The Heliocentric Theory: Nicolaus Copernicus and Galileo Galilei


By Robert M. Hazen, Ph.D., George Mason University

In the early 1500s, Nicolas Copernicus devised a theory that the planets may be revolving around the Sun, not the Earth. Much later, the great inventor and astronomer Galileo Galilei presented an argument for the validity of the Sun-centered Copernican model, as opposed to the older Earth-centered Ptolemaic model, for which he got into trouble with the church.

A painting of Nicolaus Copernicus staring up at lightening sky with stars.
Copernicus spent a lot of time observing the heavens in an observatory that he himself built. (Image: Jan Matejko/Public domain)

Copernicus and the Heliocentric Theory

The Polish astronomer Nicolaus Copernicus lived from 1473 to 1543. He was trained in theology and spent nearly half a century working for the Catholic Church. Yet, he devoted much of his life to try to construct a mathematical model of the solar system, a model in which the Sun, not the Earth, was at the center.

Copernicus studied at the University of Krakow in Poland. He built his own astronomical observatory in his spare time. He made his own records of the positions of the stars and the Moon. However, he’s not best remembered as an observational astronomer, but as a theoretician who developed this new model.

A preliminary manuscript version of his heliocentric theory, the Sun-centered theory, was circulated as early as 1514, and a few copies of that manuscript survive. Copernicus never sought personal recognition for this theory and the theory was not published until around the time of his death in 1543.

Learn more about Ptolemy’s geocentric theory.

Copernicus’s On the Revolutions of the Celestial Spheres

Copernicus’s great work is called On the Revolutions of the Celestial Spheres. In it, he proposed essentially the modern model of the solar system where the Earth and the other planets all circle the Sun, while the Moon circles around the Earth. Copernicus noted that for the stars to orbit the Earth, they’d have to be traveling at an enormous speed. Far simpler, he thought, for the Earth to rotate on its axis once every day and that each year represents one orbit of the Earth around the Sun, and that’s the modern view.

The Copernican model explained most of the retrograde motion of planets like Mars and so forth. And he said that was merely a consequence of the fact that the Earth swings in its orbit from side to side around the Sun. But the model still relied on perfect circular orbits, and so he still had to use epicycles, although much smaller than the epicycles of Ptolemy.

The Copernican model led to a greatly simplified and more accurate prediction of planetary positions, and that led to its acceptance by many scholars. That, after all, is the true measure of a theory, whether it works, whether it leads to useful predictions.

Copernicus’ predictions were later tested and refined by other astronomers. Tycho Brahe’s refined instruments allowed for more accurate observations, which were then used by the mathematician Johannes Kepler to theorize his laws of planetary motion. But the greatest supporter of the Copernican heliocentric view was Galileo Galilei.

This is a transcript from the video series The Joy of Science. Watch it now, on The Great Courses Plus.

From Copernicus to Galileo

Galileo Galilei lived from 1564 to 1642. Many people remember Galileo for his pioneering use of the telescope. He built his first telescope in 1609, first a nine-power instrument, then later a 30-power instrument, after he heard about an invention in Holland.

Galileo's sketches of the Moon.
Galileo published detailed illustrations of the surface of the Moon. (Image: Galileo Galilei/Public domain)

His first observations were published in a book called The Starry Messenger in 1610. Throughout the book, Galileo emphasizes the importance of modern observations over ancient authority.

He promises to reveal “great, unusual and remarkable spectacles opening these to the consideration of every man, and especially philosophers and astronomers”.

Galileo was the first to observe the craters in the mountains of the Moon. He saw the compound nature of Saturn, what scientists now realize are rings.

He saw the moons of Jupiter. He saw the phases of Venus. He saw 80 new stars in Orion’s belt alone. He realized that the Milky Way is a band of countless thousands of stars that can’t be seen with the unaided eye.

Galileo’s Detailed Astronomical Observations

Historian of science I. Bernard Cohen describes the power of Galileo’s work as follows:

Not only did Galileo describe the appearance of mountains on the Moon, but he also measured them. It is characteristic of Galileo as a scientist of the modern school that as soon as he found any kind of phenomenon, he wanted to measure it. It is all very well to be told that the telescope discloses that there are mountains on the Moon, just as there are mountains on Earth. But how much more extraordinary it is, and how much more convincing to be told that there are mountains on the Moon and that they are exactly four miles high.

Galileo’s publication of these discoveries and his bold support of the controversial Copernican system ultimately led to his famous and frequently oversimplified heresy trial, which took place in 1633.

Learn more about Galileo’s contributions to celestial and terrestrial mechanics.

Cause for Galileo’s Trial for Heresy

Galileo had earlier promised church officials that he would not advocate the Copernican system, at least not publicly. But in a 1632 book, which was published in Italian, he supposedly presents an even-handed account of both the Earth-centered and the Sun-centered view.

The book adopts the literary form of a debate, a conversation among observers of Copernican and Ptolemaic leanings, with an educated layperson asking questions of these people. Galileo puts the arguments of the Ptolemaic viewpoint in the mouth of a narrow-minded Aristotelian and someone who bears resemblance to Pope Urban VIII, who was the Pope at the time.

Galileo’s readers knew exactly what his point of view was, and the Pope took great offense, as is not a big surprise. So, Galileo was convicted of heresy. He was forced to recant the Copernican view, and he was placed under house arrest for the rest of his life until his death in 1642.

Common Questions about Nicolaus Copernicus, Galileo Galilei, and the Heliocentric Theory

Q. Whose work further refined the Copernican model?

The scientific instruments developed by Tycho Brahe, and his accurate observations of the movement of planets refined the Copernican model. The work of Johannes Kepler further developed this.

Q. What instrument did Galileo refine?

People remember Galileo for his pioneering use of the telescope. He built his first telescope in 1609, first a nine-power instrument, then later a 30-power instrument.

Q. Why did the Catholic Church try Galileo for heresy?

Galileo had promised church officials that he would not advocate the Copernican system, at least not publicly. But in a 1632 book, which was published in Italian, he presents an account of both the Earth-centered and the Sun-centered view. The church then tried him for heresy.

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