A compendium of astronomical and mathematical knowledge written by Ptolemy in about ad 150. It incorporates the star catalogue compiled by Hipparchus, upon whose work Ptolemy may have drawn for other parts of the book. It is the most complete surviving treatise on ancient astronomy and contains descriptions of the 48 Greek constellations on which our present-day constellation system is based. Its original Greek title was Syntaxis; Almagest, the name it acquired when translated into Arabic in about ad 820, means ‘the greatest’.