Heliocentrism

Heliocentric Solar System

In astronomy, heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System. The word is derived from the Greek (Helios = "Sun" and kentron = "Center"). Historically, heliocentrism is opposed to geocentrism and currently to modern geocentrism, which places the earth at the center. (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 center of a major dispute.

Development of the idea

To anyone who stands and looks at the sky, it seems clear that the earth stays in one place while everything in the sky 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.

The counter-intuitive idea that it is the Earth, not the heavens, that is actually moving, was suggested at least as early as 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.

Heliocentrism was first proposed by Aristarchus (c. 270 BC). By the time he was writing, the size of the Earth had been calculated accurately by Eratosthenes, and he himself measured the size and distance of the Moon and Sun; while his figures for the Moon were fairly decent, those he obtained for the Sun were quite far off by modern standards, but a serious start. Perhaps, as many people have suggested, paying attention to these numbers led him to think that it made more sense for the Earth to be moving than for the huge Sun to be moving around it.

Aristarchus' 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 center. 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 center, 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 stellar parallax would be beyond current observers' ability to detect, as it was in fact. [1] This treatment shows that the problem of stellar parallax was well understood, though it produces no information on whether the Earth's motion was a reality.

Many ancient Indian astronomers have also referred to the concept of heliocentrism. In ancient Vedic literature the Sun is referred to as the centre of spheres, and the word Surya is used to refer to both the sun and the stars. In the 5th century CE, Aryabhata (apparently independently of Aristarchus) propounded in his treatise Āryabhatīya, a theory 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 orbits of the planets around the sun are ellipses. Bhaskara (1114-1185) also made references to heliocentrism in his magnum opus Siddhanta-Shiromani. Although these works were not translated into Latin until after Copernicus had written De revolutionibus orbium coelestium, Arabic translations of the Āryabhatīya were available in the Middle East by the 10th century, before the Crusades took place, so it's likely that Aryabhata's work had an influence on Copernicus' ideas.

Copernicus also drew on the ideas of the (possibly semi-mythical) Egyptian philosopher Hermes Trismegistus in De revolutionibus orbium coelestium but it is not clear to what extent Trismegistus was actually proposing a heliocentric world view. Accurate dating is impossible, but Trismegistus probably lived in pharaonic Egypt, although actual texts we have that constitute the hermetic tradition probably date from about the time of Christ.

For many centuries, Heliocentrism was countered in Europe with 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.

In the 16th century the theory 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 Tychonian 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 the Arab astronomer Ibn al-Shatir and the Persian Nasir al-Din al-Tusi; they had resolved significant problems in the Ptolemaic system, though retaining an essentially geocentric arrangement.

Religious disputes over heliocentrism

As early as the time of Aristarchus, the heliocentric idea was denounced as being against religion. 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 center 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 Tychonian 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.[2]

Catholic scientists also:

appreciated that the reference to heresy in connection with Galileo or Copernicus had no general or theological significance, (Heilbron 1999).

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 center 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.

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 center 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 not an issue.

Even if the discussion is limited to the solar system, the sun is not at the geometric center 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 center of the Sun. (The masses of the planets, mostly Jupiter, amount to 0.14% of that of the Sun.) Therefore an astronomer on a hypothetical extrasolar planet would observe a "wobble".

Giving up the whole concept of a center of the universe and even of being "at rest" is related to the principle of relativity. While 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 center 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)

References

• Fantoli, Annibale (2003). Galileo—For Copernicanism and the Church, third English edition, tr. George V. Coyne, SJ. Vatican Observatory Publications, Notre Dame, IN. ISBN 88-209-7427-4
• Heilbron, J.L. (1999).The Sun in the Church: Cathedrals as Solar Observatories. Harvard University Press, Cambridge, MA.
• Hoyle, Sir Fred (1973). Nicolaus Copernicus. Heinemann Educational Books Ltd., London.
• Koestler, Arthur (1959). The Sleepwalkers: a history of man's changing vision of the universe. Hutchinson, London.