2007 Schools Wikipedia Selection. Related subjects: Religious disputes; Space (Astronomy)

Heliocentric Solar System
Heliocentric Solar System
Heliocentrism (lower panel) in comparsion to the geocentric model (upper panel)
Heliocentrism (lower panel) in comparsion to the geocentric model (upper panel)

In astronomy, heliocentrism is the belief that the Sun is at the centre of the Universe and/or the Solar System. The word is derived from the Greek ( Helios = "Sun" and kentron = "Centre"). Historically, heliocentrism is opposed to geocentrism and currently to modern geocentrism, which places the earth at the centre. (The distinction between the Solar System and the Universe was not clear until modern times, but extremely important relative to the controversy over cosmology and religion.) In the 16th and 17th centuries, when the theory was revived and defended by Copernicus, Galileo, and Kepler, it became the centre of a major dispute.

Development of heliocentrism

To anyone who stands and looks at the sky, it seems clear that the earth stays in one place while everything in the sky rises and sets or goes around once every day. Observing over a longer time, one sees more complicated movements. The Sun makes a slower circle over the course of a year; the planets have similar motions, but they sometimes turn around and move in the reverse direction for a while ( retrograde motion). As these motions became better understood, they required more and more elaborate descriptions, the most famous of which was the Ptolemaic system, formulated in the 2nd century.

Ancient India

The earliest traces of a counter-intuitive idea that it is the Earth that is actually moving and the Sun that is at the centre of the solar system (hence the concept of heliocentrism) is found in several Vedic Sanskrit texts written in ancient India. Yajnavalkya (c. 9th– 8th century BC) recognized that the Earth is spherical and believed that the Sun was "the centre of the spheres" as described in the Vedas at the time. In his astronomical text Shatapatha Brahmana ( he states: "The sun strings these worlds - the earth, the planets, the atmosphere - to himself on a thread." He recognized that the Sun was much larger than the Earth, which would have influenced this early heliocentric concept. He also accurately measured the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, close to the modern measurements of 107.6 for the Sun and 110.6 for the Moon. He also described a solar calendar in the Shatapatha Brahmana.

The Vedic Sanskrit text Aitareya Brahmana (2.7) (c. 9th–8th century BC) also states: "The Sun never sets nor rises thats right. When people think the sun is setting, it is not so; they are mistaken." This indicates that the Sun is stationary (hence the Earth is moving around it), which is elaborated in a later commentary Vishnu Purana (2.8) (c. 1st century), which states: "The sun is stationed for all time, in the middle of the day. [...] Of the sun, which is always in one and the same place, there is neither setting nor rising." (See Haug, Martin and Basu (1974), Joseph (2000), Kak (2000) in Selin (2000), Teresi (2002) and Blavatsky (1877) for further information.)

Ancient Greece

In the 4th century BC, in Chapter 13 of Book Two of his On the Heavens, Aristotle wrote that "At the centre, they [the Pythagoreans] say, is fire, and the earth is one of the stars, creating night and day by its circular motion about the centre." The reasons for this placement were philosophic based on the classical elements rather than scientific; fire was more precious than earth in the opinion of the Pythagoreans, and for this reason the fire should be central. However the central fire is not the Sun. The Pythagoreans believed the Sun orbited the central fire along with everything else. Aristotle dismissed this argument and advocated geocentrism.

Aristarchus's 2nd century BC calculations on the relative sizes of the Earth, Sun and Moon.
Aristarchus's 2nd century BC calculations on the relative sizes of the Earth, Sun and Moon.

