Geocentric theory
The geocentric theory (also called the geocentric model, geocentrism or Ptolemaic model) is an astronomical theory that places the Earth at the center of the universe, and the stars, including the Sun, revolving around the Earth (geo: Earth; centrism: grouped or centered).
Geocentrism was the prevailing view of the universe in many ancient civilizations, including Babylonian. In the century ii d. C. Claudio Ptolemy, in his work Almagesto, introduced a geocentric system using epicycles, deferents and equants that would be widely accepted. Ptolemy's model was in effect until the 16th century when it was superseded by Copernicus' heliocentric theory.
Geocentric theories
Pre-Socratic Philosophy
The geocentric paradigm was adopted in Greek astronomy and philosophy, in force since its inception in pre-Socratic philosophy. In the VI century B.C. C. Anaximander proposed a cosmology in which the Earth had the shape of a section of a pillar (a cylinder) floating in the center of everything. The Sun, the Moon and the planets were holes in invisible wheels that circled the Earth, through which human beings could see a hidden fire. At the same time, the Pythagoreans thought that the Earth was spherical (according to observations of eclipses) but not the center of the universe; they postulated that it was in motion around an invisible fire.
Over time, these two versions were combined; so most educated Greeks thought the Earth was a sphere at the center of the universe. In the IV century B.C. C. two influential Greek philosophers, Plato and his disciple Aristotle, wrote works based on the geocentric model.
Platonic Philosophy
According to Plato, the Earth was a sphere that rested in the center of the universe. The stars and planets revolved around the Earth in celestial circles, arranged as follows (outward from the center): Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, and the fixed stars. In the Myth of Er, a section of The Republic, Plato describes the cosmos as the "Spindle of Necessity", guarded by sirens and the three moirai
Eudoxus of Cnidus, who worked with Plato, developed a less mythical and more mathematical explanation of the motion of the planets based on Plato's discourse, stating that all phenomena in the heavens can be explained with uniform circular motion.
Aristotelian system
Aristotle developed the Eudoxus system. In the Aristotelian system, the spherical Earth was at the center of the universe, and all celestial bodies were attached to 47-55 transparent, rotating spheres that surrounded the Earth, all of them concentric with it (the number is so high because they are necessary several spheres for each planet). These spheres, known as crystalline spheres, moved at different uniform speeds to create the revolution of bodies around the Earth. These were composed of an incorruptible substance called ether. The Moon was on the closest sphere to Earth, coming into contact with the Earth area, causing dark spots (macules) and the ability to go through lunar phases.
He later described his system by explaining the natural tendencies of the terrestrial elements: earth, water, fire and air, as well as the celestial ether. His system held that the Earth was the heaviest element, with the strongest movement toward the center, thus the water formed a shell surrounding the Earth's sphere. The tendency of air and fire, on the contrary, was to move up, away from the center, with fire being lighter than air. Beyond the layer of fire, were the solid spheres of ether in which the celestial bodies themselves were also embedded entirely of ether.
Arguments in favor of geocentrism
Adherence to the geocentric model was largely due to several important observations. First of all, if the Earth were to move, then one should be able to observe the displacement of the fixed stars due to stellar parallax. In short, if the Earth moved, the shapes of the constellations would change considerably over the course of a year. Because the stars were actually much further away than Greek astronomers postulated (making the motion extremely subtle), stellar parallax was not detected until the XIX. Therefore, the Greeks chose the simpler of the two explanations. The absence of any observable parallax was considered a fatal flaw in any non-geocentric theory.
Another observation used in favor of the geocentric paradigm of the time was the apparent consistency of Venus's luminosity, which implied that it was usually the same distance from Earth, which in turn is more consistent with geocentrism than with geocentrism. heliocentrism. This is actually because the loss of light caused by Venus's phases makes up for the increase in apparent size caused by its varying distance from Earth. Objectors to heliocentrism observed that terrestrial bodies naturally tend to rest as close to the center of the Earth as possible. Despite the chance of falling closer to the center, terrestrial bodies tend not to move unless forced by an outside object, or transformed to a different element by heat or moisture.
