{{Otheruses4|the historical term|modern geocentrism|Modern geocentrism}}
{{Redirect|Geocentric|orbits around the Earth|Geocentric orbit}}
In
astronomyAstronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
, the
geocentric model or the Ptolemaic worldview of the
universeThe Universe comprises everything that physically exists, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them...
is the theory, now superseded, that the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
is the center of the universe and other objects go around it. Belief in this system was common in
ancient GreeceAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
. It was embraced by both
AristotleAristotle was a Greek philosopher, a student of Plato and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology.Together with Plato and Socrates , Aristotle is one of...
(see
Aristotelian physicsThe Greek philosopher Aristotle developed many theories on the nature of physics. These involved what Aristotle described as the four elements...
) and
PtolemyClaudius Ptolemaeus , known in English as Ptolemy , was a Roman citizen of Greek ancestry. He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology. He lived in Egypt under the Roman Empire, and is believed to have been born in the town of...
, and most, but not all,
Ancient Greek philosophersGreek philosophy focused on the role of reason and inquiry. Many philosophers today concede that Greek philosophy has shaped the entire Western thought since its inception...
assumed that the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
,
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
,
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
s, and naked eye planets circle the Earth. Similar ideas were held in
ancient ChinaAstronomy in China has a very long history, and historians consider that, 'they were the most persistent and accurate observers of celestial phenomena anywhere in the world before the Arabs'....
.
Two common observations were believed to support the idea that the Earth is in the center of the Universe: The first observation is that the stars, sun, and planets appear to revolve around the Earth each day, with the stars circling around the pole and those stars nearer the
equatorThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
rising and setting each day and circling back to their rising point; the second is the common sense perception that as the Earth is solid and stable it is not moving—but is at rest.
The geocentric model was usually combined with a
spherical EarthThe concept of a spherical Earth dates back to around the 6th century BCE in ancient Greek philosophy. It remained a matter of philosophical speculation until the 3rd century BCE when Hellenistic astronomy established the spherical shape of the earth as a physical given.The concept of a spherical...
by ancient Greek and medieval philosophers. It is not the same as the older
flat EarthThe Flat Earth model is a view that the Earth's shape is a flat plane.Various cultures have had conceptions of a flat Earth, including ancient Babylon, Ancient Egypt, pre-Classical Greece and pre-17th century China. This view contrasts with the realization first recorded around the 4th century BC...
model implied in some
mythologyMythology is the study of myths and or of a body of myths. For example, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece. The term "myth" is often used colloquially to refer to a false story;...
. The ancient Greeks believed that the motions of the planets were circular and not elliptical, a view that was not challenged in
Western cultureWestern culture refers to cultures of European origin.The term "Western culture" is used very broadly to refer to a heritage of social norms, ethical values, traditional customs, religious beliefs, political systems, and specific artifacts and technologies...
before the 17th century.
The geocentric model held sway into the early modern age; from the late
16th centuryAs a means of recording the passage of time, the 16th century lasted from 1501 through 1600.During the 16th century, Spain and Portugal explored and conquered the world seas. Latin America became a Spanish colony, while Portugal became the master of the Indian Ocean.In Europe, the Protestant...
onward it was gradually replaced by the
heliocentric modelIn astronomy, heliocentrism is the theory that the Sun is stationary and at the center of the universe. The word came from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. Discussions on the possibility of heliocentrism date to classical...
of
CopernicusNicolaus Copernicus was the first astronomer to formulate a comprehensive heliocentric cosmology, which displaced the Earth from the center of the universe...
,
GalileoGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
and
KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and key figure in the 17th century scientific revolution. He is best known for his eponymous laws of planetary motion, codified by later astronomers based on his works Astronomia nova, Harmonices Mundi, and Epitome of...
.