Heraclides of Pontus (4th century BC) explained the apparent daily motion of the celestial sphere through the rotation of the Earth, and probably realized also that Mercury and Venus rotate around the Sun. The first Greek astronomer to propose the heliocentric system however, was Aristarchus of Samos (c. 270 BC). Unfortunately his writings on the heliocentric system are lost, but we have other authors who give us crucial information about his system (the most important among them is Archimedes, who lived in the third century BC and therefore had direct knowledge of Aristarchus's works). By the time Aristarchus was writing, the size of the Earth had been calculated accurately by Eratosthenes. Aristarchus also calculated the size of the earth, and measured the size and distance of the Moon and Sun, in a treatise which fortunately survived. His geometrical method is exact, but it requires the difficult measurement of the angle between the Sun and the Moon when the latter is at the first or last quarter, which is slightly less than 90 degrees. Aristarchus overestimated the angle and consequently underestimated the distance and size of the Sun (although his figures for the Moon are fairly good). What is important, however, is Aristarchus's scientific approach, and his result that the Sun is much larger than the Earth. Perhaps, as many people have suggested, paying attention to these numbers led Aristarchus to think that it made more sense for the Earth to be moving than for the huge Sun to be moving around it.

Aristarchus's original work on heliocentrism has not survived and is known only from others' accounts; hence the uncertainty as to his arguments on its behalf. It appears, though, that he understood the problem of stellar parallax: if the Earth moves over huge distances in circling the Sun, then the nearer of the fixed stars should be seen to move relative to the farther ones, as nearby hills move relative to distant mountains when one is traveling. Aristarchus explained the lack of any such visible effect by saying that the stars were at extremely large distances: the sphere of the fixed stars was to the Earth's orbit as the surface of a sphere is to its centre. (Archimedes described and attributed this argument to Aristachus in the introduction of The Sand Reckoner.) That would make the stars infinitely distant; whether he meant that literally, or just meant to convey an extremely large ratio, is not possible to determine now. (In the event his explanation turned out to be right, though the distances are finite; stellar parallax was observed in the 19th century.)

Aristarchus' heliocentric model was considered by Archimedes in The Sand Reckoner. The purpose of this work was to prove that extremely large numbers, even the number of grains of sand that it would take to fill the universe, could be expressed mathematically and did not have to be treated vaguely as "infinite". To this end, he took the largest existing model of the universe, which was that of Aristarchus, to calculate the amount of sand that would fill even that universe. Pointing out that mathematically it made no sense to talk of a ratio between the surface of a sphere and its centre, which has no magnitude, Archimedes made the working assumption that the distance of the fixed stars was in the same relation to the radius of the Earth's orbit as that orbit was in relation to the Earth itself. Under these conditions, we can demonstrate that stellar parallax would have been beyond then-current observers' ability to detect, as it was in fact. There is no indication, however, that either Aristarchus or Archimedes explicitly discussed the problem of stellar parallax as a way to determine whether the Earth's motion was a reality.

Hellenistic Babylonia

In Hellenistic Babylonia, the astronomer Seleucus of Seleucia (b. 190 BC) adopted the heliocentric system of Aristarchus, and according to Plutarch, even proved it. This was probably related to the phenomenon of tides. Indeed Seleucus correctly theorized that tides were caused by the Moon, although he believed that the interaction was mediated by the Earth's atmosphere. He noted that the tides varied in time and strength in different parts of the world.

Medieval India

The Indian astronomer- mathematician Aryabhata ( 476– 550), in his magnum opus Aryabhatiya, propounded a heliocentric model in which the Earth was taken to be spinning on its axis and the periods of the planets were given with respect to a stationary Sun. He was also the first to discover that the light from the Moon and the planets was reflected from the Sun, and that the planets follow an elliptical orbit around the Sun, and thus propounded an eccentric elliptical model of the planets, on which he accurately calculated many astronomical constants, such as the times of the solar and lunar eclipses, and the instantaneous motion of the Moon (expressed as a differential equation).

Bhaskara ( 1114– 1185) expanded on Aryabhata's heliocentric model in his astronomical treatise Siddhanta-Shiromani, where he mentioned the law of gravity, discovered that the planets don't orbit the Sun at a uniform velocity, and accurately calculated many astronomical constants based on this model, such as the solar and lunar eclipses, and the velocities and instantaneous motions of the planets. Arabic translations of Aryabhata's Aryabhatiya were available from the 8th century, while Latin translations were available from the 13th century, before Copernicus had written De revolutionibus orbium coelestium, so it's quite likely that Aryabhata's work had an influence on Copernicus' ideas.

