Johannes Kepler: Three Scientific Theories About The Planet Movements
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Kepler's Laws of Planetary Motion
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By using an equant, Ptolemy claimed to keep motion which was uniform and circular, although it departed from the Platonic ideal of uniform circular motion. The resultant system, which eventually came to be widely accepted in the west, seems unwieldy to modern astronomers; each planet required an epicycle revolving on a deferent, offset by an equant which was different for each planet. It predicted various celestial motions, including the beginning and end of retrograde motion, to within a maximum error of 10 degrees, considerably better than without the equant. The model with epicycles is in fact a very good model of an elliptical orbit with low eccentricity. To summarize, Ptolemy devised a system that was compatible with Aristotelian philosophy and managed to track actual observations and predict future movement mostly to within the limits of the next years of observations.
The observed motions and his mechanisms for explaining them include:. The geocentric model was eventually replaced by the heliocentric model. Copernican heliocentrism could remove Ptolemy's epicycles because the retrograde motion could be seen to be the result of the combination of Earth and planet movement and speeds. Copernicus felt strongly that equants were a violation of Aristotelian purity, and proved that replacement of the equant with a pair of new epicycles was entirely equivalent. Astronomers often continued using the equants instead of the epicycles because the former was easier to calculate, and gave the same result. It has been determined [ by whom? They are computationally equivalent.
It wasn't until Kepler demonstrated a physical observation that could show that the physical sun is directly involved in determining an orbit that a new model was required. The Ptolemaic order of spheres from Earth outward is: . Ptolemy did not invent or work out this order, which aligns with the ancient Seven Heavens religious cosmology common to the major Eurasian religious traditions. It also follows the decreasing orbital periods of the Moon, Sun, planets and stars.
Early in the 11th century Alhazen wrote a scathing critique of Ptolemy 's model in his Doubts on Ptolemy c. In the 12th century, Arzachel departed from the ancient Greek idea of uniform circular motions by hypothesizing that the planet Mercury moves in an elliptic orbit ,   while Alpetragius proposed a planetary model that abandoned the equant , epicycle and eccentric mechanisms,  though this resulted in a system that was mathematically less accurate. His alternative system spread through most of Europe during the 13th century.
Fakhr al-Din al-Razi — , in dealing with his conception of physics and the physical world in his Matalib , rejects the Aristotelian and Avicennian notion of the Earth's centrality within the universe, but instead argues that there are "a thousand thousand worlds alfa alfi 'awalim beyond this world such that each one of those worlds be bigger and more massive than this world as well as having the like of what this world has. The "Maragha Revolution" refers to the Maragha school's revolution against Ptolemaic astronomy.
The "Maragha school" was an astronomical tradition beginning in the Maragha observatory and continuing with astronomers from the Damascus mosque and Samarkand observatory. 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 geocentrism. 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.
In his book, Ibn al-Shatir, an Arab astronomer of the fourteenth century , E. 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 — AD. However, the Maragha school never made the paradigm shift to heliocentrism. The possibility that Copernicus independently developed the Tusi couple remains open, since no researcher has yet demonstrated that he knew about Tusi's work or that of the Maragha school.
Not all Greeks agreed with the geocentric model. The Pythagorean system has already been mentioned; some Pythagoreans believed the Earth to be one of several planets going around a central fire. It was revived in the Middle Ages by Jean Buridan. Heraclides Ponticus was once thought to have proposed that both Venus and Mercury went around the Sun rather than the Earth, but this is no longer accepted. He wrote a work, which has not survived, on heliocentrism , saying that the Sun was at the center of the universe, while the Earth and other planets revolved around it.
In , the geocentric system met its first serious challenge with the publication of Copernicus ' De revolutionibus orbium coelestium On the Revolutions of the Heavenly Spheres , 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 philosophy and scripture. The Copernican system was no more accurate than Ptolemy's system, because it still used circular orbits. This was not altered until Johannes Kepler postulated that they were elliptical Kepler's first law of planetary motion.
With the invention of the telescope in , observations made by Galileo Galilei such as that Jupiter has moons called into question some of the tenets of geocentrism but did not seriously threaten it. Because he observed dark "spots" on the Moon, craters, he remarked that the moon was not a perfect celestial body as had been previously conceived. This was the first time someone could see imperfections on a celestial body that was supposed to be composed of perfect aether. As such, because the Moon's imperfections could now be related to those seen on Earth, one could argue that neither was unique: rather, they were both just celestial bodies made from Earth-like material.
