If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

READ: Claudius Ptolemy

An Earth-Centered View of the Universe

Born: 85 CE; Hermiou, Egypt. Died: 165 CE; Alexandria, Egypt. Portrait of Ptolemy by Andre Thevet © Bettmann/CORBIS
By Cynthia Stokes Brown
The Earth was the center of the Universe according to Claudius Ptolemy, whose view of the cosmos persisted for 1400 years until it was overturned — with controversy — by findings from Copernicus, Galileo, and Newton.

An Astronomer in Ancient Times

Claudius Ptolemy (about 85–165 CE) lived in Alexandria, Egypt, a city established by Alexander the Great some 400 years before Ptolemy’s birth. Under its Greek rulers, Alexandria cultivated a famous library that attracted many scholars from Greece, and its school for astronomers received generous patronage. After the Romans conquered Egypt in 30 BCE (when Octavian defeated Cleopatra), Alexandria became the second-largest city in the Roman Empire and a major source of Rome’s grain, but less funding was provided for scientific study of the stars. Ptolemy was the only great astronomer of Roman Alexandria.
Ptolemy was also a mathematician, geographer, and astrologer. Befitting his diverse intellectual pursuits, he had a motley cultural makeup: he lived in Egypt, wrote in Greek, and bore a Roman first name, Claudius, indicating he was a Roman citizen — probably a gift from the Roman emperor to one of Ptolemy’s ancestors.

A Geocentric View

Ptolemy synthesized Greek knowledge of the known Universe. His work enabled astronomers to make accurate predictions of planetary positions and solar and lunar eclipses, promoting acceptance of his view of the cosmos in the Byzantine and Islamic worlds and throughout Europe for more than 1400 years.
Ptolemy accepted Aristotle’s idea that the Sun and the planets revolve around a spherical Earth, a geocentric view. Ptolemy developed this idea through observation and in mathematical detail. In doing so, he rejected the hypothesis of Aristarchus of Samos, who came to Alexandria about 350 years before Ptolemy was born. Aristarchus had made the claim that the Earth revolves around the Sun, but he couldn’t produce any evidence to back it up.
Map of the Universe according to Ptolemy, from a 17th century Dutch atlas by Gerard Valck © Bettmann/CORBIS
Based on observations he made with his naked eye, Ptolemy saw the Universe as a set of nested, transparent spheres, with Earth in the center. He posited that the Moon, Mercury, Venus, and the Sun all revolved around Earth. Beyond the Sun, he thought, sat Mars, Jupiter and Saturn, the only other planets known at the time (as they were visible to the naked eye). Beyond Saturn lay a final sphere — with all the stars fixed to it — that revolved around the other spheres.
This idea of the Universe did not fit exactly with all of Ptolemy’s observations. He was aware that the size, motion, and brightness of the planets varied. So he incorporated Hipparchus’s notion of epicycles, put forth a few centuries earlier, to work out his calculations. Epicycles were small circular orbits around imaginary centers on which the planets were said to move while making a revolution around the Earth. By using Ptolemy’s tables, astronomers could accurately predict eclipses and the positions of planets. Because real visible events in the sky seemed to confirm the truth of Ptolemy’s views, his ideas were accepted for centuries until the Polish astronomer, Copernicus, proposed in 1543 that the Sun, rather than the Earth, belonged in the center.
After the Roman Empire dissolved, Muslim Arabs conquered Egypt in 641 CE. Muslim scholars mostly accepted Ptolemy’s astronomy. They referred to him as Batlamyus and called his book on astronomy al-Magisti, or “The Greatest.” Islamic astronomers corrected some of Ptolemy’s errors and made other advances, but they did not make the leap to a heliocentric (Sun-centered) universe.
Ptolemy’s book was translated into Latin in the 12th century and known as The Almagest, from the Arabic name. This enabled his teachings to be spread throughout Western Europe.
We know few details of Ptolemy’s life. But he left one personal poem, inserted right after the table of contents in The Almagest:
Well do I know that I am mortal, a creature of one day.
But if my mind follows the wandering path of stars
Then my feet no longer rest on earth, but standing by
Zeus himself, I take my fill of ambrosia, the food of the gods.

