He produced the Almagest, a compendium of contemporary astronomical knowledge, drawing on writers, such as Plato and Hipparchus, whose works were kept in the great library at Alexandria. His Ptolemaic system was a geocentric model of the Universe. Highly contrived as it now appears, it accounted for the observed apparent motions of the planets reasonably well, and remained largely unquestioned until the sixteenth century, when it was challenged by N. Copernicus. Ptolemy’s Geography enjoyed a similar period of dominance (it convinced Columbus that he could sail westwards to India); his Tetrabiblos was an astrological treatise.