HOME - Janux - D2L - Email Wikipedia course web page Vimeo course videos Course materials at iTunes U (optional) Twitter: #ouhoscurator
History of Science Roman - Pantheon

History of Science Online

- Course Info - Time tips - Semester at a glance - Weekly assignments at a glance - Timeline -

LibraryThing: Science in Ancient Mesopotamia Week 6: Roman Science

Ptolemy, Almagest (ca. 150 A.D.)

| Source page | Hero | Cicero | Lucretius | Pliny the Elder | Ptolemy | Galen | Capella | Basil and Philoponos | Augustine | Boethius | Isidore of Seville | Benedict |


Ptolemy and Galen each wrote in Greek, and worked in the 2nd century AD during the period of the Roman empire. Take your time with them; they represent, respectively, the culmination of ancient mathematical astronomy and ancient anatomy and medicine.

  1. Review the discussion of Ptolemy's astronomy in Lindberg, Beginnings of Western Science, Chapter 5, pp. 98-105.
  2. Read Michael Crowe, "The Equant," p. 38.
  3. Watch this short video on Geometrical models.
  4. Optional: photos of Ptolemy's Almagest, first printed edition by Regiomontanus (1496).
  5. Explore these animated models of various geometrical devices to ensure that you understand them: | Equant system for outer planet on epicycle; Earth eccentric; with Sun | Craig Sean McConnell (scroll down to equant animation) | Dennis Duke, click on Ptolemy's Moon |
  6. Read Crowe, pp. 42-49, the introductory section of his chapter on Ptolemy's astronomy. Don't worry about all the geometrical details; read for the gist of Crowe's points about Ptolemy. These are the true-false study questions you will answer in this section:
    1. Ptolemy, author of the greatest book of ancient mathematical astronomy, was a king of Egypt.
    2. Ptolemy wrote the Almagest in Arabic.
    3. What Euclid's Elements was to geometry, Ptolemy's Almagest was to astronomy.
    4. Ptolemy calculated the distances to the planets in the Almagest.
    5. In addition to the Almagest, Ptolemy wrote mathematical treatises on optics, music theory, and geography.
    6. Ptolemy wrote a manual of astrology called the Tetrabiblos.
    7. In Ptolemy's astronomical system, every planet was integrated into one single model, rather than treated independently.
    8. Ptolemy employed eccentric circles and epicycles in his astronomical models.
    9. Ptolemy's lunar models accurately predicted the position of the Moon but not its apparent diameter (angular width).
    10. In an equant model, motion is uniform with respect to the center of a circle.
    11. In an equant model, motion is uniform with respect to an observer on the eccentric Earth.
    12. In Ptolemy's models, the motion of the outer planets was related to the motion of the Sun, because the radius of each planetary epicycle was set parallel to the line from the Earth to the Sun.
    13. In Ptolemy's models, the motion of the inner planets (Venus and Mercury) was related to the motion of the Sun, because the center of each planetary epicycle was set on the same line (collinear) as the line from the Earth to the Sun.
  7. Read the excerpts from the Preface and Book I of Ptolemy's Almagest found in Crowe, pp. 50-65. These are the true-false study questions you will answer in this section:
    1. Ptolemy believed that the mathematical astronomer teaches beautiful theories.
    2. Ptolemy classified theoretical sciences into theology, physics, and mathematics, where mathematics falls in between the other two.
    3. Ptolemy argued that astronomy, physics and theology each could attain certain knowledge in their own subject areas.
    4. Ptolemy argued that astronomy could help both theology and physics in achieving knowledge.
    5. Ptolemy argued that astronomy inculcates a love of the eternal and unchanging, and thereby a love of divine beauty.
    6. Ptolemy argued that the fixed stars that daily rise and set move like a rotating sphere.
    7. Ptolemy argued that the rising and setting of fixed stars provides evidence that the Earth is spherical.
    8. Ptolemy argued that the Earth lies at the center of the universe because the celestial equator, the plane of the horizon, and other great circles bisect the heavens into equal hemispheres.
    9. Ptolemy argued that the universe is relatively small compared to the size of the Earth.
    10. Ptolemy argued that earth falls downward from all sides toward the center of the universe.
  8. Would you have used the equant, or not?
    The equant was a very powerful device, by which Ptolemy was able to achieve an unparalleled degree of accuracy in quantitative predictions of the positions of the planets. However, this accuracy came at a price, because there was a notable problem with the equant that bothered many astronomers. In a rotating sphere, the axis of rotation runs through the center of the sphere. Yet Ptolemy’s equant requires a sphere to rotate uniformly around a point not on its axis, which is mechanically impossible for a rigid sphere. Review the definitions of an equant model provided by Lindberg and Crowe; do you see why this is the case? What ramifications might this have held for the concept of solid celestial spheres?


"I know that I am mortal and living but a day.
But when I search for the numerous turning spirals of the stars,
I no longer have my feet on the Earth,
But am beside Zeus himself,
filling myself with god-nurturing ambrosia."
Anonymous epigraph often attributed to Ptolemy

University of Oklahoma logo

HSCI 3013. History of Science to 17th centuryCreative Commons license
Kerry Magruder, Instructor, 2004
Brent Purkaple, TA

Report typos or broken links

Go to this course at

spellcheck.net | wordcounter.net

Many thanks to the pedagogical model developed in Mythology and Folklore and other online courses by Laura Gibbs, which have been an inspiration for this course.

Academic Calendar

College of Arts and Sciences Online



This course is currently undergoing major reconstruction to bring it into alignment with the new version of the course at Janux