Galileo could also see the moons of Jupiter, which he dedicated to Cosimo II de' Medici , and stated that they orbited around Jupiter, not Earth. In December , Galileo Galilei used his telescope to observe that Venus showed all phases , just like the Moon. 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:. If Venus is between Earth and the Sun, the phase of Venus must always be crescent or all dark. If Venus is beyond the Sun, the phase of Venus must always be gibbous or full. 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 's Tychonic system 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. Johannes Kepler analysed Tycho Brahe 's famously accurate observations and afterwards constructed his three laws in and , based on a heliocentric view where the planets move in elliptical paths. Using these laws, he was the first astronomer to successfully predict a transit of Venus for the year The change from circular orbits to elliptical planetary paths dramatically improved the accuracy of celestial observations and predictions.
Because the heliocentric model devised by Copernicus was no more accurate than Ptolemy's system, new observations were needed to persuade those who still adhered to the geocentric model. However, Kepler's laws based on Brahe's data became a problem which geocentrists could not easily overcome. In , Isaac Newton stated the law of universal gravitation , described earlier as a hypothesis by Robert Hooke and others. His main achievement was to mathematically derive Kepler's laws of planetary motion from the law of gravitation, thus helping to prove the latter. This introduced gravitation as the force which both kept the Earth and planets moving through the universe and also kept the atmosphere from flying away.
The theory of gravity allowed scientists to rapidly construct a plausible heliocentric model for the Solar System. In his Principia , Newton explained his theory of how gravity, previously thought to be a mysterious, unexplained occult force, directed the movements of celestial bodies, and kept our Solar System in working order. His descriptions of centripetal force  were a breakthrough in scientific thought, using the newly developed mathematical discipline of differential calculus , finally replacing the previous schools of scientific thought, which had been dominated by Aristotle and Ptolemy.
However, the process was gradual. Several empirical tests of Newton's theory, explaining the longer period of oscillation of a pendulum at the equator and the differing size of a degree of latitude, would gradually become available between and In addition, stellar aberration was observed by Robert Hooke in , and tested in a series of observations by Jean Picard over a period of ten years, finishing in However, it was not explained until , when James Bradley provided an approximate explanation in terms of the Earth's revolution about the Sun. In , astronomer Friedrich Wilhelm Bessel measured the parallax of the star 61 Cygni successfully, and disproved Ptolemy's claim that parallax motion did not exist.
This finally confirmed the assumptions made by Copernicus, providing accurate, dependable scientific observations, and conclusively displaying how distant stars are from Earth. 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 frame is most useful in those cases, galactic and extragalactic astronomy is easier if the Sun is treated as neither stationary nor the center of the universe, but rather rotating around the center of our galaxy, while in turn our galaxy is also not at rest in the cosmic background.
If this can be done, our difficulties will be over. We shall then be able to apply the laws of nature to any CS. The struggle, so violent in the early days of science, between the views of Ptolemy and Copernicus would then be quite meaningless. Either CS could be used with equal justification. The two sentences, 'the sun is at rest and the Earth moves', or 'the sun moves and the Earth is at rest', would simply mean two different conventions concerning two different CS. Could we build a real relativistic physics valid in all CS; a physics in which there would be no place for absolute, but only for relative, motion? This is indeed possible! Despite giving more respectability to the geocentric view than Newtonian physics does,  relativity is not geocentric.
Rather, relativity states that the Sun, the Earth, the Moon, Jupiter, or any other point for that matter could be chosen as a center of the Solar System with equal validity. Relativity agrees with Newtonian predictions that regardless of whether the Sun or the Earth are chosen arbitrarily as the center of the coordinate system describing the Solar System, the paths of the planets form roughly ellipses with respect to the Sun, not the Earth. With respect to the average reference frame of the fixed stars , the planets do indeed move around the Sun, which due to its much larger mass, moves far less than its own diameter and the gravity of which is dominant in determining the orbits of the planets in other words, the center of mass of the Solar System is near the center of the Sun.
The Earth and Moon are much closer to being a binary planet ; the center of mass around which they both rotate is still inside the Earth, but is about 4, km 2, mi or What the principle of relativity points out is that correct mathematical calculations can be made regardless of the reference frame chosen, and these will all agree with each other as to the predictions of actual motions of bodies with respect to each other. It is not necessary to choose the object in the Solar System with the largest gravitational field as the center of the coordinate system in order to predict the motions of planetary bodies, though doing so may make calculations easier to perform or interpret.