Atmospheric explanations were used for many phenomena because the Eudoxo-Aristotelian model based on perfectly concentric spheres was not intended to explain changes in the brightness of the planets due to a change in distance. concentric models were abandoned, since it was impossible to develop a sufficiently accurate model under that ideal. However, while it provided similar explanations, the later deferent and epicycle model was flexible enough to accommodate observations over many centuries.
Ptolemaic system
A major flaw in Eudoxus's system of concentric spheres was that they could not account for changes in the brightness of the planets caused by a change in distance. This honor was reserved for the Ptolemaic system, supported and founded by the Hellenistic astronomer Claudius Ptolemy of Alexandria (Egypt) in the century ii d. C. the main astronomical book of his, The Almagest , was the culmination of centuries of work by Greek astronomers; it was accepted for more than a millennium as the correct cosmological paradigm by European and Muslim astronomers. Because of its influence, it is sometimes considered identical with the geocentric model.
In the Ptolemaic model, each planet is moved by two or more spheres: one sphere is its deferential that is centered on the Earth, and the other sphere is the epicycle that fits into the deferential. The planet fits into the sphere of the epicycle. The vas rotates around Earth while the epicycle rotates within the vas, causing the planet to move toward and away from Earth at various points in its orbit, including slowing down, stopping, and moving in the opposite direction. opposite (in retrograde motion). The epicycles of Venus and Mercury are always centered on a line between the Earth and the Sun, which explains why they are always close to it in the sky. The order of the Ptolemaic spheres from Earth is: Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn and fixed stars.
The deferent-and-epicycle model had been used by Greek astronomers for centuries, as had the eccentric idea. In the illustration, the center of the vas is not Earth but X, making it eccentric.
Unfortunately, the system that was in place in Ptolemy's day did not agree with the measurements, even though it was a considerable improvement over Aristotle's system. Sometimes the size of a planet's retrograde spin (most notably Mars') was smaller and sometimes larger. This prompted him to generate the idea of an equant.
The equant was a point near the center of the planet's orbit at which, if one stood there and looked, the center of the planet's epicycle would appear to be moving at the same speed. Therefore, the planet was actually moving at different speeds when the epicycle was at different positions on its deferential. Using an equant, Ptolemy claimed to maintain a uniform, circular motion, but many people disliked him because they thought he disagreed with Plato's dictum of "uniform circular motion." The resulting system, which eventually achieved wide acceptance in the West, was seen as too complicated in the eyes of modernity; it required that each planet have an epicycle revolving around a deferent, displaced by a different equant for each planet. But the system predicted various celestial motions, including the start and end of retrograde motion, fairly well for the time it was developed.
Islamic Astronomy and Geocentrism
Muslim astronomers generally accepted the Ptolemaic system and the geocentric model, but in the X century they began to texts appear that questioned Ptolemy (Shukūk). Several Muslim scholars questioned the apparent immobility of the Earth and its centrality within the universe. Some Muslim astronomers believed that the Earth revolved around its axis, like Abu Sa'id al-Sikhzi (circa 1020). According to Al-Biruni, Sikhzi invented an astrolabe called al-zūraqī based on a belief held by some of his contemporaries "that the motion we see is due to the motion of the Earth and not the motion of the sky." The prevalence of this view is confirmed by a century-old reference XIII which says:
According to the geometres (muhandisīn), the Earth is in constant circular motion, and what seems to be the movement of the heavens is actually due to the movement of the Earth and not to that of the stars.
In the early 11th century, Alhacén wrote a scathing critique of Ptolemy's model in his Doubts about Ptolemy (c. 1028), which some have interpreted as implicitly criticizing Ptolemy's geocentrism, but most agree that he was criticizing the details of Ptolemy's model rather than its geocentrism.
