Everything about Heliocentrism totally explained
In
astronomy,
heliocentrism is the theory that the sun is at the center of the
Solar System. The word came from the
Greek (ήλιος
Helios = sun and κέντρον
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 wasn't clear until modern times, but extremely important relative to the controversy over
cosmology and
religion.) Although many early cosmologists such as
Aristarchus speculated about the motion of the Earth around a stationary Sun, it wasn't until the
16th century that
Copernicus presented a fully predictive mathematical model of a heliocentric system, which was later elaborated by
Kepler and defended by
Galileo, becoming the center 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, which, though considered incorrect today, still manages to calculate the correct positions for the planets to a moderate degree of accuracy, though Ptolemy's demand that all epicycles be not eccentric causes needless problems for the motions of Mars and especially Mercury. It is interesting to note that
Ptolemy, himself, in his Almagest points out that any model for describing the motions of the planets is merely a mathematical device, and, since there's no actual way to know which is true, the simplest model that gets the right numbers should be used; however, he himself chose the epicyclic geocentric model and in his ultimate work
Planetary Hypotheses treated his models as sufficiently real that the distances of moon, sun, planets and stars were determinable by treating orbits' celestial spheres as contiguous realities. This made the stars' distance less than 20
Astronomical Units — a subtraction from science since Aristarchus's heliocentric scheme had already centuries earlier
necessarily placed the stars at least two orders of magnitude more distant.
Philosophical discussions
Philosophical arguments on heliocentrism involve general statements that the Sun is at the center of the universe or that some or all of the planets revolve around the Sun, and arguments supporting these claims. These ideas can be found in a range of
Sanskrit,
Greek,
Arabic and
Latin texts. Few of these early sources, however, develop techniques to compute any observational consequences of their proposed heliocentric ideas.
Ancient India
According to
theosophists, the earliest traces of a
counter-intuitive idea that it's the Earth that's actually moving and the Sun that's 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:
Vishnu Purana (2.8) (c. 1st century BC).
Yajnavalkya recognized that the Sun was much larger than the Earth, which would have influenced this early heliocentric concept.
The
Aitareya Brahmana (2.7) (c. 9th–8th century BC) also states:
Seleucus of Seleucia (b. 190 BC) adopted the heliocentric system of Aristarchus, and according to
Plutarch, may have even proved it. His proposed proof may have been related to his observations of 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.
In the medieval
Islamic civilization, due to the scientific dominance of the Ptolemaic system in early
Islamic astronomy, most
Muslim astronomers accepted the geocentric model. However, several Muslim scholars had discussions on whether the Earth moved and tried to explain how this might be possible. but many agree that he was actually criticizing the details of Ptolemy's model rather than his geocentrism. Alhacen did, however, later propose the
Earth's rotation on its axis in
The Model of the Motions (c. 1038). In 1030,
al-Biruni discussed the Indian astronomical theories of
Aryabhata,
Brahmagupta and
Varahamihira in his
Indica. Al-Biruni agreed with the
Earth's rotation about its own axis, and while he was initially neutral regarding the heliocentric and
geocentric models, he noted that heliocentrism was a philosophical problem, rather than a mathematical problem. Abu Said
al-Sijzi, a contemporary of al-Biruni, suggested the possible movement of the Earth around the Sun, which Biruni didn't reject.
Qutb al-Din (b. 1236), in his
The Limit of Accomplishment concerning Knowledge of the Heavens, also discussed whether heliocentrism was a possibility.
Medieval Europe
Heliocentric ideas were known in Europe before Copernicus. Explorers and traders were increasingly venturing out beyond Europe and introducing the West to the
Indian heliocentric traditions as detailed above (cf. the
Silk Road and Muslim commentators). Scholars were also aware of the arguments of
Aristarchus and
Philolaus, as well as several other thinkers who had proposed (or were alleged to have proposed) heliocentric or quasi-heliocentric views, such as
Hicetas,
Heraclides Ponticus, and
Martianus Capella.
During the
Late Middle Ages, Bishop
Nicole Oresme discussed the possibility that the Earth rotated on its axis, while Cardinal
Nicholas of Cusa in his
Learned Ignorance asked whether there was any reason to assert that the Sun (or any other point) was the center of the universe. In parallel to a mystical definition of God, Cusa wrote that "Thus the fabric of the world (
machina mundi) will
quasi have its center everywhere and circumference nowhere."
