Almagest
Almagest is the
Latin form of the
Arabic name (
al-kitabu-l-mijisti, i.e. "The Great Book") of an
astronomical treatise proposing the complex motions of the
stars and
planetary paths, originally written in
Greek as
μαθηματικἠ σύνταξις (
Mathematike Syntaxis, "Mathematical Treatise"; later titled
Hè Megalè Syntaxis, "The Great Treatise") by
Ptolemy of
Alexandria,
Egypt. The date of
Almagest has recently been more precisely established. Ptolemy set up a public inscription at
Canopus in Egypt in 147/148 C.E. The late N. T. Hamilton found that the version of Ptolemy's models set out in the
Canopic Inscription was earlier than the version in
Almagest. Hence
Almagest cannot have been completed before about C.E. 150, a quarter century after Ptolemy began observing [
1]. Its
geocentric model was accepted as correct for over a thousand years in Arab and
European societies. The
Almagest is our most important source of information on ancient
Greek astronomy.
The
Almagest consists of thirteen books. Their subject matter can be summarized as follows:
* Book I contains an outline of
Aristotelian cosmology, a set of
chord tables, and an introduction to
spherical trigonometry.
* Book II covers problems associated with the daily motion attributed to the heavens, namely risings and settings of celestial objects, and the length of daylight.
* Book III covers the motion of the
Sun.
* Books IV and V cover the motion of the
Moon, lunar
parallax, and the sizes and distances of the Sun and Moon relative to the
Earth.
* Book VI covers solar and lunar
eclipses.
* Books VII and VIII cover the motions of the fixed
stars, including
precession of the
equinoxes. They also contain a
star catalogue.
* Book IX addresses general issues associated with creating models for the five
naked eye planets, as well as the motion of Mercury.
* Book X covers the motions of Venus and Mars.
* Book XI covers the motions of Jupiter and Saturn.
* Book XII covers stations and
retrogradations, which occur when planets appear to pause, then briefly reverse their motion against the background of the
zodiac. Ptolemy understood these terms to apply to Mercury and Venus as well as the outer planets.
* Book XIII covers motion in latitude (the deviation of planets from the
ecliptic, the apparent path of the Sun through the stars.
Ptolemy's Cosmos
The cosmology of the
Almagest includes five main points, each of which is the subject of a chapter in Book I. What follows is a close paraphrase of Ptolemy's own words from Toomer's translation.
* The celestial realm is spherical, and moves as a sphere (i.e., by rotating).
* The earth is a sphere.
* The earth is at the center of the cosmos.
* The earth is a point with respect to the heavens.
* The earth does not move. This statement applies to both motion from place to place and rotation on an axis.
Ptolemaic Planetary Models
Ptolemy assigned the following order to the
planetary spheres, beginning with the innermost:
# Moon# Mercury# Venus# Sun# Mars# Jupiter# Saturn# Sphere of fixed stars
Other classical writers suggested different sequences.
Plato (c. 427-c. 347 BCE) made the Sun next in order after the Moon, while
Martianus Capella (5th century CE) put Mercury and Venus in motion around the Sun. Ptolemy's authority was preferred by most Islamic and late medieval European astronomers.
Ptolemy inherited from his Greek predecessors a geometrical toolbox and a partial set of models for predicting where the planets would appear in the sky.
Apollonius of Perga (c. 262-c. 190 BCE) had introduced the
deferent and epicycle and the
eccentric deferent to astronomy.
Hipparchus (2nd century BCE) had crafted mathematical models of the motion of the Sun and Moon. Hipparchus had some knowledge of
Mesopotamian astronomy, and he felt that Greek models should match those of the Babylonians in accuracy. He was unable to create accurate models for the remaining five planets.
In the
Almagest, Ptolemy adopted Hipparchus' solar model, which consisted of a simple eccentric deferent. For the Moon, he began with Hipparchus' epicycle-on-deferent, then added a device that historians of astronomy refer to as a
crank mechanism. He succeeded in creating models for the other planets, where Hipparchus had failed, by introducing a third device called the
equant.
The
Almagest was written by Ptolemy as a textbook of mathematical astronomy. It explained geometrical models of the planets based on combinations of circles, that could be used to predict the motions of celestial objects. In a later book, the
Planetary Hypotheses, Ptolemy explained how to transform his geometrical models into three-dimensional spheres or partial spheres. In contrast to the mathematical
Almagest, the
Planetary Hypotheses is sometimes described as a book of cosmology.
Ptolemy's comprehensive treatise of mathematical astronomy superseded most older texts of Greek astronomy. Some were more specialized and thus of less interest; others simply became outdated by the newer models. As a result, the older texts ceased to be copied and were gradually lost. Much of what we know about the work of astronomers like Hipparchus comes from references in the
Almagest.
|
Ptolemy's Almagest became an authoritative work for many centuries, as this 16th-century portrait of him as a Renaissance mathematician shows |
The Almagest in the Middle Ages and Renaissance
The first translations into Arabic were made in the
9th century, with two separate efforts, one sponsored by the
caliph Al-Ma'mun. By this time, the work was lost in
Europe, or only dimly remembered in
astrological lore. Consequently,
Western Europe rediscovered Ptolemy from translations of Arabic versions. In the
twelfth century a Spanish version was produced, later turned into
Latin under the patronage of Emperor
Frederick II. Another Latin version, this time directly from the Arabic, was produced by
Gerard of Cremona, who found his text in
Toledo in
Spain.
Gerard of Cremona was unable to translate many technical terms, even retaining the Arabic
Abrachir for Hipparchus.
In the
15th century, a Greek version appeared in Western Europe, and Johannes Müller, better known as
Regiomontanus, made an abridged Latin version at the instigation of the brilliant
Greek churchman
Johannes, Cardinal Bessarion. At the same time, a full
translation was made by
George of Trebizond. It included a commentary that was as long as the original. The work of translation, done under the patronage of
Pope Nicholas V was intended to supplant the old translation. The new manuscripts were a great improvement; the new commentary was not, and aroused much heated criticism. The Pope declined the dedication of the translation, and Regiomontanus' translation had the upper hand for the next century and more.
Commentaries on
Almagest were written by
Theon of Alexandria (extant),
Pappus (fragments), and
Ammonius (lost).
*Two translations of the
Almagest have been published in English. The first was included in volume 16 of the
Britannica Great Books series. A more recent translation, by
G. J. Toomer, (ISBN 069100260600), is almost universally thought to be superior.
*An older French translation (facing the Greek text), published in two volumes (1813 and 1816) by Nicolas B. Halma, is available online at the Gallica web site[
2].
* James Evans,
The History and Practic of Ancient Astronomy, Oxford University Press, 1998
* Olaf Pedersen,
A Survey of the Almagest, Odense University Press, 1974
* Olaf Pedersen,
Early Physics and Astronomy: A Historical Introduction, 2nd edition, Cambridge University Press, 1993
*
Online copy of the star catalog in the Almagest*
Almagest Planetary Model Animations*
Online luni-solar & planetary ephemeris calculator based on the Almagest
