Ptolemy
Name: Claudius Ptolemy
Nickname/Alias: Claudius Ptolemaeus,
Achievements: His collected works (the “Almagest”) influenced astronomical and religious conceptions for at least thirteen centuries after his death.
Born: c. 90 CE; Egypt.
Died: c. 168 CE; Alexandria, Egypt (Aged about 78)
Field & Discipline: Science; Astronomy; Geography; Astronomer; Geographer
Language: Latin; Greek
Education: Unknown
Major Work:
“Almagest” (150 CE)
Life & Times:
Ptolemy lived and worked mainly in Alexandria, Egypt, where he set up an observatory on the top floor of a temple and made astronomical observations from around 125 to 150. It is likely that he worked at the Alexandria Library and Museion, the research institution where the famous Greek geometers, Apollonius of Perga and [work related to] Euclid* had studied and taught. One of its directors, Eratosthenes measured the size of the Earth correct to within 50 miles.
At the library, Ptolemy would have had available the entire accumulated wealth of information compiled by Greek scholars over the previous four centuries. It can be assumed that the research he completed in the library, together with his own observations, provided the basis for his great work, the “Almagest,” the first attempt to provide a systematic treatment of the whole of mathematical astronomy.
It became a brilliant tool for instruction on many topics and encompassed everything from basic principles to complex mathematical explanations. In the work Ptolemy described the motion of the Sun, Moon and planets and their related phenomena, and provided a mathematical model. He also offered a guide to the use of these tables, using numerical parameters from observational data. When appropriate he also described the construction of the observational instruments used.
Ptolemy researched works by other astronomers and scholars like Hipparchus and Aristarchos in order to create the Ptolemaic system: a geocentric scheme for the universe. Some of the scholars he studied had already proposed a heliocentric solar system but Ptolemy wrongly followed the views of [followed views of] Aristotle*.
The “Almagest” was published in 150 CE, under its Greek title of “He Mathematike Syntaxis” (“The Mathematical Collection”) but it soon become known as “The Great Compilation.”
Years later, in the 9th century, Islamic scholars like [translated by] Al-Farghani* started translating Greek works into Arabic and “The Great Compilation” became “al-majisti” (“The Greatest”), or (in the West: “Almagest”). From Arabic the work was translated into Latin, Ptolemy’s ideas soon reached Western Europe.
The “Almagest” is made up of thirteen different books dealing with various astronomical concepts. In the first book, he reasoned that if Earth’s gravity drew everything towards its centre, then this was also true for the celestial bodies around the Earth.
This brought him to support Aristotle’s theory that Earth was at the centre of the cosmos with everything orbiting it. The Ptolemaic system assumed that all orbits were circular but not necessarily with Earth at the centre of them. Ptolemy worked on the basis that the orbits of Mercury, Venus and Mars were epicyclical. This means that a planet was orbiting a point which, itself, was orbiting the Earth.
In the second book Ptolemy noted that the planets were much closer to the Earth than the "fixed" stars, and he seems to have believed in the physical existence of eight nested
crystalline rotating spheres.
Seven of these each carried one heavenly bodies. The fixed stars were carried on the eighth. The spheres were moved by the “prime mover.” This system became dogma in Western Christendom until the 16th century, when it was replaced by the heliocentric system of the Polish astronomer [theory superseded by] Copernicus*.
In the third book, Ptolemy observed solstices and equinoxes which meant that he could predict the length of the seasons and design a model for the Sun, showing it moving in a circular motion of uniform velocity. He found that the Earth was not in fact in the centre of the circle, but at a distance to one side, called the “eccentricity.”
Books four and five contained his theory of his theory of the Moon. Ptolemy explained three periods of lunar motion (the times taken for it to return to the same longitude, velocity and latitude).
He explained Hipparchus’ epicycle model for the Moon, but noted that there were discrepancies between the model and what could actually be seen.
