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Gerbert of Aurillac was the most prominent personality of the tenth century:astronomer, organ builder and music theoretician, mathematician, philosopher,and finally pope with the name of Silvester II (999-1003). Gerbert introducedfirstly the arabic numbers in Europe, invented an abacus for speeding thecalculations and found a rational approximation for the equilateral trianglearea, in the letter to Adelbold here discussed. Gerbert described a semi-sphereto Constantine of Fleury with built-in sighting tubes, used for astronomicalobservations. The procedure to identify the star nearest to the North celestialpole is very accurate and still in use in the XII century, when "Computatrix"was the name of Polaris. For didactical purposes the Polaris would have beenprecise enough and much less time consuming, but here Gerbert was clearlyaligning a precise equatorial mount for a fixed instrument for accurate daytimeobservations. Through the sighting tubes it was possible to detect equinoxesand solstices by observing the Sun in the corresponding days. The horalogium ofMagdeburg was probably a big and fixed-mount nocturlabe, always pointing thestar near the celestial pole.

Gerbert of Aurillac: astronomy and geometry in tenth century Europe