For Further Discussion

Think about the following question and write your response and any additional questions you have in the Questions Area below.
Even though Ptolemy’s system was wrong, people believed in it. Why?

Want to join the conversation?

  • female robot grace style avatar for user 00015651
    Why couldn't Aristarchus find evidence for his heliocentric theory, he made a claim?
    (30 votes)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user Eric Waldstein
      Aristarchus believed the stars to be very far away, and that in consequence there was no observable parallax. He used this to reasoning to help substantiate his view. While this speculation was accurate, stellar parallax is only detectable with telescopes. As such, his speculation was not able to be proved at the time.
      (32 votes)
  • spunky sam blue style avatar for user brivera21
    What made Ptolemy important?
    (8 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Mildred Mobus
    I think science is really hard to understand, and if your only experience in life is where you live, then it is the center of YOUR universe, and maybe that means they all thought it was actually the center. Also, we tend to believe authorities in different fields when they give out theories...for something like this, a lot of people probably didn't have a whole lot of knowledge so when a scholar told them the Earth was the center it was probably easy to believe and go along with.
    (13 votes)
    Default Khan Academy avatar avatar for user
  • aqualine tree style avatar for user Jurgen Gjonçari
    How is it possible that they (scientists) were able to see planets with naked eye and study their movement? How did they know that what they were seeing was not a star?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Catherine C
    Meant to add more specifically, how many people were educated to be critical thinkers, and how many were encouraged to think differently out loud? Given history is rewritten by conquerors, based on what I have read so far about history in those times, I think not many people dared speak their thought(s) openly.

    Then there are those people who, apparently, are gifted with an ability to persuade and convince if even if mostly through persistence and repetition. Who and what is heard most and how often?

    None of my comments intend to diminish Ptolemy's works. I lack education to do such a thing :.) However, again given history, it seems that quite often a majority of people are moved to believe and think what a handful of more authoritative figures say. Individual autonomy becomes suppressed by a simple cycle of accepting, believing, then perpetuating someone else's ideas above their own.

    I think it is important to question everything with respect, including those who put forth educationally elevated theories, be those simple or complex. Exploring plausible possibilities would seem to lead to more discoveries and perhaps improving investigative methods.
    (4 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user Izzy
    Lay people then, and now, don't necessarily have the ability to test certain theories out. It sounds like Ptolemy belonged to a long line of already respected people. This probably made other political and cultural elites more accessible to him, giving him the chance to convince and influence them of his theories. He backed his ideas with reproducible observations which common sense at the time agreed with. The deck was stacked in Ptolemy's favor, so to speak.
    (4 votes)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Catherine C
    How many people were educated then? And of those, how many people were educated as individuals?

    Though we are taught to respect education, how many people are sufficiently educated to appreciate what is being told, being that requires an equal level of education to fully comprehend?

    It seems many people are often awed by what they do not know, and I think perhaps that is more correct and appreciable than claiming to 'know'; opposed to humbly putting forth theories that might be advanced - or not - with more open-minded willingness to explore possibilities.
    (2 votes)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Catherine C
    Not that Wikipedia is boss, it states about Philolaus: "He is also credited with originating the theory that the Earth was not the center of the Universe."

    Discovered this while learing about Aristarchus, who Wikipedia states was influenced by Philolaus. Though Aristarchus seems to have originated heliocentric thinking, seems only fair to add Philolaus's influence.
    (2 votes)
    Default Khan Academy avatar avatar for user
  • leafers sapling style avatar for user Avinasen [bio pls] 🔛
    Because this model is based on the point of view of the naked eye, lots of people accepted it because that was what they saw too. It's a bit like looking at the physical features of a person and assuming you know all about him and his relationships.

    Even though there were minor (and major) flaws in the geocentric model, people either ignored it or changed the model itself a little without changing the major positions.

    It was only until when there were advanced scietific models (which is a long time after his claim), his theory was defeated and replaced with the heliocentric model.

    This comes to show that you have to understand more to mispercept less.
    (2 votes)
    Default Khan Academy avatar avatar for user
  • leaf red style avatar for user Aslan Feb
    Because at the time it looked right, and It made us feel good to know that we were the center of the universe.
    (2 votes)
    Default Khan Academy avatar avatar for user