Ptolemy

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Astronomer
Mathematician
Geographer

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How did Ptolemy explain retrograde motion?

Medieval illustration of Claudius Ptolemaeus, known as Ptolemy, wearing a crown and holding an armillary sphere, with a scroll bearing his name.

Diagram illustrating Ptolemy's equant model, showing a stationary Earth at the center with a planet moving in a small circle called an epicycle, which itself moves along a larger circle, the deferent. The equant point is marked off-center from Earth, and the positions of apogee and perigee are labeled. Small stars are scattered in the background.

Open illuminated manuscript showing a woodcut world map from Claudius Ptolemy's Cosmographia, with landmasses outlined in color and surrounded by blue seas.

Open book showing a two-page woodcut world map from Claudius Ptolemy's Cosmographia, 1482, with blue oceans, labeled continents, and windhead figures around the border.

frontispiece to Galileo's Dialogue Concerning the Two Chief World Systems, Ptolemaic & Copernican

Map of the Greco-Roman world highlighting the birthplaces of notable ancient mathematicians, marked with the pi symbol and labeled with their names. Cities such as Athens, Alexandria, Syracuse, and Miletus are shown around the Mediterranean, with modern country boundaries outlined. An inset provides detail for the region of Greece and western Anatolia.

Ptolemy's map of the world, as printed at Ulm, Ger., 1482.

How we understood the universe throughout history

At a Glance

Ptolemy summary

Quizzes

View of the Andromeda Galaxy (Messier 31, M31).

Astronomy and Space Quiz

Michael Faraday (L) English physicist and chemist (electromagnetism) and John Frederic Daniell (R) British chemist and meteorologist who invented the Daniell cell.

Faces of Science

Numbers and Mathematics

Nicolaus Copernicus. Nicolas Copernicus (1473-1543) Polish astronomer. In 1543 he published, forward proof of a Heliocentric (sun centered) universe. Coloured stipple engraving published London 1802. De revolutionibus orbium coelestium libri vi.

All About Astronomy

Scientists and Their Inventions Quiz

Related Questions

About how many places did Ptolemy record in his Guide to Geography?
How did Claudius Ptolemy contribute to mathematics?
How did Claudius Ptolemy contribute to the development of geography?
How did Claudius Ptolemy contribute to the study of optics?
How did Copernicus challenge the Ptolemaic system?
How did Islamic astronomers preserve and use the Almagest?
How did Kepler’s laws replace Ptolemy’s circular planetary motions?
How did Ptolemy explain the apparently irregular movements of the planets?
How did Ptolemy explain the daily rising and setting of the Sun, Moon, and stars?
How did Ptolemy use eccentric circles to explain the Sun’s changing speed in the zodiac?
How did Ptolemy use his model to predict solar and lunar eclipses?
How did Ptolemy use mathematics to predict the positions of the Sun, Moon, and planets?
How did Ptolemy’s lunar theory improve on earlier Greek astronomy?
How did the Ptolemaic system explain retrograde motion of the planets?
How large did Ptolemy estimate the universe to be?
How many books are included in the Almagest?
How many stars were listed in Ptolemy’s star catalog?
What did Claudius Ptolemy write about musical theory?
What did Ptolemy argue about mirrors and reflected images?
What did Ptolemy’s Tetrabiblos teach about astrology?
What did Tycho Brahe question about Ptolemy’s astronomical observations?
What evidence did Ptolemy use to argue that Earth is spherical?
What information did Ptolemy’s Handy Tables provide to astronomers?
What is Ptolemy’s Guide to Geography about?
What is a deferent in the Ptolemaic system?
What kind of trigonometry appears in the Almagest?
What regions were included in Ptolemy’s map of the known world?
What role did the equant point play in Ptolemy’s planetary model?
What was the original Greek title of the Almagest?
What were Claudius Ptolemy’s most important achievements in astronomy?
When was Ptolemy’s Almagest completed?
Which earlier geographer’s writings and maps did Ptolemy use for his geographic work?
Which fields of study did Claudius Ptolemy contribute to?
Why did Ptolemy believe the planets were attached to nested spheres?
Why did Ptolemy place Earth at the center of the universe?
Why have historians questioned some of Ptolemy’s reported astronomical observations?
Why is Ptolemy’s astronomical book called the Almagest?
What is Ptolemy best known for?
How did Ptolemy impact the world?
What were Ptolemy’s achievements?
Why is Hippocrates famous?
What is Hippocrates remembered for?
What was Carl Friedrich Gauss’s childhood like?
What awards did Carl Friedrich Gauss win?
How was Carl Friedrich Gauss influential?
What accomplishments was Archimedes known for?
What specific works did Archimedes create?

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External Websites

Planet Facts - Claudius Ptolemy
World History Encyclopedia - Claudius Ptolemy
BBC Sounds - In Our Time - Ptolemy and Ancient Astronomy
Maricopa Open Digital Press - MCC AST - Hipparchus and Ptolemy
Famous Scientists - Claudius Ptolemy
Molecular Expressions - Claudius Ptolemy
Perseus Digital Library - A Dictionary of Greek and Roman biography and mythology - Ptolemaeus, Clau'dius

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Articles from Britannica Encyclopedias for elementary and high school students.

Ptolemy - Children's Encyclopedia (Ages 8-11)
Ptolemy - Student Encyclopedia (Ages 11 and up)

Ptolemy

Egyptian astronomer, mathematician, and geographer

Also known as: Claudius Ptolemaeus, Ptolemy of Alexandria

Written by

Alexander Raymond Jones

Professor of Classics, University of Toronto. Author of Pappus of Alexandria.

Alexander Raymond Jones

Fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

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Last updated

Oct. 23, 2026 •History

Ptolemy (born c. 100 ce—died c. 170 ce) was an Egyptian astronomer, mathematician, and geographer of Greek descent who flourished in Alexandria during the 2nd century ce. In several fields his writings represent the culminating achievement of Greco-Roman science, particularly his geocentric (Earth-centred) model of the universe now known as the Ptolemaic system. Virtually nothing is known about Ptolemy’s life except what can be inferred from his writings. His first major astronomical work, the Almagest,...

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