Classical Greece
The geocentric model entered
Greek astronomyGreek astronomy is the astronomy of those who wrote in the Greek language in classical antiquity; for example, Aristarchus of Samos Greek astronomer/mathematician and his heliocentric model of the solar system. Greek astronomy is understood to include the ancient Greek, Hellenistic, Greco-Roman,...
and philosophy at an early point; it can be found in
Pre-Socratic philosophyThe pre-Socratic Greek philosophers were active before Socrates or contemporaneously, but expounding knowledge developed earlier. The popularity of the term originates with Hermann Diels' work Die Fragmente der Vorsokratiker...
. In the
6th centuryThe 6th century is the period from 501 to 600 in accordance with the Julian calendar in the Christian/Common Era. In the West this century marks the end of Classical Antiquity and the beginning of the Middle Ages.- Overview :...
BC,
AnaximanderAnaximander was a pre-Socratic Greek philosopher who lived in Miletus, a city of Ionia. He belonged to the Milesian school and learned the teachings of his master Thales. He succeeded him and became the second master of that school where he counted Anaximenes and Pythagoras amongst his...
proposed a cosmology with the Earth shaped like a section of a pillar (a cylinder), held aloft at the center of everything. The Sun, Moon, and planets were holes in invisible wheels surrounding the Earth; through the holes, humans could see concealed fire. About the same time, the Pythagoreans thought that the Earth was a sphere (in accordance with observations of eclipses), but not at the center; they believed that it was in motion around an unseen fire. Later these views were combined, so most educated Greeks from the 4th century BC on thought that the Earth was a sphere at the center of the universe.
In the
4th centuryAs a means of recording the passage of time, the 4th century was that century which lasted from 301 to 400.- Overview :...
BC, two influential Greek philosophers wrote works based on the geocentric model. These were
PlatoPlato , was a Classical Greek philosopher, mathematician, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world...
and his student
AristotleAristotle was a Greek philosopher, a student of Plato and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology.Together with Plato and Socrates , Aristotle is one of...
. According to Plato, the Earth was a sphere, stationary at the center of the universe. The stars and planets were carried around the Earth on
spheres or circlesThe celestial spheres, or celestial orbs, were the fundamental celestial entities of the cosmological celestial mechanics first invented by Eudoxus, and developed by Aristotle, Ptolemy, Copernicus and others...
, arranged in the order (outwards from the center): Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, fixed stars. In the "
Myth of ErThe Myth of Er is an eschatological legend that concludes Plato's dialogue known as The Republic . The story includes an account of the cosmos and the afterlife that for many centuries greatly influenced religious, philosophical and scientific thought.The story begins as a man named Er dies in battle...
," a section of the
Republic, Plato describes the cosmos as the Spindle of Necessity, attended by the Sirens and turned by the three Fates.
Eudoxus of CnidusEudoxus of Cnidus was a Greek astronomer, mathematician, scholar and student of Plato. Since all his own works are lost, our knowledge of him is obtained from secondary sources, such as Aratus's poem on astronomy...
, who worked with Plato, developed a less mythical, more mathematical explanation of the planets' motion based on Plato's
dictumIn legal terminology, dictum is a statement of opinion or belief considered authoritative because of the dignity of the person making it....
stating that all phenomena in the heavens can be explained with uniform circular motion. Aristotle elaborated on Eudoxus' system. In the fully developed Aristotelian system, the spherical Earth is at the center of the universe. All heavenly bodies are attached to 56 concentric spheres which rotate around the Earth . (The number is so high because several transparent spheres are needed for each planet.) The Moon is on the innermost sphere. Thus it touches the realm of Earth, which contaminates it, causing the dark spots (
maculaThe sonya or dean is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
) and the ability to go through lunar phases. It is not perfect like the other heavenly bodies, which shine by their own light.{{Facts|date=August 2009}}
Adherence to the geocentric model stemmed largely from several important observations. First of all, if the Earth did move, then one ought to be able to observe the shifting of the fixed stars due to
parallaxParallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines...
