Almagest

Summary

The Almagest is Ptolemy's complete mathematical account of the motions of the Sun, Moon, planets and fixed stars, written in Greek at Alexandria about AD 150 under the title Mathematike Syntaxis (Mathematical Compilation). Its familiar name comes from the Arabic al-majisti, from a Greek word meaning the greatest. The library stores no text of it and gives no quotations, because no public-domain English translation exists: the English versions by R. Catesby Taliaferro (1952) and G. J. Toomer (1984) are both under copyright. This record describes the work in its own words. The thirteen books proceed from first principles to detailed tables. Book I, after a preface on why mathematics gives the surest knowledge and leads the mind toward the divine, argues that the heavens are spherical and turn as a sphere, that the Earth is spherical, lies at the centre and does not move, and is as a point compared with the heavens; it then builds the trigonometry needed, a table of chords in steps of half a degree. Book II treats spherical astronomy for different latitudes: day lengths, rising times of the zodiac and the shadows of the gnomon. Book III sets out the theory of the Sun, fixing the length of the year with Hipparchus and showing that an eccentric circle and an epicycle on a concentric circle can produce the same motion. Books IV and V give the theory of the Moon, adding a second irregularity found by Ptolemy himself, describe an armillary instrument, and compute parallax and the distances of Sun and Moon. Book VI treats eclipses of the Sun and Moon. Books VII and VIII discuss precession, which Ptolemy sets at one degree in a hundred years, and give a catalogue of 1,022 stars in forty-eight constellations with their longitudes, latitudes and magnitudes. Books IX to XIII take up the five planets: their order, their models of epicycle and eccentric, the equant point about which the epicycle's centre moves uniformly, the tables of their positions, their retrograde motions and stations, their greatest elongations and their latitudes. Throughout Ptolemy states observations by date, many his own and many from Hipparchus and Babylonian records, derives parameters from them, and builds tables a reader can use to predict positions for any date. The work was translated into Arabic in ninth-century Baghdad and into Latin in the twelfth century, and it remained the foundation of astronomy until Copernicus, who followed its structure even as he moved the Sun to the centre.

Top Lessons

• Saving the appearances

  Ptolemy's aim is to find combinations of uniform circular motions that reproduce what is actually seen in the sky. He shows that different geometric models, such as an eccentric and an epicycle, can give the same appearance, so the choice between them is guided by simplicity and fit. The Almagest is a lesson in what a mathematical model can and cannot claim.

• Reasons for a central, motionless Earth

  Ptolemy argues in Book I that a rotating Earth would leave clouds and thrown objects behind, and that heavy bodies everywhere fall toward the centre. These were serious arguments from the physics of his day, answered only when a new account of motion was developed in the seventeenth century.

• Astronomy as a path to the divine

  In his preface Ptolemy says that the study of the unchanging heavenly motions makes its followers love their divine beauty and grow in nobility of character. For him precise calculation and contemplation belong together.

• Building on the records of others

  Ptolemy uses Babylonian eclipse records and Hipparchus's observations stretching back centuries, and dates every result. The work shows astronomy as a long collaboration across cultures and generations.

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Medical Lens

The Almagest itself contains no medicine. Its importance for the history of medicine is indirect: Ptolemy's companion work, the Tetrabiblos, set out the astrology that late antique, Islamic and medieval physicians used to time bloodletting and purges and to interpret the critical days of fevers, and that astrology relied on the planetary positions computed by Almagest methods. University medical curricula in medieval Europe often included astronomy for this reason. These connections belong to the reception of the work rather than to its text.