Everything You Think You Know About the Dark Ages is Wrong
…ative to lines of celestial longitude and latitude. He (or more likely his best student) wrote a book on the astrolabe, an instrument for telling time and making measurements by the sun or stars. You could even use it to calculate the circumference of the earth, which Gerbert and his peers knew very well was not flat like a disc but round as an apple. Much of this science Gerbert learned as a youth living on the border of Islamic Spain, then an e…
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