. In short, the shapes of the
constellationIn modern astronomy, a constellation is an area of the celestial sphere, defined by exact boundaries.The term "constellation" can also be used loosely to refer to just the more prominent visible stars that seem to form a pattern in that area.-Definitions:...
s should change considerably over the course of a year, or else the stars are so much further away than the Sun and the planets that this motion would be undetectable. Stellar parallax was not detected until the 19th century as the distances from the Earth to the stars made the effect extremely subtle, so the Greeks chose the simpler of the two explanations (either the Earth is not moving and so no effect exists, or the stars are so far away the effect was undetectable). The lack of any observable parallax was considered a fatal flaw of any non-geocentric theory.
Another important observation was that Venus stays about the same brightness most of the time, implying that it is usually about the same distance from Earth, which is more consistent with geocentrism than heliocentrism. In reality, that is because the loss of light caused by its phases compensates for the increase in apparent size caused by its varying distance from Earth. Other objections included the idea, put forward by Aristotle, that the natural state of heavy objects like the Earth was at rest, and that some force was required to move them. It was also believed by some that the Earth's rotation on its axis would cause the air and objects in it (such as birds or clouds) to be left behind.
A major flaw in the Eudoxan and Aristotelian models based on concentric spheres was that they could not explain the changes in brightness of the planets caused by a change in distance.
Claudius Ptolemy
Although the basic tenets of Greek geocentrism were established by the time of Aristotle, the details of his system did not become standard. This honor was reserved for the
Ptolemaic system, espoused by the
HellenisticHellenization is a term used to describe the spread of ancient Greek culture, and to a lesser extent, language. It is mainly used to describe the spread of Hellenistic civilization during the Hellenistic period following the campaigns of Alexander the Great of Macedon...
astronomer
Claudius PtolemaeusClaudius Ptolemaeus , known in English as Ptolemy , was a Roman citizen of Greek ancestry. He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology. He lived in Egypt under the Roman Empire, and is believed to have been born in the town of...
in the 2nd century AD. His main astronomical book, the
AlmagestAlmagest is the Latin form of the Arabic name of a mathematical and astronomical treatise proposing the complex motions of the stars and planetary paths, originally written in Greek as by Ptolemy of Alexandria, Egypt, written in the 2nd century...
, was the culmination of centuries of work by
HellenicAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
,
HellenisticHellenistic civilization represents the zenith of Greek influence in the ancient world from 323 BC to about 146 BC ; note, however that Koine Greek language and Hellenistic philosophy and religion are also indisputably elements of the Roman era till Late Antiquity...
and Babylonian astronomers; it was accepted for over a millennium as the correct cosmological model by European and
Islamic astronomersIn the history of astronomy, Islamic astronomy or Arabic astronomy refers to the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age , and mostly written in the Arabic language. These developments mostly took place in the Middle East, Central Asia,...
. Because of its influence, the Ptolemaic system is sometimes considered identical with the
geocentric model.
Ptolemy argued that the Earth was in the center of the universe from the simple observation that half the stars were above the horizon and half were below the horizon at any time, and the assumption that the stars were all at some modest distance from the center of the universe. If the Earth were substantially displaced from the center, this division into visible and invisible stars would not be equal.
Ptolemaic system
In the Ptolemaic system, each planet is moved by two or more spheres: one sphere is its deferent. The deferent was a circle centered around a point halfway between the
equantEquant is a mathematical concept developed by Claudius Ptolemy in the 2nd century AD to account for the observed motion of heavenly bodies....
and the earth. Another sphere is the
epicycle which is embedded in the deferent. The planet is embedded in the epicycle sphere. The deferent rotates around the Earth while the epicycle rotates within the deferent, causing the planet to move closer to and farther from Earth at different points in its orbit, and even to slow down, stop, and move backward (in retrograde motion). The epicycles of Venus and Mercury are always centered on a line between Earth and the Sun (Mercury being closer to Earth), which explains why they are always near it in the sky. The Ptolemaic order of spheres from Earth outward is:
- Moon
The Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
- Mercury
For the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three...
- Venus
Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6...
- Sun
The Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
- Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
- Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
- Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant...
