ALMAGEST

astronomical and mathematical encyclopaedia compiled about AD 140 by Ptolemy (Claudius Ptolemaeus of Alexandria). It served as the basic guide for Arab and European astronomers until about the beginning of the 17th century. The name, corrupted from the Arabic, means the greatest; among other names given to the work were The Great Treatise, The Great Astronomer, and The Mathematical Collection. It was first translated into Arabic in 827 and retranslated from Arabic to Latin in the last half of the 12th century. The Almagest is divided into 13 books. Book One gives, in broad outline, the geocentric, or Ptolemaic, plan of the solar system. Book Two contains the earliest surviving work on trigonometry: a table giving the values of chords of a circle at intervals of 1/2, accurate to at least five places; and studies in the solution of spherical triangles. Book Three deals with the motion of the Sun and the length of the year; Book Four deals with the Moon and the month, as does Book Five, which also takes up the distances of Sun and Moon and tells how to construct an astrolabe. Eclipses are treated in Book Six, as also are planetary conjunctions and oppositions. Books Seven and Eight mainly concern the fixed stars, giving ecliptic coordinates and magnitudes for 1,022 of them. This star catalog is based on that of Hipparchus (129 BC); those two books also discuss precession and the construction of celestial globes. The remaining five books, the most original, set forth in detail the Ptolemaic system outlined in Book One.