Nasir al-Din Tusi, 13th century, resolved significant problems in the Ptolemaic system
Nasir al-Din Tusi, 13th century, resolved significant problems in the Ptolemaic system

Islamic World

Although scholars from the Islamic World never actually proposed a heliocentric model, some astronomers did criticize the geocentric model. For example, Ibn al-Haitham (Alhazen) in the 12th century wrote a scathing critique of Ptolemy's geocentric model: "Ptolemy assumed an arrangement that cannot exist, and the fact that this arrangement produces in his imagination the motions that belong to the planets does not free him from the error he committed in his assumed arrangement, for the existing motions of the planets cannot be the result of an arrangement that is impossible to exist."

The Persian scientist Nasir al-Din Tusi ( 1201– 1274) resolved significant problems in the Ptolemaic system by developing the Tusi-couple as an alternative to the physically problematic equant introduced by Ptolemy. Muslim scientist Mu'ayyad al-Din al-'Urdi (c. 1250) developed the Urdi lemma. Arab Muslim astronomer Ibn al-Shatir ( 1304– 1375), in his treatise Kitab Nihayat as-Sul fi Tashih al-Usul (A Final Inquiry Concerning the Rectification of Planetary Theory), eliminated the need for an equant by introducing an extra epicycle, departing from the Ptolemaic system in a way very similar to what Copernicus later also did. Ibn al-Shatir proposed a system that was only approximately geocentric, rather than exactly so, having demonstrated trigonometrically that the Earth was not the exact centre of the universe. His rectification was later used in the Copernican model, along with the Tusi-couple and Urdi lemma. Their theorems played an important role in the Copernican model of heliocentrism.

Renaissance Europe

Nicolaus Copernicus, 16th century, made great advances on the heliocentric planetary model
Nicolaus Copernicus, 16th century, made great advances on the heliocentric planetary model

It should be noted that the popular belief that in the West, before Copernicus, the doctrine of heliocentrism was unheard of, or incomprehensible, is simply false. Not only were Arabic texts increasingly translated into Latin after the 11th century (as a result of the increasing contact with the Muslim world through Islamic Spain and the Crusades), but explorers and traders were increasingly venturing out beyond Europe (facilitated by the Pax Mongolica) and introducing the West to the Indian heliocentric traditions as detailed above. And of course scholars were well aware of the arguments of Aristarchus and Philolaus, as well as the numerous other classical thinkers who had proposed (or were alleged to have proposed) heliocentric or quasi-heliocentric views, such as Hicetas and Heraclides Ponticus (Copernicus certainly was). Moreover, a few European thinkers also discussed heliocentrism in the so called 'Middle Ages': for example Nicolas Oresme and Nicholas of Cusa. However, for most scholars in this period, heliocentrism had one extremely major and obvious problem: the apparent common sense view that, if the Earth were spinning and moving around the Sun, people and objects would tend to fall off or spin out into space; an object dropped from a tower would fall behind the tower as the latter rotated with the Earth and would land to the West; and so on. A response to these objections required much better understanding of physics.

Despite these problems in the 16th century the theory of heliocentrism was revived by Nicolaus Copernicus, in a form consistent with then-current observations. This theory resolved the issue of planetary retrograde motion by arguing that such motion was only perceived and apparent, rather than real: it was a parallax effect, as a car that one is passing seems to move backwards against the horizon. This issue was also resolved in the geocentric Tychonic system; the latter, however, while eliminating the major epicycles, retained as a physical reality the irregular back-and-forth motion of the planets, which Kepler characterized as a " pretzel." In developing his theories of planetary motion, Copernicus was probably indebted to the earlier work of Indian astronomer Aryabhata for his work on heliocentrism, and the Muslim scientists/astronomers Tusi, al-Urdi, and Ibn al-Shatir for resolving significant problems in the Ptolemaic system.

Religious disputes over heliocentrism

As early as the time of Aristarchus, the heliocentric idea was denounced as being against religion in Europe. The issue did not assume any importance, however, for nearly 2,000 years.