- Fixed Stars
- Sphere of Prime Mover
The deferent-and-epicycle model had been used by Greek astronomers for centuries, as had the idea of the
eccentric (a deferent which is slightly off-center from the Earth). In the illustration, the center of the deferent is not the Earth but X, making it eccentric (from the Latin
ex- or
e- meaning "from," and
centrum meaning "center"). Unfortunately, the system that was available in Ptolemy's time did not quite match
observationObservation is either an activity of a living being , consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments. The term may also refer to any datum collected during this activity.-Observation in science:A scientific method...
s, even though it was considerably improved over Aristotle's system. Sometimes the size of a planet's retrograde loop (most notably that of Mars) would be smaller, and sometimes larger. This prompted him to come up with the idea of an
equantEquant is a mathematical concept developed by Claudius Ptolemy in the 2nd century AD to account for the observed motion of heavenly bodies....
. The equant was a point near the center of a planet's orbit which, if you were to stand there and watch, the center of the planet's epicycle would always appear to move at the same speed. Therefore, the planet actually moved at different speeds when the epicycle was at different points on its deferent. By using an equant, Ptolemy claimed to keep motion which was uniform and circular, but many people did not like it because they did not think it was true to Plato's dictum of "uniform circular motion." The resultant system which eventually came to be widely accepted in the west was an unwieldy one to modern eyes; each planet required an epicycle revolving on a deferent, offset by an equant which was different for each planet. But it predicted various celestial motions, including the beginnings and ends of retrograde motion, fairly well at the time it was developed.
Geocentrism and rival systems
Not all Greeks agreed with the geocentric model. The
PythagoreanPythagorean means of or pertaining to the ancient Ionian mathematician, philosopher, and music theorist Pythagoras. See:-Philosophy:* Pythagoreanism is a term used for the esoteric and metaphysical beliefs purported to have been held by Pythagoras....
system has already been mentioned; some Pythagoreans believed the Earth to be one of several planets going around a central fire.
HicetasHicetas was a Greek philosopher of the Pythagorean School. He was born in Syracuse. Like his fellow Pythagorean Ecphantus and the Academic Heraclides Ponticus, he believed that the daily movement of permanent stars was caused by the rotation of the Earth around its axis...
and Ecphantus, two Pythagoreans of the 5th century BC, and
Heraclides PonticusHeraclides Ponticus , also known as Herakleides, was a Greek philosopher who lived and died at Heraclea Pontica, now Karadeniz Ereğli, Turkey....
in the 4th century BC, believed that the Earth rotated on its axis but remained at the center of the universe. Such a system still qualifies as geocentric. It was revived in the
Middle AgesThe Middle Ages of European history is a period of European history covering roughly a millennium in the 5th century through 16th centuries. More specific starting and ending points are sometimes adopted by scholars to suit their respective specializations or current focus...
by
Jean BuridanJean Buridan was a French priest who sowed the seeds of the Copernican revolution in Europe. Although he was one of the most famous and influential philosophers of the late Middle Ages, he is today among the least well known...
. Heraclides Ponticus is also sometimes said to have proposed that both Venus and Mercury went around the Sun rather than Earth, but the evidence for this claim is not clear.
Martianus CapellaMartianus Minneus Felix Capella was a pagan writer of Late Antiquity, one of the earliest developers of the system of the seven liberal arts that structured Early Medieval education...
definitely put Mercury and Venus on epicycles around the Sun.
Aristarchus of SamosAristarchus was a Greek astronomer and mathematician, born on the island of Samos, in Greece. He was the first person to present an explicit argument for a heliocentric model of the solar system, placing the Sun, not the Earth, at the center of the known universe...
was the most radical. He wrote a work, which has not survived, on
heliocentrismIn astronomy, heliocentrism is the theory that the Sun is stationary and at the center of the universe. The word came from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. Discussions on the possibility of heliocentrism date to classical...
