The word "helios" in Greek means "sun." Heliocentric means that the sun is at the center. A heliocentric system is one in which the planets revolve around a fixed sun. Thus Mercury, Venus, the Earth, Mars, Jupiter and Saturn all revolve around the sun. The moon is the only celestial sphere in this system which revolves around the earth, and, together with it, around the sun.

This theory was first proposed by Nicolaus Copernicus. Copernicus was a Polish astronomer. He first published the heliocentric system in his book: De revolutionibus orbium coelestium, "On the revolutions of the heavenly bodies," which appeared in 1543. Copernicus died the same year his book was published. After 1,400 years, Copernicus was the first to propose a theory which differed from Ptolemy's geocentric system, according to which the earth is at rest in the center with the rest of the planets revolving around it. The claim that all planets revolve around the sun had been raised in ancient times, but Copernicus was the first to succeed in describing the movements of the planets using an astronomical theory which placed the sun at the center.

Copernicus conducted his studies over many years and was well acquainted with the Ptolemaic theory. In order to explain the exact movements of the planets, it was necessary to add more and more spheres along which the planets move. Copernicus noticed that all of the planets, apart from the sun, have the same annual movement, and he thought that this movement might be explained by the annual movement of the earth around the sun. His hypothesis that all the planets revolve around the sun was reinforced by the unique characteristics of the sun, which gives light and heat to all the other planets. Hence, it was plausible that the sun served as the center of the planetary system.

The daily movement of all the stars around the earth, claimed Copernicus, could be explained by the rotation of the earth on its axis within a 24-hour period. The view experienced from an object revolving around itself is identical to the view experienced when all the other objects revolve around it.

In his book, Copernicus explained the movement of the planets and the stars in a simpler way than the Ptolemaic theory did. However, one cannot point out any observational difference between the two theories. Both of them predict identical planetary movements.

How does one determine what is revolving and what is being revolved around? Does the earth revolve about the sun or does the sun revolve about the earth?

The way to prove this is by means of physical experiments and arguments rather than observation proofs. Galileo contributed a great deal to such arguments in refutation of the Ptolemaic theory, but it was Newton who first produced convincing proof supporting the geocentric theory.

Copernicus remained loyal to the Ptolemaic tools and used the spheres in order to explain the movements of the stars. He too thought that the movements of the planets are composed of a number of uniform circular movements, which eventually create a non-circular path. Copernicus was obliged to abandon the idea that the planets are made of a special material, ether, because for him the earth itself is a planet and is obviously not made of ether. He claimed that the movements of the planets are uniform and circular because of their spherical shape. By adhering to this notion of circular movement, Copernicus continued an astronomical tradition of two thousand years which dated back to ancient Greece, and continued through the Arab astronomy of the Middle Ages and right up to the Christian astronomy. Galileo too upheld this tradition by maintaining the claim that circular movement was the characteristic movement of the celestial bodies. Johannes Kepler was the first to discover that the planets move in an elliptical orbit around the sun.

In 1539, the Protestant leader Martin Luther denounced the new theory. The Catholic Church disregarded Copernicus' book until 1616, when it was included in the "Index": A list of prohibited books. The Catholic church even used Copernicus' book to correct the calendar (which we still use today). The Church did not object to the theory so long as the book was treated as no more than a mathematical explanation,(about which he had heard considerable rumors). Which does not really claim that the earth rotates around the sun. This explanation also allowed Protestant astronomers to use the theory.

However, there were other objections to the theory that were not religious in nature. Most astronomers and natural philosophers of that period claimed that Copernicus' theory was scientifically implausible and raised many counter claims.

In his play "Life of Galileo," Bertolt Brecht attempts to explain how Galileo teaches Andrea the earth movement (p. 6-7) :

Galileo : Did you figure out what I told you yesterday?
Andrea : What? You mean Kippernick and all that turning business?
Galileo : Yes.
Andrea : No. Why do you want me to figure it out? It's too hard for me, I'll only be eleven in October.
Galileo : I want you to understand it, you in particular. To make everybody understand, that's why I work and buy expensive books instead of paying the milkman.
Andrea : But I can see that the sun's not in the same place in the evening and morning. So it can't stand still. It just can't.
Galileo : You "see"! What do you see? You see nothing at all. You're just gaping. Gaping isn't seeing. (He places the iron washstand in the center of the room.) Now, that's the sun. Sit down. (Andrea sits down in the only chair. Galileo stands behind him) Where is the sun, right or left?
Andrea : Left.
Galileo : And how does it get to the right?
Andrea : When you carry it over to the right. Naturally.
Galileo : Only then? (He picks up the chair with him in it and urns it halfway around.) Where's the sun now?
Andrea : On the right.
Galileo : Has it moved?
Andrea : I guess it hasn't.
Galileo : What moved?
Andrea : Me.
Galileo : Wrong! Stupid! the chair!
Andrea : But me with it!
Galileo : Obviously. The chair is the earth. You're sitting on it.
(From : BERTOLT BRECHT Collected Plays Vol. 5 ,Life of Galileo, Editing By Ralph Manheim and John Willett ,1972)

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