Now that he had explained the motion of the Sun and the Moon, Ptolemy went on to apply these motions to create a theory of eclipses, which he described in book six. Books seven and eight concentrated mainly on the fixed stars which, Ptolemy believed, always kept the same positions relative to each other. Here, Ptolemy included a catalogue of over a thousand stars.
Up to this point, Ptolemy based his work on ideas researched from his predecessors, but in the last five books of the “Almagest,” he discussed his own planetary theory. This involved a highly complicated model which explained observational data and which had not been available before his time.
This was a geocentric system in which the Earth was at the centre of the universe with the planets moving in a circle around it. He produced a graphic representation in which large circles (called “deferents”) represented the planets. Each deferent traveled round the circumference of a smaller circle, called an “epicycle,” (whose centre revolved on the deferent).
According to Ptolemy, this model provided the basis for mathematical computations that would explain all the strange things astronomers observed, such as the way the brightness of the planets waxed and waned, and the way in which they appeared to stop moving, or go backwards and forwards in a loop (like Mars).
The speed of the planetary movements also seemed to vary and Ptolemy dealt with this anomaly by creating an imaginary point in the sky called the “equant” from where the speed could be seen to be steady and circular.
Most of the tables that Ptolemy scattered throughout the “Almagest” were published under the title “Handy Tables,” which he updated when new data became available. He also wrote “Planetary Hypothesis,” which replaced his geometrical theories with mechanical ones, aimed at readers who could not grasp mathematical computations.
Another of Ptolemy's major works, the “Guide to Geography,” mapped out the known world, gave the latitudinal and longitudinal coordinates of major locations and showed how to construct maps using projective geometry which would eventually become the basis for the Renaissance use of perspective .
Even as late as 1775, everybody also believed Ptolemy’s early claim that a southern continent bounded the Indian Ocean, until [influenced] Cook* returned from the Southern Hemisphere having failed to find an Antarctic continent.
“Optica” was Ptolemy’s next work, in which he discussed color, reflection, refraction and mirrors of various shapes. This was probably based on the refraction of light from celestial bodies he observed at different altitudes. This led him to discuss the change in direction of light when it passes obliquely from a medium of one density into another. It was the first recorded attempt at a solution to this observational problem and was later taken up and explained by the great 11th-century Arabic mathematician, [influenced] Al Hazen*.
Extra Connections:
His work related to that of [work related to] Maestlin*, [work related to] Apian*, [work related to] Gerard of Cremona*, [work related to] Peuerbach*, [work related to] Al Battani*, [ideas questioned by] ben Gershom*, [work related to] Behaim*, [work related to] Plato of Tivoli*, [work related to] Alberti*, [work related to] Kepler*, [work related to] Galileo*, [work related to] Brahe*, [work related to] Roger Bacon*, [work related to] Bessarion* and [work related to] Plethon* [work related to] Witelo*, [work related to] Rheinhold*, [work related to] Waldsemuller*, [work related to] Halley*, [work related to] Vespucci* and [work related to] Snell*,
Assessment: “The Almagest” was not superseded until a century after Copernicus published his heliocentric theory in 1543.
Oneliner: Ptolemy, Greek astronomer who produced a system for the cosmos
Bibliography:
- Dreyer, J. L. E., A History of Astronomy from Thales to Kepler, New York: Dover Publications Inc., 1953.
- Grasshoff, G., The History of Ptolemy's Star Catalogue, New York, 1990.
- Ronan, Colin A., Discovering the Universe, London: Heinemann, 1972.
- Walker, Christopher, Astronomy Before the Telescope, London: British Museum Press, 1996.
Multimedia:
James Burke Connections #2 - Death in the Morning
"Death in the Morning" examines the standardization of precious metal with the touchstone in the ancient world. This innovation stimulated trade from Greece to Persia, ultimately causing the construction of a huge commercial center and library at Alexandria which included Ptolemy's star tables. This wealth of astronomical knowledge aided navigators 14 centuries later after the development of lateen sails and sternpost rudders.
Father: Unknown
Mother: Unknown
Siblings: Unknown
Spouse: Unknown
Children: Unknown