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Precision sundial. Flechet's solar chronometer, a sundial claimed to be accurate to within 15 seconds. It consists of a curved disc free to turn on the axis denoted by C and D. This disc is numbered with the hours of the day. The user turns the disc so that the lens (top centre) focuses the Sun's image on the line mn on plate II. The time is then read from point A. Classical sundials are only accurate to about 15 minutes since the speed of the Earth varies during its orbit around the Sun. Flechet's device corrects this because the device is tilted on the axis E so that CD is parallel to the Earth's axis. Image from Les Recreations Scientifique by G. Tissadier (1881).
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