In the 12th century, Azarquiel moved away from the ancient Greek idea of uniform circular motions by hypothesizing that the planet Mercury moved in an elliptical orbit, while Alpetragio proposed a planetary model that abandoned the mechanisms of equants, epicycles, and deferents, although this resulted in a system that was less mathematically exact. Alpetragio also declared the Ptolemaic model to be a model imaginary that was accurate in predicting planetary positions but not real or physical. The alternative system of it spread through most of Europe during the XIII century .
Fakhr al-Din al-Razi (1149-1209), in dealing with his conception of physics and the physical world in his Matalib, rejected the Aristotelian and Avicenian notion of the centrality of Earth within the universe, arguing that there are "thousands of worlds (alfa alfi 'awalim) beyond this world, so that each of those worlds can be larger and more enormous than this world, as well as having the same of what this world has. To support his theological argument, he quotes the Qur'anic verse: "All praise belongs to God, Lord of the Worlds," emphasizing the term "Worlds."
The "Maraghe revolution" refers to the critique of the Maraghe school against Ptolemaic astronomy. The "Maraghe school" was an astronomical tradition that began at the Maraghe observatory and continued with astronomers at the Damascus mosque and the Samarkand observatory. Like their Andalusian predecessors, Maraghe's astronomers attempted to solve the problem of the equant (the circle around whose circumference a planet or the center of an epicycle was conceived to move uniformly) and produced alternative configurations to the Ptolemaic model without abandoning geocentrism. They were more successful than their Andalusian predecessors in producing non-Ptolemaic configurations that eliminated equants and epicycles, were more accurate than the Ptolemaic system in predicting numerical planetary positions, and were more in agreement with empirical observations. Maraghe's most important astronomers included Mo'ayyeduddin Urdi (d. 1266), Nasir al-Din al-Tusi (1201-1274), Qutb al-Din al-Shirazi (1236-1311), Ibn al-Shatir (1304–1375), Ali Qushji (c. 1474), Al-Biryandi (c. 1525), and Shams al-Din al-Khafri (1550).
Ibn al-Shatir, the Damascus astronomer (1304-1375) who worked at the Umayyad Mosque, wrote an important book entitled Kitab Nihayat al-Sul fi Tashih al-Usul (< i>A Final Inquiry into the Correctness of the Planetary Theory) on a theory that is largely based on the Ptolemaic system known at the time. In his book Ibn al-Shatir, an Arab astronomer of the 14th century, E. S. Kennedy wrote: "What is most interesting, however, is that Ibn al-Shatir's lunar theory, except for trivial differences in parameters, is identical to that of Copernicus (1473-1543)." The discovery that Ibn al-Shatir's models are mathematically identical to Copernicus's suggests the possible transmission of these models to Europe. At the Maraghe and Samarkand observatories, the rotation of the Earth was discussed by Al-Tusi and Ali. Qushji (1403); the arguments and evidence they used resemble those used by Copernicus to support the movement of the Earth.
However, the Maraghe school never brought about the paradigm shift to heliocentrism. The influence of the Maraghe school on Copernicus remains speculative, as there is no documentary evidence to prove it. The possibility that Copernicus independently developed the Tusi coupling remains open, as no researcher has shown that he was aware of Tusi's work or that of the Maraghe school.
Other geocentric systems
Hicetas and Ecfanthus (two Pythagoreans from the 5th century BC), and Heraclides Pontus (from the style="font-variant:small-caps;text-transform:lowercase">IV BC), believed that the Earth rotates on its axis but remains at the center of the universe. Such a system still qualifies as geocentric. It was revived in the Middle Ages by Jean Buridan. Heraclides Ponticus is also sometimes cited for having proposed that Venus and Mercury did not circle the Earth but the Sun, but the evidence for this theory was not clear. Marciano Capella definitely put Mercury and Venus in epicycles around the Sun.
Rival theories
Not all Greeks accepted the geocentric model. Some Pythagoreans believed that the Earth could be one of several planets circling in a central fire.
First heliocentrism
The first heliocentric was Aristarchus of Samos (II century BC) was the most radical. He wrote a book, which has not been preserved, on heliocentrism, saying that the Sun was the center of the universe, while the Earth and other planets revolved around it. His theory was not popular, and he only had one known follower, Seleucus of Seleucia.
