During the many thousand years that human beings have been looking up at the stars, our concept of what the Universe looks like has changed dramatically. At one time, the magi and sages of the world believed that the Universe consisted of a flat Earth (or a square one, a zigarrut, etc.) surrounded by the Sun, the Moon, and the stars. Over time, ancient astronomers became aware that some stars did not move like the rest, and began to understand that these too were planets.
In time, we also began to understand that the Earth was indeed round, and came up with rationalized explanations for the behavior of other celestial bodies. And by classical antiquity, scientists had formulated ideas on how the motion of the planets occurred, and how all the heavenly orbs fit together. This gave rise to the Geocentric model of the universe, a now-defunct model that explained how the Sun, Moon, and firmament circled around our planet.
The notion that the Earth was the center of the Universe is certainly an understandable one. To ancient people, looking up at the skies, it seemed evident that the Sun, the Moon and the stars rotated around the Earth once a day. For the Earth-bound observer, the ground that they stood on seemed like a fixed point of reference, a flat plane from which to watch the circling cosmos.
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And over time, thanks to centuries of record-keeping by various civilizations – from ancient Babylonian and Egyptian astronomers to contemporary Mediterranean ones – a formalized system began to emerge that put the Earth at the center of all things. This would continue to endure well into the 17th and 18th centuries, by which point, the model’s inherent inconsistencies would lead to it being abandoned in favor of the heliocentric model.
The earliest recorded example of a geocentric universe comes from around the 6th century BCE. It was during this time that Pre-Socratic philosopher Anaximander proposed a cosmological system where a cylindrical Earth was held aloft at the center of everything. Meanwhile, the Sun, Moon, and planets were holes in invisible wheels surrounding the Earth, through which humans could see concealed fire.
During this same century, the Pythagoreans began to propose that the Earth was circular, based on observation of eclipses (and in all likelihood, observations of the zodiac from different latitudes). By the 4th century BCE, this idea combined with the concept of a geocentric universe to create the cosmological system that most Greeks subscribed to.
It was also during the 4th century BCE that Plato and Aristotle would create works on the geocentric universe that would secure its place as the predominant cosmological theory. According to Plato, the Earth was a sphere and the stationary center of the universe. The stars and planets were carried around the Earth on spheres or circles, arranged in the order of distance from the center. These were the Moon, the Sun, Venus, Mercury, Mars, Jupiter, Saturn, fixed stars, and the fixed stars.
His system was expanded by Eudoxus of Cnidus, a contemporary of Plato’s who developed a less mythical, more mathematical explanation of the planets’ motion based on the Platonic idea of uniform circular motion. Aristotle elaborated on Eudoxus’ system, placing a spherical Earth at the center and all other heavenly bodies arranged in concentric crystalline (i.e. transparent) spheres around it.
These spheres all moved at different uniform speeds to create the rotation of bodies around the Earth and were composed of an incorruptible substance called “aether”. He further described his system by explaining the natural tendencies of the terrestrial elements, which in accordance with contemporary belief were earth, water, fire, air; though Aristotle included a fifth element of “celestial aether”.
Earth was the heaviest element, hence why it moved towards the center; whereas water, fire and air formed layers around it. Beyond these layers, the solid spheres of aether in which the celestial bodies were embedded lay. Another important aspect of his model was the inclusion of the “Prime Mover”, a sort of deistic concept whereby all motion in the Universe is initiated by a being or force that is themselves “unmoved”.
Support for this cosmological principle was based on a number of accepted theories. For one, if the Earth were to move, scholars believed that there would be an observable change in the positions of the fixed stars and constellations (aka. stellar parallax). This could be explained by reasoning that they were either motionless, or much further away than believed. Naturally, they chose to believe the former, as it was the simpler explanation.
Another observation that supported geocentric theory was the apparent consistency in Venus’ luminosity, which was interpreted to mean that it was the same distance from Earth at any given time. While this would later come to be explained as the result of Venus’ phases compensating for its increase in apparent size, ancient astronomers lacked the means to see this taking place (i.e. telescopes).
