Geocentrism and Heliocentrism

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 appear 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 a stationary earth and the sun, stars, planets, and moon orbiting around the earth (geocentrism). Below is a diagram of geocentrism:

Ancient people also pondered the same question. In ancient Greek, some believed in geocentrism and others believed in heliocentrism. You can click on this page to learn more about ancient Greek astronomy and astronomers.

Around 150 CE 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 ( planets seem to move backward among the stars). Ptolemy settled the question and people in Europe believed in it for over 1,000 years.

Around 1500 CE Nicolaus Copernicus (1473-1543), 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. Only a small number of people at that time believed him. One of them was Galileo Galilei (1564-1642), who was subjected to the Inquisition by the Catholic Church and put under house arrest by Pope Urban VIII for the rest of Galileo's life. The Catholic Church also declared heliocentrism false and its teaching banned. Today many people believe that the rejection of heliocentrism during Galileo's time was a triumph of religious ideology over science. Below is a painting showing Copernicus:

The reality is that many people, including scientists, did not believe in heliocentrism at that time -- there was not sufficient evidence to counter our everyday experience of a stationary earth. In 1651 an Italian astronomer Giovanni Battista Riccioli (1598-1671) published a massive 1,500 pages treatise on astronomy titled “Almagestum Novum.” 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.

Scientific evidence developed slowly. It was not until the 18th century that we had good evidence to support heliocentrism. We look at two pieces of evidence:

(1)   Copernicus postulated that the earth is rotating around a north-south polar axis. This means that the rotational speed at the equator (about 1,600 kilometer per hour) is much faster than the speed at the poles (almost zero). If a person standing near north pole throws a ball to another person near the equator, the ball will be deflected as shown in the diagram below.

Riccioli did fire cannons and wanted to detect the deflection. He failed. This is one of the reasons Riccioli argued against heliocentrism.

The problem with Riccioli’s experiment was that the deflection was beyond the capability of the instruments at that time. The deflection does occur in atmospheric winds and weather patterns, which was explained based on the calculation of a French scientist Gaspard-Gustave de Coriolis (1792-1843). Today it is called the Coriolis effect in honor of him.

For more information about Coriolis effect, please click on the article “Winds and the Coriolis Effect” on the Physical Geography page.

(2)  If the earth is orbiting the sun, astronomers should be able to observed an effect known as “parallax.” It is the apparent shift of position of a distant object when the position of an observer changed.  As shown in the diagram below, the apparent position of a distant star should change as the position of the earth changes from one end of the orbit (e.g., in December) to another end (e.g., June). 

Astronomers at that time used the best telescopes to observe parallax of the stars (stellar parallax). But the position of the stars appeared to be unchanged. This is another reason people at that time, such as Riccioli, were skeptical of  heliocentrism. 

Stellar parallax is so difficult to detect that its existence was the subject of much debate in astronomy for hundreds of years. It was finally conclusively detected by experiments carried out in 1832-1838.

In the search for stellar parallax, scientists discovered a related effect: stellar aberration. This effect depends on the motion of the earth in orbiting sun instead of its position, as explained in the diagram below:

Just like straight rainfall appears to be falling at an angle as we observe it in a moving car, the position of a star appears to be at a different position as the earth orbits around the sun.

The stellar aberration effect was detected in 1727 by James Bradley (1692-1762). Stellar aberration is easier to detect and as a result it was detected earlier.

For more information about stellar parallax and stellar aberration, please click on the appropriate sections on the astronomy and physics page.