 The thirteen books of the "Almagest" are the most influential and significant trigonometric work of all antiquity.A theorem that was central to Ptolemy's calculation of chords was what is still known today as Ptolemy's theorem, that the sum of the products of the opposite sides of a cyclic quadrilateral is equal to the product of the diagonals.A special case of Ptolemy's theorem appeared as proposition 93 in Euclid's "Data".