The Copernican system
In 1543 the geocentric theory faced its first serious questioning with the publication of De revolutionibus orbium coelestium by Copernicus, which claimed that the Earth and the other planets, contrary to the official doctrine of the time, rotated around the Sun. However, the geocentric system was maintained for several years, since the Copernican system did not offer better predictions of cosmic ephemeris than the previous one, and it also posed a problem for natural philosophy, as well as for religious education.
Copernicus' theory established that the Earth revolves around itself once a day, and that once a year it made a complete revolution around the Sun. He also affirmed that the Earth, in its rotational movement, tilted on its axis (like a top). However, he still maintained some principles of ancient cosmology, such as the idea of the spheres within which the planets were and the outer sphere where the stars were immobile, which is false by astronomical verifications made today, thanks to technology and its advances.
Gravitation: Newton and Kepler
Johannes Kepler, after analyzing the observations of Tycho Brahe, built his three laws in 1609 and 1619, based on a heliocentric view where the planets move in elliptical paths. Using these laws, he was the first astronomer to successfully predict a transit of Venus (circa 1631).
In 1687, Isaac Newton devised his law of universal gravitation, which introduced gravitation as the force that keeps planets in orbit, allowing scientists to quickly construct a plausible heliocentric model for the planets. Solar system. Using the universal law of gravitation, the orbits of all the planets in the solar system can be calculated with precision, with the exception of Mercury, whose perihelion had a precession that cannot be explained by Newton's laws of gravitation. Despite this problem, the scientific community believed so much in Newton's laws that the existence of a planet, Vulcan, was even postulated to justify Mercury's orbit. Mercury's perihelion precession could not be explained until Albert Einstein's general theory of relativity in 1915.
Geocentrism today
Astronomy
However, a geocentric framework is useful to astronomers in many scientific ways. For the study of objects outside the solar system, where the distances are much greater than the distance from the Earth to the Sun, their study is simplified by taking the Earth as the center.
The solar system is still of interest to planetarium designers since, for technical reasons, giving the planet Ptolemaic-style motion has advantages over Copernican-style motion.
Religion
Some religious fundamentalists, mostly creationists, still interpret their scriptures to indicate that the Earth is the physical center of the universe; this is called modern geocentrism or neogeocentrism.
The Contemporary Association for Biblical Astronomy, led by physicist Gerhardus Bouw, upholds a modified version of Tycho Brahe's model, which they call geocentricity. However, most religious groups today accept the heliocentric paradigm.
On October 31, 1992, Pope John Paul II rehabilitated Galileo 359 years after he was condemned by the Church. While this does not mean that heliocentrism has been declared to be an absolute truth, he does dismiss any notion that there is heresy in believing in the heliocentric theory. It should be noted that the aim was primarily to reconcile the notion that science and faith can be united, and Galileo's earlier rejection of heliocentrism should not continue to be construed as a disagreement between the two.
Astrology
For their part, astrologers, while they may not believe in geocentrism as a principle, still employ the geocentric model in their calculations to predict horoscopes.
There are some elements that we can apply to contrast them with the geocentric system: the leap year system, the inclination of the axis of rotation, and the phase cycle of the Moon.
If the Earth does not move around the Sun, it would be the Sun that would revolve around the Earth once every 24 hours, so that the Earth would not rotate around its axis of rotation either. The idea that the Sun would go around the Earth in 24 hours means that it would have to go around the Earth 365.25 times to complete one year, but some institutional authority would have had to establish that number, and the rational thing would be a integer. The leap year day system devised by the Church would be the adaptation of the rational calendar to the assumption that every 4 cycles of 365.25 turns around the Earth, the Sun would accumulate one turn, the 366th or 366th day. But according to Nature, the 365
,25 turns (365.25 days) is the number of turns that the planet has time to make during its orbit around the Sun, and therefore it is a measure given by the universe.