, saying that the Sun was at the center of the universe, while the Earth and other planets revolved around it. His theory was not popular, and he had one named follower,
Seleucus of SeleuciaSeleucus of Seleucia was a Hellenistic astronomer and philosopher from the Seleucia region of Mesopotamia who supported the heliocentric theory of planetary motion. Seleucus is known from the writings of Plutarch, Strabo and Aetius...
.
Maragha system
The "Maragha Revolution" refers to the Maragheh school's revolution against Ptolemaic astronomy. The "Maragha school" was an astronomical tradition beginning in the Maragheh observatory and continuing with astronomers from Damascus and Samarkand. Like their Andalusian predecessors, the Maragha astronomers attempted to solve the equant problem (the circle around whose circumference a planet or the center of an epicycle was conceived to move uniformly) and produce alternative configurations to the Ptolemaic model without abandoning the geocentric model. They were more successful than their Andalusian predecessors in producing non-Ptolemaic configurations which eliminated the equant and eccentrics, were more accurate than the Ptolemaic model in numerically predicting planetary positions, and were in better agreement with empirical observations.[83] The most important of the Maragha astronomers included Nasīr al-Dīn al-Tūsī (1201-1274), Qutb al-Din al-Shirazi (1236–1311), al-Birjandi (d. 1525) and Shams al-Din al-Khafri (d. 1550).{{Citation needed|date=November 2008}}.
Ibn Al-Shatir, the Damascene astronomer (1304–1375 A.D), wrote a major book entitled "Kitab Nihayat al-Sul fi Tashih al-Usul" ( A Final Inquiry Concerning the Rectification of Planetary Theory) on a theory which departs largely from the Ptolemaic system known at that time. In his book "Ibn al-Shatir, an Arab astronomer of the fourteenth century," E.S.Kennedy wrote "what is of most interest, however, is that Ibn al-Shatir's lunar theory, except for trivial differences in parameters, is identical with that of Copernicus (1473–1543 A.D)." The discovery that the models of Ibn al-Shatir are mathematically identical to those of Copernicus raised the very interesting question of a possible transmission of these models to Europe.
Copernican system
{{Main|Copernican heliocentrism}}
In 1543, the geocentric system met its first serious challenge with the publication of Copernicus'
De revolutionibus orbium coelestiumNicolai Copernici Torinensis De revolutionibus orbium coelestium, Libri VI , first printed in 1543 in Nuremberg, is the seminal work on heliocentric theory and the masterpiece of astronomer Nicolaus Copernicus...
, which posited that the Earth and the other planets instead revolved around the Sun. The geocentric system was still held for many years afterwards, as at the time the Copernican system did not offer better predictions than the geocentric system, and it posed problems for both
natural philosophyNatural philosophy or the philosophy of nature , is a term applied to the study of nature and the physical universe that was dominant before the development of modern science...
and scripture.
With the invention of the
telescopeA telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century...
in 1609, observations made primarily by
Galileo GalileiGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
(such as that
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
has moons) called into question some of the tenets of geocentrism but did not seriously threaten it.
In December 1610, Galileo Galilei used his telescope to observe that
VenusVenus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6...
showed all
phaseThe phases of the planet Venus are the different variations of lighting seen on the planet's surface, similar to lunar phases. The first recorded observations of them were telescopic observations by Galileo Galilei in 1610...
s, just
like the MoonA lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun...
. He thought that while this observation was incompatible with the Ptolemaic system, it was a natural consequence of the heliocentric system.
However, Ptolemy placed Venus' deferent and epicycle entirely inside the sphere of the Sun (between the Sun and Mercury), but this was arbitrary; he could just as easily have swapped Venus and Mercury and put them on the other side of the Sun, or made any other arrangement of Venus and Mercury, as long as they were always near a line running from the Earth through the Sun, such as placing the center of the Venus epicycle near the Sun. In this case, if the Sun is the source of all the light, under the Ptolemaic system:
{{quote|If Venus is between Earth and the Sun, the phase of Venus must always be
crescentIn art and symbolism, a crescent is generally the shape produced when a circular disk has a segment of another circle removed from its edge, so that what remains is a shape enclosed by two circular arcs of different diameters which intersect at two points .In astronomy, a crescent is...
or all dark.
