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Heliocentrism

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

   Heliocentric Solar System
   Enlarge
   Heliocentric Solar System
   Heliocentrism (lower panel) in comparsion to the geocentric model
   (upper panel)
   Enlarge
   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 (8.7.3.10) 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.
   Enlarge
   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
   Enlarge
   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
   Enlarge
   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
   Enlarge
   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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