This is not to say, however, that the Eudoxian-Artisotelian model was without its share of flaws. For example, the apparent luminosity of Mercury, Mars and Jupiter were subject to change over time. In addition, the passage of Mars and Jupiter through the sky was subject to “retrograde motion“, a phenomena where they would appear to slow down, move backwards, and then move forwards again through the zodiac.
All of this contradicted the belief in uniform circular motion, which would have meant that there should be changes in apparent luminosity or changes in the apparent motion of the planet’s across the sky. Resolving these issues, and standardizing the many aspects of the Aristotelian system, would become the work of Egyptian-Greek astronomer Claudius Ptolemaeus (aka. Ptolemy).
In his treatise Almagest, which was released in the 2nd century CE, Ptolemy unveiled his concept for a geocentric universe, which would remain the accepted view for the next 1500 years. Drawing on centuries of astronomical traditions, ranging from Babylonian to modern times, Ptolemy argued that the Earth was in the center of the universe, the planets and Sun revolved around it, and the stars were all at a modest distance from the center.
Each planet in this system is also moved by a system of two spheres – a deferent and an epicycle. The deferent is a circle whose center point is removed from the Earth, which was used to account for the differences in the lengths of the seasons. The epicycle is embedded in the deferent sphere, acting as a sort of “wheel within a wheel”. The purpose of he epicycle was to account for retrograde motion, where planets in the sky appear to be slowing down, moving backwards, and then moving forward again.
Unfortunately, these explanations did not account for all the observed behaviors of the planets. Most noticeably, the size of a planet’s retrograde loop (especially Mars) were sometimes smaller, and larger, than expected. To alleviate the problem, Ptolemy developed the equant – a geometrical tool located near the center of a planet’s orbit that causes it to move at a uniform angular speed.
To an observer standing at this point, a planet’s epicycle would always appear to move at uniform speed, whereas it would appear to be moving at non-uniform speed from all other locations.While this system remained the accepted cosmological model within the Roman, Medieval European and Islamic worlds for over a thousand years, it was unwieldy by modern standards.
Granted, it did manage to predict planetary motions with a fair degree of accuracy, and was used to prepare astrological and astronomical charts until replaced by the heliocentric model of the Universe. At the same time, however, every planet in the model required an epicycle revolving on a deferent and offset by an equant, which were also different for each planet. In time, these complexities would come to be challenged.
During the Middle Ages, the geocentric model gained new power and as it became synthesized with Christian theology to become an essential canon. As part of a general trend whereby classical knowledge was being rediscovered by the 13th century and after, the adoption of the Aristotelian-Ptolemiac model of the Universe was part of a marriage between Faith and Reason champion by scholars like St. Thomas Aquinas.
For starters, the separation of the Universe into the “heavens” and the Earth, with the Earth at the center of creation and the heavens beyond, agreed with the Christian view of mankind being the pinnacle of God’s creation. Second, the Prime Mover of Aristotle’s cosmos was interpreted as being the God of Christian theology, and the outermost sphere of the Prime Mover was equated with the Christian Heaven.
As a result of this, challenging the view that the heavens revolved around the Earth was not merely a scientific matter, but a matter of heresy. Hence why it did not come to be challenged until the 16th century with the publication of Nicolaus Copernicus‘ De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), which he only published posthumously to avoid controversy. It is also why support for the heliocentric model of the Universe was also carefully tempered and its adoption gradual.
The geocentric view of the Universe was also the accepted cosmological model in the Islamic world during the Middle Ages. However, beginning in the 10th century CE, there were several Muslim astronomers who challenged one of more aspects of Ptolemy’s work. For instance, Iranian astronomer Abu Sa’id al-Sijzi (ca. 945 – ca. 1020) contradicted the Ptolemaic model by asserting that the Earth revolved on its axis, thus explaining the apparent diurnal cycle and the rotation of the stars relative to Earth.
In the early 11th century, Egyptian-Arab astronomer Alhazen wrote a critique entitled Doubts on Ptolemy (ca. 1028) in which he criticized many aspects of his model. Around the same time, Iranian philosopher Abu Rayhan Biruni (973 – 1048) discussed the possibility of Earth rotating about its own axis and around the Sun – though he considered this a philosophical issue and not a mathematical one.