Nicolaus Copernicus published the definitive statement of his system in De Revolutionibus in 1543. Copernicus began to write it in 1506 and finished it in 1530, but did not publish it until the year of his death. Although he was in good standing with the Church and had dedicated the book to Pope Paul III, the published form contained an unsigned preface by Osiander stating that the system was a pure mathematical device and was not supposed to represent reality. Possibly because of that preface, the work of Copernicus inspired very little debate on whether it might be heretical during the next 60 years.

The term at that time for such a purely fictitious computing trick was hypothesis. In order to understand the disputes of the following 100 years, it is necessary to remember that the modern meaning, an idea that is to be confirmed or disproved by experiment, did not arise until later.

There was an early suggestion among Dominicans that the teaching should be banned, but nothing came of it at the time. Some Protestants, however, voiced strong opinions during the 16th century. Martin Luther once said:

"There is talk of a new astrologer who wants to prove that the earth moves and goes around instead of the sky, the sun, the moon, just as if somebody were moving in a carriage or ship might hold that he was sitting still and at rest while the earth and the trees walked and moved. But that is how things are nowadays: when a man wishes to be clever he must needs invent something special, and the way he does it must needs be the best! The fool wants to turn the whole art of astronomy upside-down. However, as Holy Scripture tells us, so did Joshua bid the sun to stand still and not the earth."

This was reported in the context of dinner-table conversation and not a formal statement of faith. Melanchthon, however, opposed the doctrine over a period of years.

Over time, however, the Catholic Church began to become more adamant about protecting the geocentric view. Pope Urban VIII, who had approved the idea of Galileo's publishing a work on the two theories of the world, became hostile to Galileo; it is said that he believed Galileo mocked him in his Dialogue Concerning the Two Chief World Systems, though evidence of this is scant or lacking. (The character who represents traditional views in the dialogue is named "Simplicio", after the classic philosopher Simplicius, who was admired at the time by followers of neoplatonism.) Over time, the Catholic Church became the primary opposition to the Heliocentric view.

The favored system had been that of Ptolemy, in which the Earth was the centre of the universe and all celestial bodies orbited it. (The Catholic support for geocentricism should not be confused with the idea of a flat earth, which the Church never supported.) A geocentric compromise was available in the Tychonic system, in which the Sun orbited the Earth, while the planets orbited the Sun as in the Copernican model. The Jesuit astronomers in Rome were at first unreceptive to Tycho's system; the most prominent, Clavius, commented that Tycho was "confusing all of astronomy, because he wants to have Mars lower than the Sun." (Fantoli, 2003, p. 109) But as the controversy progressed and the Church took a harder line toward Copernican ideas after 1616, the Jesuits moved toward Tycho's teachings; after 1633, the use of this system was almost mandatory. For advancing heliocentric theory Galileo was put under house arrest for the last several years of his life.

Theologian and pastor Thomas Schirrmacher, however, has argued:

Contrary to legend, Galileo and the Copernican system were well regarded by church officials. Galileo was the victim of his own arrogance, the envy of his colleagues, and the politics of Pope Urban VIII. He was not accused of criticising the Bible, but disobeying a papal decree.

Catholic scientists also:

appreciated that the reference to heresy in connection with Galileo or Copernicus had no general or theological significance, (Heilbron 1999).
In the 17th century AD Galileo Galilei opposed the Roman Catholic Church by his strong support for heliocentrism
In the 17th century AD Galileo Galilei opposed the Roman Catholic Church by his strong support for heliocentrism

Whether these modern interpretations of theology were generally held in the Church in Galileo's time may be judged from the words of the Inquisition when it tried and condemned Galileo in 1633. In the Inquisition's formal charges he was not accused of violating a papal decree; rather, the charges condemned his holding of "a false doctrine taught by many, namely, that the sun is immovable in the centre of the world, and that the earth moves". During formal questioning by the Inquisition, Galileo was asked (first day) what orders he had been given in 1616 (clearly a reference to the supposed decree); but he was also questioned (fourth day) on his Copernican beliefs. The final verdict was on exactly the same lines as the indictment: he had rendered himself "vehemently suspected of heresy", but there was no mention of disobedience to a specific order.

