Early Astronomers: Ptolemy, Aristotle, Copernicus, and Galileo

You know, because you've been told, that the Earth revolves around the Sun. You also probably know that planets other than our own have moons, and the way to test to see whether or not something is true is by experimenting. Thousands of years ago, these things were not widely known. The heavens above were anyone's guess, and the way things were was just the way the gods had made them. It was felt there was no need to truly understand them or put them in any kind of order.

The Greek scholars changed much of that. They were famous for their schools of higher knowledge, which were rather different than ours. Students would gather around a teacher, perhaps in a beautiful grove, and ask questions and discuss among themselves what might be the answers and the best ways to figure out those answers. Many of today's colleges still aspire to this way of learning.


Ptolemy was an astronomer and mathematician. He believed that the Earth was the center of the Universe. The word for earth in Greek is geo, so we call this idea a "geocentric" theory. Even starting with this incorrect theory, he was able to combine what he saw of the stars' movements with mathematics, especially geometry, to predict the movements of the planets. His famous work was called the Almagesti. In order to make his predictions true, he worked out that the planets must move in epicycles, smaller circles, and the Earth itself moved along an equant. None of this was true, but it made the math work for his predictions. This flawed view of the Universe was accepted for many centuries.


He is sometimes called the grandfather of science. He studied under the great philosopher Plato and later started his own school, the Lyceum at Athens. He, too, believed in a geocentric Universe and that the planets and stars were perfect  spheres though Earth itself was not. He further thought that the movements of the planets and stars must be circular since they were perfect and if the motions were circular, then they could go on forever. Today, we know that none of this is the case, but Aristotle was so respected that these wrong answers were taught for a very long time. Aristotle, outside of astronomy, was a champion observer. He was one of the first to study plants, animals, and people in a scientific way, and he did believe in experimenting whenever possible and developed logical ways of thinking. This is a critical legacy for all the scientists who followed after him.


Well over a thousand years later, Nicolaus Copernicus came up with a radical way of looking at the Universe. His heliocentric system put the Sun (helio) at the center of our system. He was not the first to have this theory. Earlier starwatchers had believed the same, but it was Copernicus who brought it to the world of the Renaissance and used his  own observations of the movements of the planets to back up his idea. His ideas, including the revelation that the Earth rotates on its axis, were too different for most of the scholars of his time to accept. They used only parts of his theory. Those who did study his work intact often did so in secret. They were called Copernicans.


Born in Pisa, Italy approximately 100 years after Copernicus, Galileo became a brilliant student with an amazing genius for invention and observation. He had his own ideas on how motion really worked, as opposed to what Aristotle had taught, and devised a telescope that could enlarge objects up to 20 times. He was able to use this telescope to prove the truth of the Copernican system of heliocentrism. He published his observations which went against the established teaching of the Church. He was brought to trial and, although he made a confession of wrong-doing, he was still kept  under house arrest for the rest of his life. But it was too late to lock away the knowledge that Galileo shared.Other scientists, including Sir Isaac Newton and Johannes Kepler, seized its importance and were able to learn even more about the ways of the world and the heavens beyond.

These early scientists' legacy continues to this day. As time goes on, we use our instruments, science, math, reasoning, and creativity to learn more about the secrets of the Universe. In this way, we are directly linked to the astronomers of centuries ago who gave us direction to discover more about the dances of the planets and the nature of the stars.


In the Library and on the Web

Discovery of the Solar System
This site from NASA follows the development of ideas about the solar system from Ptolemy to Copernicus to Galileo.

Makers of Science
Volume one of this reference set has information on Aristotle, Copernicus, and Galileo. 

Recentering the Universe
An 88-page round-up of how these early scientists' work changed everything about people understood their world.

Articles on Ptolemy, Copernicus, Galileo, and Aristotle can be found in our online databases: Biography in Context and Encyclopaedia Britannica (English, Spanish, and children's versions).


Aristotle Leads the Way
Captures the life and times of Aristotle, the ancient Greek philosopher who studied a wide range of subjects and helped shape early scientific beliefs. In our young adult collection.

Contents: Among the trees of Athens -- Life in ancient Athens -- Life in the king's court -- Investigating the natural world -- Return to Greece -- Thinking about thinking -- On Earth and the heavens above -- On right and wrong -- Impact of Aristotle -- Life and times -- Life at a glance.

The Great Thinker
Stresses the importance of Aristotle's intellectual discoveries on the future of science.

Biography for Kids: Aristotle
A short online biography of Aristotle good for upper elementary grades that touches on some important points.


An enjoyable 128-page biography of the 16th-century Polish scientist that includes activities on retrograde motion, the solar system, parallax, and an astrolabe.

Nicolaus Copernicus
The center of the universe -- A childhood in Poland -- A long-term student -- In the service of the church -- Look to the stars -- The Copernican theory -- Late in life -- A scientific revolution -- Late in life -- Life at a glance.

The History News in Space
Uses a newspaper format to take a look at developments that led from the ideas of Copernicus and other early scientists to the technological advances that enabled man to venture to the moon and beyond.

Nicolaus Copernicus: Making the Earth a Planet
This eBook from Oxford University Press is targeted to high school students and above. Click here for information on accessing our eBook collection.


Galileo: Astronomer and Physicist
A biography of seventeenth-century Italian astronomer and physicist Galileo that includes related activities for readers.

Galileo for Kids
This biography has experiments and activities as well as his life story.

Starry Messenger
A beautiful retelling, in picture book format, of Galileo's story which can be used with both younger and older audiences.

The Galileo Project
A hypertext source of information on the life and work of Galileo Galilei (1564-1642) and the science of his time. Includes a biography, letters from his daughter, and a tour of his home. From Rice University.

Galileo's Battle for the Heavens
A timeline of his life, articles on his place in science and his telescope, his mistakes on predicting the tides, a teachers' guide, and interactives on his experiments with falling objects, projectiles, inclined planes, and pendulums.


Ptolemy: Roman Scholar Claudius Ptolemaeus
A bit on Ptolemy's life and more about his accomplishments.

Ptolemy, the Man
An overview of Ptolemy's achievements and links to biographical information.

Ptolemy's World
Lots of illustrations of Ptolemy's geography, a note on his principle of simplicity, and links to his texts.