If Venus is beyond the Sun, the phase of Venus must always be gibbous or full.}}
But Galileo saw Venus at first small and full, and later large and crescent.
This showed that with a Ptolemaic cosmology, the Venus epicycle can be neither completely inside nor completely outside of the orbit of the Sun. As a result, Ptolemaics abandoned the idea that the epicycle of Venus was completely inside the Sun, and later 17th century competition between astronomical cosmologies focused on variations of
Tycho Brahe'sTycho Brahe, born Tyge Ottesen Brahe , was a Danish nobleman known for his accurate and comprehensive astronomical and planetary observations...
Tychonic systemThe Tychonic system was a model of the solar system published by Tycho Brahe in the late 16th century which combined what he saw as the mathematical benefits of the Copernican system with the philosophical and "physical" benefits of the Ptolemaic system. The model may have been inspired by Paul...
(in which the Earth was still at the center of the universe, and around it revolved the Sun, but all other planets revolved around the Sun in one massive set of epicycles), or variations on the Copernican system.
Gravitation
Johannes KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and key figure in the 17th century scientific revolution. He is best known for his eponymous laws of planetary motion, codified by later astronomers based on his works Astronomia nova, Harmonices Mundi, and Epitome of...
, after analysing Tycho Brahe's observations, constructed his
three lawsIn astronomy, Kepler's three laws of planetary motion are:#The orbit of every planet is an ellipse with the sun at a focus.#A line joining a planet and the sun sweeps out equal areas during equal intervals of time....
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
transitThe term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, as seen by an observer at some particular vantage point.* A transit occurs when a celestial...
of Venus (for the year 1631).
In 1687,
Isaac NewtonSir Isaac Newton FRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian who is perceived and considered by a substantial number of scholars and the general public as one of the most influential men in history...
devised his law of universal gravitation, which introduced gravitation as the force that both kept the Earth and planets moving through the heavens and also kept the air from flying away, allowing scientists to quickly construct a plausible heliocentric model for the solar system.
In 1838, astronomer Friedrich Wilhelm Bessel successfully measured the
parallaxParallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines...
of the star
61 Cygni61 Cygni,
[Not to be confused with 16 Cygni, a more distant system containing two G-type stars harboring the gas giant planet 16 Cygni Bb.] sometimes called Bessel's Star or Piazzi's Flying Star, is a binary star system in the constellation Cygnus...
, disproving Ptolemy's assertion that parallax motion did not exist.
A geocentric frame is useful for many everyday activities and most laboratory experiments, but is a less appropriate choice for solar-system mechanics and space travel. While a
heliocentric frameIn astronomy, heliocentrism is the theory that the Sun is stationary and at the center of the universe. The word came from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. Discussions on the possibility of heliocentrism date to classical...
is most useful in those cases, galactic and extra-galactic astronomy is easier if the sun is treated as neither stationary nor the center of the universe, but rotating around the center of our galaxy, and in turn our galaxy is also not at rest in the cosmic background.
Modern geocentrism
{{Main|Modern geocentrism}}
Individuals of some religions interpret their scriptures literally as stating that the Earth is the physical center of the universe. This requires the Sun to revolve around the Earth instead of the other way around because if the Earth were moving it could not continuously be in the center of the universe. This is known as modern geocentrism.
AstrologersPlanets in astrology have a meaning different from the modern astronomical understanding of what a planet is. Before the age of telescopes, the night sky was observed to consist of two very similar components: fixed stars, which remained motionless in relation to each other, and wandering stars, ...
, while they may not believe in geocentrism as a principle, still employ the geocentric model in their calculations.{{Citation needed|date=June 2007}}
The contemporary Association for Biblical Astronomy, led by physicist Dr. Gerardus Bouw, holds to a modified version of the model of Tycho Brahe, which they call geocentricity.
