The story of our gaining an understanding of the planetary motions now begins. It allows us to examine how science works, in slow motion (about 15 centuries worth)

The heavens themselves, the planets, and this centre
Observe degree, priority, and place,
Insisture, course, proportion, season, form,
Office, and custom, in all line of order.

-- William Shakespeare, Troilus and Cressida

Painting of Ptolemy


lived around 150 AD, and elaborated the geocentric (earth-centered) model of the solar system that prevailed until around 1500 AD. His model lasted for such a long time because it was able to match the existing observations of the positions of the planets reasonably well. (pictures from Bill Arnett, Graham Hart See, also

Well do I know that I am mortal, a creature of one day.
But if my mind follows the winding paths of the stars
Then my feet no longer rest on earth, but standing by
Zeus himself I take my fill of ambrosia, the divine dish.

- Ptolemy's epigram, from the Almagest, probably written by himself

ptolemyastrologymed.jpg (51799 bytes)

Essentials of the Ptolemaic Solar System Model

Key point: Development of a true scientific theory for the motions of the planets

    He started with a large body of observations and a small number of "simple" assumptions for planet motions that had been proposed by earlier Greek astronomers (rejecting Aristotle's crystalline spheres).

almagest.jpg (102039 bytes) His work is described in the "Almagest" - the Arabic title of the work, meaning roughly "Great Work." To fit the motions well, he had to use complex combinations of motions around circles that were moving on circles, and so forth. Working all of this out was a triumph of detailed mathematics. This illustration is from a translation of Ptolemy's Almagest from Arabic to Latin by Gerard of Cremona in 1175. The image shows the motion of the superior planets—Mars, Jupiter, and Saturn. (From Library of Congress Vatican Exhibit
In this scheme, a retrograde episode looked like this (from Journey Through Astronomy,
animation of retrograde motion with epicycles

However, his accurate calculations demonstrated that the positions of the planets could only be explained if the circles at the basis of planetary motions were offset from the earth, so the constant motion was around a point in space rather than the earth. By precise adjustments of this nature, he was able to find a system that reproduced the planetary positions relatively accurately. Although we tend to present his model as simple sketches, his real work was a huge body of accurate calculations that allowed him to adjust the model to provide an accurate fit to the observations of the positions of the planets.

In common with another great astronomer, Kepler, Ptolemy also wrote on astrologybuttonex.jpg (1228 bytes)

Was Ptolemy's Theory a Good Model by Modern Standards?

It had convincing strong points:

Thus, it was a good scientific theory for its time. With 20/20 hindsight, it had some weaknesses too:

Ptolemy's work was saved from being lost and forgotten because an Islamic center of intellectual inquiry developed in Bagdad, where scholars translated many of the Greek works into Arabic. Note that the drawing of the orbits of Mars, Jupiter, and Saturn reproduced above was translated from the Arabic, and thus was taken from the Islamic translation 1000 years after Ptolemy died. The Islamic scientists made a number of important contributions to our understanding as well as preserving older work. (from "Islam and the Arab World", Edited by Bernard Lewis American heritage Publishing Co., Inc. New York. P.200 )


islamobs.jpg (64176 bytes)
urania.jpg (8078 bytes)






Urania, Greek muse of astronomy, by Simon Vouet,

sirtflaunch.jpg (4413 bytes)

bayeuxa.jpg (32323 bytes)






Comet Halley in the Bayeux Tapestry

Click to return to syllabus

Click for Greek Astronomy

hypertext copyright.jpg (1684 bytes) G. H. Rieke

Click to go to Medieval Astronomy