Mathematical astronomy
In
mathematical astronomy, computational models of heliocentrism involve mathematical computational systems that are tied to a heliocentric model and where positions of the planets can be computed. The first computational system explicitly tied to a heliocentric model was the
Copernican model described by
Copernicus, but there were earlier computational systems that may have implied some form of heliocentricity, notably
Aryabhata's model, which has astronomical parameters which some have interpreted to imply a form of heliocentricity. Several
Muslim astronomers also developed computational systems with astronomical parameters compatible with heliocentricity, as stated by
Biruni, but the concept of heliocentrism was considered a philosophical problem rather than a mathematical problem. Their astronomical parameters, however, were later adapted in the Copernican model in a heliocentric context.
Medieval India
Aryabhata (
476–
550), in his magnum opus
Aryabhatiya, propounded a computational system based on a planetary model in which the Earth was taken to be
spinning on its axis and the periods of the planets were given with respect to the Sun. Some have interpreted this to be a heliocentric model,
but this view has been disputed by others.
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
elliptical orbits, on which he accurately calculated many astronomical constants, such as the periods of the planets, times of the
solar and
lunar eclipses, and the instantaneous motion of the Moon (expressed as a
differential equation). Early followers of Aryabhata's model included
Varahamihira,
Brahmagupta, and
Bhaskara II.
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 possible that Aryabhata's work had an influence on Copernicus' ideas.
Nilakantha Somayaji (1444-1544), in his
Aryabhatiyabhasya, a commentary on Aryabhata's
Aryabhatiya, developed a computational system for a partially heliocentric planetary model, in which the planets orbit the Sun, which in turn orbits the Earth, similar to the
Tychonic system later proposed by
Tycho Brahe in the late 16th century. Nilakantha's system, however, was mathematically more effient than the Tychonic system, due to correctly taking into account the equation of the centre and
latitudinal motion of Mercury and Venus. Most astronomers of the
Kerala school of astronomy and mathematics who followed him accepted his planetary model.
Middle East
In the 2nd century BC, the Babylonian astronomer
Seleucus of Seleucia is said to have proved the heliocentric theory. According to
Bartel Leendert van der Waerden, Seleucus may have proved the heliocentric theory by determining the constants of a
geometric model for the heliocentric theory and by developing methods to compute planetary positions using this model. He may have used
trigonometric methods that were available in his time, as he was a contemporary of
Hipparchus.
In the 9th century, the Afghan astronomer
Ja'far ibn Muhammad Abu Ma'shar al-Balkhi developed a planetary model which can be interpreted as a heliocentric model. This is due to his
orbital revolutions of the planets being given as heliocentric revolutions rather than
geocentric revolutions, and the only known planetary theory in which this occurs is in the heliocentric theory. His work on planetary theory hasn't survived, but his astronomical data was later recorded by al-Hashimi and
Abū Rayhān al-Bīrūnī.
Al-Biruni discussed the possibility of whether the Earth rotated about its own axis and around the Sun, but in his
Masudic Canon, he set forth the principles that the Earth is at the center of the universe and that it has no motion of its own. He was aware that if the Earth rotated on its axis and around the Sun, this would be consistent with his astronomical parameters, but he considered this a philosophical problem rather than a mathematical problem. 'Umar al-Katibi al-
Qazwini (d. 1277), who also worked at the
Maragheh observatory, in his
Hikmat al-'Ain, wrote an argument for a heliocentric model, but later abandoned the model. while the works of
Alhacen and
Biruni were also known in Europe at the time. In fact, however, Aristarchus's heliocentrism appears to have attracted little attention, religious or otherwise, until Copernicus revived and elaborated it.
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 didn't 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 wasn't 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.
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:
Melanchthon, however, opposed the doctrine over a period of years.
Some years after the publication of
De Revolutionibus John Calvin preached a sermon in which he denounced those who "pervert the course of nature" by saying that "the sun doesn't move and that it's the earth that revolves and that it turns". On the other hand, Calvin isn't responsible for another famous quotation which has often been misattributed to him: It has long been established that this line can't be found in any of Calvin's works. It has been suggested that the quotation was originally sourced from the works of
Lutheran theologian
Abraham Calovius.
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. 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. 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:
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:
The official opposition of the Church to heliocentrism didn't 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; for example, 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 couldn't 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 wasn't 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 no longer an issue.
Even if the discussion is limited to the
solar system, the sun isn't 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 can't 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 a hypothetical astronomer on an
extrasolar planet would observe a "wobble" in his perception of the Sun's motion.
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's 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 mass, solar mass or in the center of mass of solar system are frequently selected. The adjectives may be used in this context. However, such selection of coordinates has no philosophical or physical implications.
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