Cardinal Robert Bellarmine himself considered that Galileo's model made "excellent good sense" on the ground of mathematical simplicity; that is, as a hypothesis (see above). And he said:

If there were a real proof that the Sun is in the centre of the universe, that the Earth is in the third sphere, and that the Sun does not go round the Earth but the Earth round the Sun, then we should have to proceed with great circumspection in explaining passages of Scripture which appear to teach the contrary, and we should rather have to say that we did not understand them than declare an opinion false which has been proved to be true. But I do not think there is any such proof since none has been shown to me. (Koestler 1959, pp. 447–448)

Therefore, he supported a ban on the teaching of the idea as anything but hypothesis. In 1616 he delivered to Galileo the papal command not to "hold or defend" the heliocentric idea. In the discussions leading to the ban, he was a moderate, as the Dominican party wished to forbid teaching heliocentrism in any way whatever. Galileo's heresy trial in 1633 involved making fine distinctions between "teaching" and "holding and defending as true".

The official opposition of the Church to heliocentrism did not by any means imply opposition to all astronomy; indeed, it needed observational data to maintain its calendar. In support of this effort it allowed the cathedrals themselves to be used as solar observatories called meridiane; i.e., they were turned into "reverse sundials", or gigantic pinhole cameras, where the Sun's image was projected from a hole in a window in the cathedral's lantern onto a meridian line.

In 1664, Pope Alexander VII published his Index Librorum Prohibitorum Alexandri VII Pontificis Maximi jussu editus which included all previous condemnations of geocentric books. An annotated copy of Principia by Isaac Newton was published in 1742 by Fathers le Seur and Jacquier of the Franciscan Minims, two Catholic mathematicians with a preface stating that the author's work assumed heliocentrism and could not be explained without the theory. Pope Benedict XIV suspended the ban on heliocentric works on April 16, 1757 based on Isaac Newton's work. Pope Pius VII approved a decree in 1822 by the Sacred Congregation of the Inquisition to allow the printing of heliocentric books in Rome.

The view of modern science

The realization that the heliocentric view was also not true in a strict sense was achieved in steps. That the Sun was not the centre of the universe, but one of innumerable stars, was strongly advocated by the mystic Giordano Bruno; Galileo made the same point, but said very little on the matter, perhaps not wishing to incur the church's wrath. Over the course of the 18th and 19th centuries, the status of the Sun as merely one star among many became increasingly obvious. By the 20th century, even before the discovery that there are many galaxies, it was no longer an issue.

Even if the discussion is limited to the solar system, the sun is not at the geometric centre of any planet's orbit, but rather at one focus of the elliptical orbit. Furthermore, to the extent that a planet's mass cannot be neglected in comparison to the Sun's mass, the center of gravity of the solar system is displaced slightly away from the centre of the Sun. (The masses of the planets, mostly Jupiter, amount to 0.14% of that of the Sun.) Therefore a hypothetical astronomer on an extrasolar planet would observe a "wobble".

Giving up the whole concept of being "at rest" is related to the principle of relativity. While, assuming an unbounded universe, it was clear there is no privileged position in space, until postulation of the special theory of relativity by Albert Einstein, at least the existence of a privileged class of inertial systems absolutely at rest was assumed, in particular in the form of the hypothesis of the luminiferous aether. Some forms of Mach's principle consider the frame at rest with respect to the masses in the universe to have special properties.

Modern use of geocentric and heliocentric

In modern calculations, the origin and orientation of a coordinate system often have to be selected. For practical reasons, systems with their origin in the center of Earth's mass, solar mass or in the centre of mass of solar system are frequently selected. The adjectives geocentric or heliocentric may be used in this context. However, such selection of coordinates has no philosophical or physical implications.

Fred Hoyle wrote:

The relation of the two pictures [geocentricity and heliocentricity] is reduced to a mere coordinate transformation and it is the main tenet of the Einstein theory that any two ways of looking at the world which are related to each other by a coordinate transformation are entirely equivalent from a physical point of view. (Hoyle, 1973, p. 78)
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