A study done in 2005 by Dr. Jon D. Miller of
Northwestern University{{Infobox university|name = Northwestern University|image_name = NU seal.png|motto = Quaecumque sunt vera |mottoeng =Whatsoever things are true |established = 1851|type = Private|calendar = Quarter...
, an expert in the public understanding of science and technology, found that one adult American in five thinks the Sun revolves around the Earth.
Planetariums
The geocentric (Ptolemaic) model of the
solar systemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
is still of interest to
planetariumA planetarium is a theatre built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation...
makers, as, for technical reasons, a Ptolemaic-type motion for the planet light apparatus has some advantages over a Copernican-type motion. The
celestial sphereIn astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
, still used for teaching purposes and sometimes for navigation, is also based on a geocentric system.
Geocentric models in science fiction
Alternate history
science fictionScience fiction is a genre of fiction. It differs from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically-established or scientifically-postulated laws of nature...
has produced some literature of interest on the proposition that some alternate universes and Earths might indeed have laws of physics and cosmologies that are Ptolemaic and Aristotelian in design. This subcategory began with
Philip Jose Farmer'sPhilip José Farmer was an American author, principally known for his award-winning science fiction and fantasy novels and short stories....
short storyA short story is a work of fiction that is usually written in prose, often in narrative format. This format or medium tends to be more pointed than longer works of fiction, such as novellas and novels or books...
,
Sail On! Sail On!"Sail On! Sail On!" is a alternate history short story from Philip José Farmer, originally published in 1952. In this alternative 1492, the Earth is flat, despite scepticism from scientists and philosophers over this geological provenance...
(1952), where Columbus has access to
radioRadio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
technology, and where his
SpanishSpain , officially the Kingdom of Spain , is a country located in southwestern Europe on the Iberian Peninsula.
[The Spanish constitution does not establish any official denomination of the country, even though España , Estado español and Nación española are used interchangeably...]
-financed exploratory and trade fleet sail off the edge of the (flat) world in his geocentric alternate universe in 1492, instead of discovering
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
and
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
.
Richard Garfinkle'sRichard Garfinkle is an American writer of science fiction.He is best known as the author of Celestial Matters, a novel published by Tor Books, which was nominated for the Hugo Award and Nebula Award and won the Compton Crook Award in 1997....
Celestial MattersCelestial Matters is a science fantasy novel, set in an alternate universe with different laws of physics, written by Richard Garfinkle and published by Tor Books in 1996...
(1996) is set in a more elaborated geocentric cosmos, where Earth is divided by two contending factions, the
Classical GreeceClassical Greece was a culture that was highly advanced and which heavily influenced the cultures of Ancient Rome and still has an enduring effect on Western Civilization. Much of modern politics, artistic thought, scientific thought, literature, and philosophy derives from this ancient society...
-dominated
Delian LeagueThe Delian League was an association of approximately 150 5th-century BC Greek city-states under the leadership of Athens, whose purpose was to continue fighting the Persian Empire after the Greek victory in the Battle of Plataea at the end of the Greco–Persian Wars...
and the (Chinese)
Middle KingdomThe Middle Kingdom may refer to*China*The Middle Kingdom of Egypt*A group of midwest U.S. states associated with the Society for Creative Anachronism*A part of Ocean Park Hong Kong*Middle Francia*The Middle Kingdom, an album by Celtic metal band Cruachan...
, both of which are capable of flight within an alternate universe based on Ptolemaic astronomy,
Aristotle'sAristotle was a Greek philosopher, a student of Plato and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology.Together with Plato and Socrates , Aristotle is one of...
physics and Taoist thought. Unfortunately, both superpowers have been fighting a thousand-year war since the time of
Alexander the GreatAlexander III of Macedon, popularly known as Alexander the Great , was an Ancient Greek king of Macedon who created one of the largest empires in ancient history...
.
External links
{{Greek astronomy}}
{{DEFAULTSORT:Geocentric Model}}