A geocentric coordinate system can be more convenient when dealing only with bodies mostly influenced by the gravity of the Earth such as artificial satellites and the Moon , or when calculating what the sky will look like when viewed from Earth as opposed to an imaginary observer looking down on the entire Solar System, where a different coordinate system might be more convenient.
The Ptolemaic model of the solar system held sway into the early modern age ; from the late 16th century onward it was gradually replaced as the consensus description by the heliocentric model. Geocentrism as a separate religious belief, however, never completely died out. In the United States between and , for example, various members of the Lutheran Church—Missouri Synod published articles disparaging Copernican astronomy and promoting geocentrism. Graebner observed that the synod had no doctrinal position on geocentrism, heliocentrism, or any scientific model, unless it were to contradict Scripture.
He stated that any possible declarations of geocentrists within the synod did not set the position of the church body as a whole. Articles arguing that geocentrism was the biblical perspective appeared in some early creation science newsletters pointing to some passages in the Bible , which, when taken literally, indicate that the daily apparent motions of the Sun and the Moon are due to their actual motions around the Earth rather than due to the rotation of the Earth about its axis. For example, in Joshua , the Sun and Moon are said to stop in the sky, and in Psalms the world is described as immobile.
Contemporary advocates for such religious beliefs include Robert Sungenis author of the book Galileo Was Wrong. Most contemporary creationist organizations reject such perspectives. The famous Galileo affair pitted the geocentric model against the claims of Galileo. In regards to the theological basis for such an argument, two Popes addressed the question of whether the use of phenomenological language would compel one to admit an error in Scripture. Both taught that it would not. There can never, indeed, be any real discrepancy between the theologian and the physicist, as long as each confines himself within his own lines, and both are careful, as St.
Augustine warns us, "not to make rash assertions, or to assert what is not known as known". If dissension should arise between them, here is the rule also laid down by St. Augustine, for the theologian: "Whatever they can really demonstrate to be true of physical nature, we must show to be capable of reconciliation with our Scriptures; and whatever they assert in their treatises which is contrary to these Scriptures of ours, that is to Catholic faith, we must either prove it as well as we can to be entirely false, or at all events we must, without the smallest hesitation, believe it to be so.
Ordinary speech primarily and properly describes what comes under the senses; and somewhat in the same way the sacred writers-as the Angelic Doctor also reminds us — "went by what sensibly appeared", or put down what God, speaking to men, signified, in the way men could understand and were accustomed to. Maurice Finocchiaro, author of a book on the Galileo affair, notes that this is "a view of the relationship between biblical interpretation and scientific investigation that corresponds to the one advanced by Galileo in the " Letter to the Grand Duchess Christina ".
Hence with grave words did he proclaim that there is no error whatsoever if the sacred writer, speaking of things of the physical order "went by what sensibly appeared" as the Angelic Doctor says, speaking either "in figurative language, or in terms which were commonly used at the time, and which in many instances are in daily use at this day, even among the most eminent men of science". For "the sacred writers, or to speak more accurately — the words are St. Augustine's — the Holy Spirit, Who spoke by them, did not intend to teach men these things — that is the essential nature of the things of the universe — things in no way profitable to salvation"; which principle "will apply to cognate sciences, and especially to history", that is, by refuting, "in a somewhat similar way the fallacies of the adversaries and defending the historical truth of Sacred Scripture from their attacks".
In , Pope Alexander VII republished the Index Librorum Prohibitorum List of Prohibited Books and attached the various decrees connected with those books, including those concerned with heliocentrism. He stated in a Papal Bull that his purpose in doing so was that "the succession of things done from the beginning might be made known [ quo rei ab initio gestae series innotescat ]".
The position of the curia evolved slowly over the centuries towards permitting the heliocentric view. In , during the papacy of Benedict XIV, the Congregation of the Index withdrew the decree which prohibited all books teaching the Earth's motion, although the Dialogue and a few other books continued to be explicitly included. In , the Congregation of the Holy Office, with the pope's approval, decreed that Catholic astronomer Giuseppe Settele was allowed to treat the Earth's motion as an established fact and removed any obstacle for Catholics to hold to the motion of the Earth:. His Holiness has decreed that no obstacles exist for those who sustain Copernicus' affirmation regarding the Earth's movement in the manner in which it is affirmed today, even by Catholic authors.
