the difference in the elevation of the noontime sun at two different locations, he was able to measure the angular difference between the vertical directions at those two locations. This angular difference told him what fraction of the way around the earth separated the two locations. He then used this fraction and the measured distance between the two locations to estimate the distance around the earth (a.k.a. the circumference). As the story goes, old Eratosthenes learned that at noon on one day a year (the summer solstice, but that's not really important), the Sun shone directly down a deep vertical well in a town called Syene, about 500 miles south of his home in Alexandria. Eratosthenes reasoned that the Sun must be directly overhead at noon in Syene at this time. He knew that the Sun was not directly overhead in Alexandria on this day, so he figured that the vertical direction in Alexandria was different from the vertical direction in Syene.

Photo credit: Enigma machine. Photograph: Linda Nylind for the Guardian

How did the Enigma machine work?

Like all the best cryptography, the Enigma machine is simple to describe, but infuriating to break. Straddling the border between mechanical and electrical, Enigma looked from the outside like an oversize typewriter. Enter the first letter of your message on the keyboard and a letter lights up showing what it has replaced within the encrypted message. At the other end, the process is the same: type in the “ciphertext” and the letters which light are the decoded missive.

Development Of The Digital Computer

A typical digital computer system has four basic functional elements: (1) input-output equipment, (2) main memory, (3) control unit, and (4) arithmetic-logic unit. Any of a number of devices is used to enter data and program instructions into a computer and to gain access to the results of the processing operation. Common input devices include keyboards and optical scanners; output devices include printers and monitors.

Illustrations and Explanations of Wonderful Machines

Qi qi tu shuo (Illustrations and explanations of wonderful machines) is the first Chinese translation of a work that introduced Western mechanics and machine engineering to China. It was dictated by Deng Yuhan and recorded and translated by Wang Zheng (1571–1644).


Cryptology, science concerned with data communication and storage in secure and usually secret form. It encompasses both cryptography and cryptanalysis.

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