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The four-cubit dioptra of Archimedes (3rd c. B.C.)

It was an early astronomical instrument suitable for measuring very small angles. It was used to calculate the apparent angle and the distance of celestial bodies from the Earth but also the distance between them. It consisted of a long (four-cubit) calibrated rod which had a plate with an eyepiece and a sliding component. The component had either a narrow plate or a larger plate with a large hole or two smaller ones. The operator of the instrument aimed the celestial body by moving the sliding component until it covered its circumference. Then he immediately read the apparent angle or by using ratios calculated the required distance.