He has, moreover, suggested the insertion of several notations into this work, aimed at demonstrating that the above mentioned affirmation [of Copernicus], as it has come to be understood, does not present any difficulties; difficulties that existed in times past, prior to the subsequent astronomical observations that have now occurred. He is now appointed the task of bringing to an end any concerns and criticisms regarding the printing of this book, and, at the same time, ensuring that in the future, regarding the publication of such works, permission is sought from the Cardinal Vicar whose signature will not be given without the authorization of the Superior of his Order.
In , the Congregation of the Holy Office removed the prohibition on the publication of books treating of the Earth's motion in accordance with modern astronomy and Pope Pius VII ratified the decision:. The most excellent [cardinals] have decreed that there must be no denial, by the present or by future Masters of the Sacred Apostolic Palace, of permission to print and to publish works which treat of the mobility of the Earth and of the immobility of the sun, according to the common opinion of modern astronomers, as long as there are no other contrary indications, on the basis of the decrees of the Sacred Congregation of the Index of and of this Supreme [Holy Office] of ; and that those who would show themselves to be reluctant or would disobey, should be forced under punishments at the choice of [this] Sacred Congregation, with derogation of [their] claimed privileges, where necessary.
The edition of the Catholic List of Prohibited Books for the first time omits the Dialogue from the list. The Pope declared the incident to be based on a "tragic mutual miscomprehension". He further stated:. Cardinal Poupard has also reminded us that the sentence of was not irreformable, and that the debate which had not ceased to evolve thereafter, was closed in with the imprimatur given to the work of Canon Settele. The error of the theologians of the time, when they maintained the centrality of the Earth, was to think that our understanding of the physical world's structure was, in some way, imposed by the literal sense of Sacred Scripture.
Let us recall the celebrated saying attributed to Baronius "Spiritui Sancto mentem fuisse nos docere quomodo ad coelum eatur, non quomodo coelum gradiatur". In fact, the Bible does not concern itself with the details of the physical world, the understanding of which is the competence of human experience and reasoning. There exist two realms of knowledge, one which has its source in Revelation and one which reason can discover by its own power. To the latter belong especially the experimental sciences and philosophy. The distinction between the two realms of knowledge ought not to be understood as opposition.
A few Orthodox Jewish leaders maintain a geocentric model of the universe based on the aforementioned Biblical verses and an interpretation of Maimonides to the effect that he ruled that the Earth is orbited by the Sun. The Zohar states: "The entire world and those upon it, spin round in a circle like a ball, both those at the bottom of the ball and those at the top. All God's creatures, wherever they live on the different parts of the ball, look different in color, in their features because the air is different in each place, but they stand erect as all other human beings, therefore, there are places in the world where, when some have light, others have darkness; when some have day, others have night.
While geocentrism is important in Maimonides' calendar calculations,  the great majority of Jewish religious scholars, who accept the divinity of the Bible and accept many of his rulings as legally binding, do not believe that the Bible or Maimonides command a belief in geocentrism. After the translation movement led by the Mu'tazila , which included the translation of Almagest from Latin to Arabic, Muslims adopted and refined the geocentric model of Ptolemy , which they believed correlated with the teachings of Islam.
Prominent cases of modern geocentrism are very isolated. Very few individuals promoted a geocentric view of the universe. He rejected the heliocentric model and wrote a book  that explains the movement of the sun, moon and other planets around the Earth. The geocentric Ptolemaic model of the solar system is still of interest to planetarium makers, as, for technical reasons, a Ptolemaic-type motion for the planet light apparatus has some advantages over a Copernican-type motion.
However this effect is negligible at the scale of accuracy that applies to a planetarium. All Islamic astronomers from Thabit ibn Qurra in the ninth century to Ibn al-Shatir in the fourteenth, and all natural philosophers from al-Kindi to Averroes and later, are known to have accepted From Wikipedia, the free encyclopedia. Redirected from Geocentrism.
Superseded description of the Universe with Earth at the center. For orbits around Earth, see Geocentric orbit. For the coordinate system, see Geocentric coordinates. Main articles: Maragheh observatory , Astronomy in medieval Islam , and Islamic cosmology. The examples and perspective in this article may not include all significant viewpoints. Please improve the article or discuss the issue. June Learn how and when to remove this template message. Main article: Copernican heliocentrism. A good idea of the similarly primitive state of Hebrew astronomy can be gained from biblical writings, such as the Genesis creation story and the various Psalms that extol the firmament, the stars, the sun, and the earth. The Hebrews saw the Earth as an almost flat surface consisting of a solid and a liquid part, and the sky as the realm of light in which heavenly bodies move.