In the 11th and 12th centuries several Andalusian astronomers, centered in the Almohad (Moorish) territory of Spain, challenged the geocentric model of the Universe as well. For instance, 11th century astronomer Abu Ishaq Ibrahim al-Zarqali (aka. Arzachel) departed from the ancient Greek idea of uniform circular motions by hypothesizing that the planet Mercury moves in an elliptic orbit.
In the 12th century, fellow Andalusian Nur ad-Din al-Bitruji (aka. Alpetragius) proposed a planetary model that abandoned the equant, epicycle and eccentric mechanisms. This was followed by Fakhr al-Din al-Razi’s (1149–1209) publication of his treatise Matalib, which dealt with conceptual physics. In it, he rejected the notion of the Earth’s centrality within the universe and instead proposed a cosmology in which there were a “thousand thousand worlds beyond this world…”
And at the Maragha Observatory in eastern Iran, the Damascus mosque, and the Ulugh Beg (aka. Samarkand) Observatory in modern-day Kazakhstan, the Earth’s rotation was discussed by several generations of astronomers between the 13th and 15th centuries. Though these were largely philosophical in nature and did not result in the adoption of heliocentrism, many of the arguments and evidence put forward resembled those used later by Copernicus.
The Heliocentric Model:
In the 16th century, Nicolaus Copernicus began devising his version of the heliocentric model, which represented the culmination of years worth of research. Like others before him, Copernicus built on the work of a number classical astronomers who did not support the geocentric view, as well as paying homage to the Maragha school and several notable philosophers from the Islamic world.
By 1514 century, Copernicus summarized his ideas in a short treatise titled Commentariolus (“Little Commentary”), which he began circulating to friends. This forty-page manuscript described his ideas about the heliocentric hypothesis, which was based on seven general principles. These principles stated that:
- Celestial bodies do not all revolve around a single point
- The center of Earth is the center of the lunar sphere—the orbit of the moon around Earth
- All the spheres rotate around the Sun, which is near the center of the Universe
- The distance between Earth and the Sun is an insignificant fraction of the distance from Earth and Sun to the stars, so parallax is not observed in the stars
- The stars are immovable – their apparent daily motion is caused by the daily rotation of Earth
- Earth is moved in a sphere around the Sun, causing the apparent annual migration of the Sun. Earth has more than one motion
- Earth’s orbital motion around the Sun causes the seeming reverse in direction of the motions of the planets
Thereafter he continued gathering data for a more detailed work, and by 1532, he had come close to completing the manuscript of his magnum opus – De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres). In it, he advanced his seven major arguments, but in more detailed form and with detailed computations to back them up.
By placing the orbits of Mercury and Venus between the Earth and the Sun, Copernicus was able to account for changes in their appearances. In short, when they are on the far side of the Sun, relative to Earth, they appear smaller but full. When they are on the same side of the Sun as the Earth, they appear larger and “horned” (crescent-shaped).
It also explained the retrograde motion of planets like Mars and Jupiter by showing that Earth astronomers do not have a fixed frame of reference but a moving one. This further explained how Mars and Jupiter could appear significantly larger at certain times than at others. In essence, they are significantly closer to Earth when at opposition than when they are at conjunction.
However, due to fears that the publication of his theories would lead to condemnation from the church (as well as, perhaps, worries that his theory presented some scientific flaws) he withheld his research until a year before he died. It was only in 1542, when he was near death, that his treatise was sent to Nuremberg to be published.
Thanks to the invention of the telescope, ongoing observations about the motions of the planets, and refined calculations, astronomers would come to understand that the Earth was not immovable. Nevertheless, for thousands of years, the geocentric model of the universe would remain the accepted cosmological system, and was used to calculate the positions of the planet, eclipses, and other astronomical phenomena.
In the end, the geocentric model of the universe succumbed to the same fate as many other accepted notions of its day. Much like the true age of the Earth, humanity’s biological origins, and astrology, the belief that the Earth was the center of the universe did not survive the expansion in learning that was taking place by the 17th century.
We have many interesting articles on the Geocentric Model of the Universe here at Universe Today. Here is What Is The Difference Between The Geocentric and Heliocentric Universe?, Where Is The Center Of The Universe, and The Earth Goes Around The Sun?
Astronomy Cast also has an episode on the subject, titled Episode 77: Where Is The Center Of The Universe