The Heliocentric Theory: Copernicus, Galileo, Kepler, Newton

by Tom Irvine, February 17, 2006

The conclusion that the "Earth circles the Sun," was reached and publicized by Copernicus, Galileo, Kepler, Newton, and Halley. This is the "heliocentric theory." To appreciate the work of these men, one must also consider the role of ancient Greek philosophers and the Roman Catholic Church.


Aristotle (384 - 322 BC), a Greek philosopher, taught that the Earth was the center of the Universe. He thus taught that the Sun, the Moon, and the planets thus orbited the Earth. This is the "geocentric theory." Aristotle set forth this theory in a book called, "On the Heavens."

On the other hand, another Greek named Aristarchus of Samos in the 3rd century BC placed the Earth and other planets in motion around the central Sun. This idea, however, was rejected by the people in favor of Aristotle's geocentric theory.

Aristarchus of Samos
Aristarchus (310 - 230 BC) was both a mathematician and astronomer. He the first to propose a sun-centered universe. Archimedes (287 - 212 BC) critized this theory in The Sand-Reckoner.

Claudius Ptolemy (85-165 AD) was a later philosopher who lived in Alexandria.

He wrote that the Earth was motionless because constant gales would sweep across it if it were in motion. Ptolemy also devised a complex system of "epicycles" to account for the apparent retrograde motion of the planets. Retrograde means backwards. Consider two objects rotating about a common point with the inner object traveling at a faster angular speed than the outer object. As the inner objects passes by the outer object, the outer object will appear to move backwards. This effect is an optical illusion. In Ptolemy's epicycle system, each planet revolved around the earth in a large circle by making a series of smaller circles, to account for this effect.

Ptolemy wrote these theories in book called "Almagest," around 140 AD.

Thomas Aquinas

The theories of Aristotle and Ptolemy were adopted by the Roman Catholic Church, which played an important role in presenting these theories to the Europeans.
Thomas Aquinas (1225-1274), a priest and philosopher, used the writings of these men to show that the heavens were "God-ordained and man-centered." The Church leaders felt that they had the duty of teaching the people about the universe, which declared the glory of God.

Nicholas Copernicus

Nicholas Copernicus (1473 - 1543) studied to become a priest in the University of Bologna in 1497. His religious training included astronomy, since one of his duties would be to warn the people about the future by what happened in the heavens.

Domenico Maria Novara was an astronomy professor who taught Copernicus. Domenico and Copernicus spent many nights studying the heavens together. They freely discussed ways for improving and simplifying Ptolemy's system of planetary motion.

Copernicus set out to improve the system devised by Ptolemy. He realized that the rising and setting of the Sun, Moon, and stars could be accounted for by a daily revolution of the Earth. Also, he found that if he put the Sun at the center of the planet's orbits he could simplify the number of epicycles from 80 in Ptolemy's system to a mere 34.

Although epicycles do not exist, Copernicus' intuition was correct. His idea that the Earth and planets orbited about the sun became know as the "heliocentric theory." He wrote about it in his book "De Revolutionibus, " which translates to "Concerning the Revolutions."

Copernicus defended his placement of the Sun at the center of the Universe by asking, "For who would place this lamp of a very beautiful temple in another or better place than this, wherefrom it can illuminate everything at the same time?"

Martin Luther, Philipp Melancthon, Andrew Osiander, Tommasco Caccini and other religious leaders were quick to denounce Copernicus' heliocentric theory.

Galileo Galilei

Galileo Galilei (1564 - 1642) lived in Italy. He used the newly-invented telescope to make his own observations. He studied mountains and craters on the Earth's moon, the phases of Venus, and the moons of Jupiter. Particularly he noted that Venus at times appears to be a crescent, just as the Earth's moon does. All of these findings supported Copernicus' heliocentric theory.

Galileo wrote about his observations and thus angered the Roman Catholic Church. The Church eventually placed him under house arrest. The Inquisition was the tribunal of the Roman Catholic Church at this time. The Inquisition made Galileo kneel before them and confess that the heliocentric theory was false.

A modern author name Preserved Smith wrote, "Though the implications of the new science were not worked out immediately, it began to be suspected that if the theories were true, man had lost his birthright as the creature for whose sake all else existed, and had been reduced to the position of a puny and local spectator of infinite forces unresponsive to his wishes and unmindful of his purposes."
Tycho Brahe
Tycho Brahe (1546-1601) was a Danish astronomer who made measurements of the planet and stars. His measurements were the most accurate that had yet been made.
Tycho began his observations in Denmark but later moved to Prague to continue his work.
Tycho proposed a system in which all of the planets except for Earth orbited about the Sun. He claimed that the Sun still orbited about the Earth, however.

Johannes Kepler

Johannes Kepler (1571 - 1630) was a German astronomer. Kepler was invited to live in Prague by Tycho Brahe. Tycho died a year after Kepler's arrival. Kepler inherited a wealth of astronomical data from Tycho. Kepler used this data to draw conclusions about the orbits of the planets.
Kepler's Three Laws can be used to describe the motion of the Planets:
1. The Planets move in orbits about the Sun that are ellipses
2 . The planets move such that the line between the Sun and the Planet sweeps out the same area in the same area in the same time no matter where in the orbit.
3. The square of the period of the orbit of a planet is proportional to the mean distance from the Sun cubed.
Isaac Newton
Isaac Newton (1642 - 1727) lived in England. Newton derived the law of gravitation between two masses. Since the Sun was the most massive object in the planetary system, all of the planets would naturally be attracted to it and revolve around it, in the same manner as the Moon revolves around the Earth.

Newton eventually wrote about gravitation and the heliocentric theory in Principia Mathematica in 1687, at the prompting of another famous astronomer, Edmund Halley (1656-1742) .
Halley used Newton's equations to predict that a comet seen in 1682 would return in 1758. The return of Halley's comet gave final proof to the heliocentric theory.

Roman Catholic Church

In 1992, the Roman Catholic Church finally repealed the ruling of the Inquisition against Galileo. The Church gave a pardon to Galileo and admitted that the heliocentric theory was correct. This pardon came 350 years after Galileo's death.


1. Colin Ranan, Galileo, Putnam, New York, 1974.

2. J. Dyer, A History of Astronomy from Thales to Kepler, Dover, Cambridge, MA, 1953.

3. William Bixby, The Universe of Galileo and Newton, American Heritage,
New York, 1964.

4. Laura Fermi et al., Galileo and the Scientific Revolution, Basic Books, New York, 1961.

5. Angus Armitage, The World of Copernicus, New American Library, New York,

6. Myron Gilmore, The World of Humanism: 1453-1517, Harper and Row, New
York, 1952.

7. Patrick Moore, Watchers of the Stars, Putnam, New York, 1973.

8. A.C. Dickens, The Age of Humanism and Reformation: Europe in the
Fourteen, Fifteenth, and Sixteen Centuries, Prentice-Hall, Englewood
Cliffs, N.J., 1972.

9. Preserved Smith, History of Modern Culture: The Great Renewal 1543-1687,
Peter Smith Gloucester, MA, 1957.

10. Peter Gay, Age of Enlightenment, Time, New York, 1966.

11. Robert Downs, Books that Change the World, American Library, Chicago,1956.

12. Willy Ley, Watchers of the Skies, Viking, New York, 1963.

13. Gravity, Nelson Doubleday, New York, 1970.

14. John Brandt, New Horizons in Astronomy, Freeman, San Francisco, 1972.
15. Thomas Kuhn, The Structures of Scientific Revolutions, University of Chicago Press, 1996.


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