The earth rested on cornerstones and could not be moved except by Jehovah as in an earthquake. According to the Hebrews, the Sun and the Moon were only a short distance from one another. The Earth is usually described as a disk encircled by water. Cosmological and metaphysical speculations were not to be cultivated in public nor were they to be committed to writing.
Rather, they were considered [ by whom? Hebrew cosmology pictured a flat Earth, over which was a dome-shaped firmament, supported above the Earth by mountains, and surrounded by waters. Holes or sluices windows, Gen. The firmament was the heavens in which God set the Sun Psalms and the stars Genesis on the fourth day of the creation. There was more water under the Earth Genesis and during the Flood the two great oceans joined up and covered the Earth; sheol was at the bottom of the Earth Isa.
It can also be used as a synonym for "heaven" Gen. The ancient Israelites also used more descriptive terms for how God created the celestial realm, and based on the collection of these more specific and illustrative terms, I would propose that they had two basic ideas of the composition of the heavenly realm. First is the idea that the heavenly realm was imagined as a vast cosmic canopy. Since the texts that mention the stretching out of the sky are typically drawing on creation imagery, it seems that the figure intends to suggest that the heavens are Yahweh's cosmic tent. One can imagine ancient Israelites gazing up to the stars and comparing the canopy of the sky to the roofs of the tents under which they lived.
In fact, if one were to look up at the ceiling of a dark tent with small holes in the roof during the daytime, the roof, with the sunlight shining through the holes, would look very much like the night sky with all its stars. The second image of the material composition of the heavenly realm involves a firm substance. The model of the universe inherited from the Hebrew Bible and the Ancient Near East of a flat Earth completely surrounded by water with a heavenly realm of the gods arching above from horizon to horizon became obsolete.
In the past the heavenly realm was for gods only. It was the place where all events on Earth were determined by the gods, and their decisions were irrevocable. The gulf between the gods and humans could not have been greater. The evolution of Jewish cosmography in the course of the Second Temple Period followed developments in Hellenistic astronomy. It represents a coherent model for the experiences of the people of Mesopotamia through that period. It reflects a world-view that made sense of water coming from the sky and the ground as well as the regular apparent movements of the stars, Sun, Moon, and planets.
There is a clear understanding of the restrictions on breeding between different species of animals and of the way in which human beings had gained control over what were, by then, domestic animals. There is also recognition of the ability of humans to change the environment in which they lived. This same understanding occurred also in the great creation stories of Mesopotamia; these stories formed the basis for the Jewish theological reflections of the Hebrew Scriptures concerning the creation of the world. The Jewish priests and theologians who constructed the narrative took accepted ideas about the structure of the world and reflected theologically on them in the light of their experience and faith.
There was never any clash between Jewish and Babylonian people about the structure of the world, but only about who was responsible for it and its ultimate theological meaning. The envisaged structure is simple: Earth was seen as being situated in the middle of a great volume of water, with water both above and below Earth. A great dome was thought to be set above Earth like an inverted glass bowl , maintaining the water above Earth in its place.
Earth was pictured as resting on foundations that go down into the deep. These foundations secured the stability of the land as something that is not floating on the water and so could not be tossed about by wind and wave. The waters surrounding Earth were thought to have been gathered together in their place. The stars, Sun, Moon, and planets moved in their allotted paths across the great dome above Earth, with their movements defining the months, seasons, and year. Almost all ancient cultures developed cosmological stories to explain the basic features of the cosmos: Earth and its inhabitants, sky, sea, Sun, Moon, and stars. The Babylonians, for example, regarded the universe as born from a primeval pair of human-like gods. Early Egyptian cosmology explained eclipses as the Moon being swallowed temporarily by a sow or as the Sun being attacked by a serpent.
As of 1 October , 4, known extrasolar planets in 3, planetary systems including multiple planetary systems , ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter , have been discovered, out of which more than planets are the same size as Earth , nine of which are at the same relative distance from their star as Earth from the Sun, i. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems.
The ambiguities inherent in defining planets have led to much scientific controversy. The five classical planets of the Solar System , being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology , religious cosmology , and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the " fixed stars ", which maintained a constant relative position in the sky. The reasons for this perception were that stars and planets appeared to revolve around Earth each day  and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest.
