Does the sun orbit around the earth (“geocentrism”) or the earth orbit around the sun (“heliocentrism”)?

What are our personal observations?

First, from anywhere on earth, the sun appears to revolve around the earth once per day. While the moon and the planets have their own motions, they also appear to revolve around the earth about once per day. The stars appeared to be fixed on a celestial sphere rotating once each day about an axis through the geographic poles of the earth.

Second, the earth seems to be unmoving; it feels solid, stable, and stationary. There is nothing in our personal experience that it is moving.

All these point to the sun, stars, planets, and moon orbiting around the earth (geocentrism).

Why do we believe that the earth orbits around the sun? Because our textbooks and teachers tell us so.

Ancient people also pondered the same question. In ancient Greek, some believed in geocentrism and others believed in heliocentrism. Our website contains several books that discuss the cosmic views of ancient Greeks. For example, visit this link:  The Copernicus of Antiquity.

Around 150 AD Ptolemy, a Roman Greek, put forth strong arguments for a geocentric model. In his model, the earth is stationary while the stars, sun, planets and moon orbit around the earth. In addition, the planets also do “epicycles”, i.e., move in small circles while orbiting the earth. This model could explain all celestial movements as seen by our eyes, such as retrograde. Ptolemy settled the question and people in Europe believed in it for over 1,000 years.

Around 1500 AD Nicolaus Copernicus, a person born in the province of Royal Prussia in the Kingdom of Poland, introduced to Europe a heliocentric model similar to one originally proposed by the ancient Greeks. He was supported by a small number of people at that time.

However, there were strong resistances to Copernicus’ theory. One of Copernicus’ supporters, Galileo Galilei, was subjected to the Inquisition. The Catholic Church declared heliocentrism false and its teaching banned.

Today heliocentrism has been proven right. Many scientific experiments confirmed it. Students nowadays are told (by their teachers? books?) that the rejection of heliocentrism during Galileo's time was a triumph of religious ideology over science.

However, the facts may be more complicated. Recently, a physicist, Christopher Graney, presented a translation and discussion of a portion of Almagestum Novum, a massive 1500 pages treatise on astronomy published by an Italian astronomer Giovanni Battista Riccioli in 1651. The year of publication was about ten years after the 1642 death of Galileo. Riccioli listed 126 arguments for and against the Copernican model (49 for, 77 against). Most of the arguments were scientific. These include arguments based on telescopic observations of stars and the inability to detect what is known today as the "Coriolis effect." In the end, Riccioli decided against the Copernican model.

According to Wikipedia:

"Observations that favored the heliocentric model over the geocentric model were lacking and not obvious at the time of Galileo's trial in the early 1600s. Direct evidence supporting heliocentrism had to wait for the emergence of Newtonian mechanics in the late 17th century, the observation of the stellar aberration of light by James Bradley in the 18th century, the analysis of orbital motions of binary stars by William Herschel in the 19th century, and the accurate measurement of the stellar parallax in the 19th century. According to physicist Christopher Graney, Galileo's own observations did not actually support the Copernican heliocentric view, but were more consistent with Tycho Brahe's hybrid model where the Earth did not move, and everything else circled around it and the Sun."

If Professor Graney is right, it appears that the rejection of the Copernican model at Galileo's time had scientific basis and was not merely a result of religious ideology.

For those who are interested in reading the 126 arguments, see this link:  Graney paper.

It should also be pointed out that substantial scientific progresses in understanding the solar system were also made outside of Europe, e.g., in the Islamic region and especially Iran. There are speculations that the knowledge gained in the Islamic region had diffused to Europe by the time Copernicus developed his model.

A paper describing such diffusion is a paper written by George Saliba titled "Critiques of Ptolemaic Astronomy in Islamic Spain". A copy of the paper is hosted on our website:

Critiques of Ptolemaic Astronomy in Islamic Spain

The first paragraph of the Introduction of the paper is copied below:

"There is now an emerging consensus that the astronomy that preceded Copernicus had a lot to do with the kind of astronomy which was later proclaimed by him, and that the foundations of Copernican astronomy were already laid in major Muslim intellectual centers, with some particular significance to al-Andalus itself, but more importantly to the cities of the Muslim east. This should not be surprising to the students of Copernicus because they know very well at least the names of Jábir ibn Aflah (first half of the twelfth century), and al-Bi.trüjí (Latin Alpetragius c. 1200), to mention only two Andalusian astronomers, were both well known to Copernicus, and some of their results were used by him either directly or through other Latin works which were based on them. The Copernican connection with the Eastern Muslim cities was discovered very recently and is still emerging."