The first civilization known to have a functional theory of the planets were the Babylonians , who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa , a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC. APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year. These would remain the only known planets until the invention of the telescope in early modern times.
The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The Pythagoreans , in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star Hesperos and morning star Phosphoros as one and the same Aphrodite , Greek corresponding to Latin Venus ,  though this had long been known by the Babylonians. In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun.
The geocentric system remained dominant until the Scientific Revolution. By the 1st century BC, during the Hellenistic period , the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye.
These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. Cicero , in his De Natura Deorum , enumerated the planets known during the 1st century BCE using the names for them in use at the time: .
In CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbits of planets are elliptical. In , Nilakantha Somayaji of the Kerala school of astronomy and mathematics , in his Tantrasangraha , revised Aryabhata's model.
Most astronomers of the Kerala school who followed him accepted his planetary model. In the 11th century, the transit of Venus was observed by Avicenna , who established that Venus was, at least sometimes, below the Sun. With the advent of the Scientific Revolution , use of the term "planet" changed from something that moved across the sky in relation to the star field ; to a body that orbited Earth or that was believed to do so at the time ; and by the 18th century to something that directly orbited the Sun when the heliocentric model of Copernicus , Galileo and Kepler gained sway.
Thus, Earth became included in the list of planets,  whereas the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably — although the latter would gradually become more prevalent in the following century. In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century such as Ceres , Pallas , Juno , and Vesta were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter the asteroid belt , and had a much smaller mass; as a result they were reclassified as " asteroids ".
In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Because there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in , there was no apparent need to have a formal definition. In the 20th century, Pluto was discovered. After initial observations led to the belief that it was larger than Earth,  the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in , Ray Lyttleton suggested that Pluto may be an escaped satellite of Neptune ,  and Fred Whipple suggested in that Pluto may be a comet.
Then, on October 6, , Michel Mayor and Didier Queloz of the Geneva Observatory announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star 51 Pegasi. The discovery of extrasolar planets led to another ambiguity in defining a planet: the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as brown dwarfs. Brown dwarfs are generally considered stars due to their ability to fuse deuterium , a heavier isotope of hydrogen.
Although objects more massive than 75 times that of Jupiter fuse hydrogen, objects of only 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets. With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a belt , or if it was large enough to generate energy by the thermonuclear fusion of deuterium.
A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System the Kuiper belt during the s and early s. Pluto was found to be just one small body in a population of thousands. Some of them, such as Quaoar , Sedna , and Eris , were heralded in the popular press as the tenth planet , failing to receive widespread scientific recognition.
Acknowledging the problem, the IAU set about creating the definition of planet , and produced one in August The number of planets dropped to the eight significantly larger bodies that had cleared their orbit Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune , and a new class of dwarf planets was created, initially containing three objects Ceres , Pluto and Eris. There is no official definition of extrasolar planets.
The positions statement incorporates the following guidelines, mostly focused upon the boundary between planets and brown dwarfs: . One definition of a sub-brown dwarf is a planet-mass object that formed through cloud collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.
For example, a planet formed by accretion around a star may get ejected from the system to become free-floating, and likewise a sub-brown dwarf that formed on its own in a star cluster through cloud collapse may get captured into orbit around a star. One study suggests that objects above 10 M Jup formed through gravitational instability and should not be thought of as planets.
The 13 Jupiter-mass cutoff represents an average mass rather than a precise threshold value. Large objects will fuse most of their deuterium and smaller ones will fuse only a little, and the 13 M J value is somewhere in between. Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure. After much debate and one failed proposal, a large majority of those remaining at the meeting voted to pass a resolution. The resolution defines planets within the Solar System as follows: .
Under this definition, the Solar System is considered to have eight planets. Bodies that fulfill the first two conditions but not the third such as Ceres, Pluto, and Eris are classified as dwarf planets , provided they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include c as a criterion. This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects.
As described by astronomer Steven Soter :. The IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and because the criteria of roundness and orbital zone clearance are not presently observable. Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star. The table below lists Solar System bodies once considered to be planets but no longer considered as such by the IAU, as well as whether they would be considered planets under alternative definitions, such as Soter's definition  that favors dynamical dominance or Stern's  and definitions  that favor having a shape dominated by gravity.
Ceres was subsequently classified by the IAU as a dwarf planet in The reporting of newly discovered large Kuiper belt objects as planets — particularly Eris — triggered the August IAU decision on what a planet is. The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece , the two great luminaries the Sun and the Moon were called Helios and Selene , two ancient Titanic deities; the slowest planet Saturn was called Phainon , the shiner; followed by Phaethon Jupiter , "bright"; the red planet Mars was known as Pyroeis , the "fiery"; the brightest Venus was known as Phosphoros , the light bringer; and the fleeting final planet Mercury was called Stilbon , the gleamer.
The Greeks also assigned each planet to one among their pantheon of gods, the Olympians and the earlier Titans :. The Greek practice of grafting their gods' names onto the planets was almost certainly borrowed from the Babylonians. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. Unlike Ares, Nergal was also god of pestilence and the underworld. Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods.
Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church , use the Roman Latin names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans , shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic , Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.
Uranus is unique in that it is named for a Greek deity rather than his Roman counterpart. Some Romans , following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt , believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. Because each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected — and still preserved in many modern languages. Earth is the only planet whose name in English is not derived from Greco-Roman mythology.
Because it was only generally accepted as a planet in the 17th century,  there is no tradition of naming it after a god. The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets. Many of the Romance languages retain the old Roman word terra or some variation of it that was used with the meaning of "dry land" as opposed to "sea". Non-European cultures use other planetary-naming systems.
China and the countries of eastern Asia historically subject to Chinese cultural influence such as Japan, Korea and Vietnam use a naming system based on the five Chinese elements : water Mercury , metal Venus , fire Mars , wood Jupiter and earth Saturn. It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion a process of sticky collision dust particles in the disk steadily accumulate mass to form ever-larger bodies.
Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets. When the protostar has grown such that it ignites to form a star , the surviving disk is removed from the inside outward by photoevaporation , the solar wind , Poynting—Robertson drag and other effects. Protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.
The energetic impacts of the smaller planetesimals as well as radioactive decay will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core. With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity —an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 helium —is now thought to determine the likelihood that a star will have planets. According to the IAU definition , there are eight planets in the Solar System, which are in increasing distance from the Sun :.
Jupiter is the largest, at Earth masses, whereas Mercury is the smallest, at 0. The number of geophysical planets in the Solar System is unknown - previously considered to be potentially in the hundreds, but now only estimated at only the low double digits. An exoplanet extrasolar planet is a planet outside the Solar System. As of 1 October , there are 4, confirmed exoplanets in 3, planetary systems , with systems having more than one planet.
These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of giant planets that survived the supernova and then decayed into their current orbits. The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October , when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury.
There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes , which could be rocky like Earth or a mixture of volatiles and gas like Neptune—a radius of 1. Another possible type of planet is carbon planets , which form in systems with a higher proportion of carbon than in the Solar System. A study, analyzing gravitational microlensing data, estimates an average of at least 1. On 20 December , the Kepler Space Telescope team reported the discovery of the first Earth-size exoplanets , Keplere  and Keplerf ,  orbiting a Sun-like star , Kepler Around 1 in 5 Sun-like stars have an "Earth-sized" [d] planet in the habitable [e] zone, so the nearest would be expected to be within 12 light-years distance from Earth.
There are exoplanets that are much closer to their parent star than any planet in the Solar System is to the Sun, and there are also exoplanets that are much farther from their star. Mercury , the closest planet to the Sun at 0. The Kepler system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. Neptune is 30 AU from the Sun and takes years to orbit, but there are exoplanets that are hundreds of AU from their star and take more than a thousand years to orbit, e. A planetary-mass object PMO , planemo ,  or planetary body is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium to be rounded under its own gravity , but not enough to sustain core fusion like a star.
These include dwarf planets , which are rounded by their own gravity but not massive enough to clear their own orbit , planetary-mass moons , and free-floating planemos, which may have been ejected from a system rogue planets or formed through cloud-collapse rather than accretion sometimes called sub-brown dwarfs. A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite; it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape usually a spheroid , but has not cleared the neighborhood of other material around its orbit. Planetary scientist and New Horizons principal investigator Alan Stern , who proposed the term 'dwarf planet', has argued that location should not matter and that only geophysical attributes should be taken into account, and that dwarf planets are thus a subtype of planet.
The IAU accepted the term rather than the more neutral 'planetoid' but decided to classify dwarf planets as a separate category of object. Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space. Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Binary systems of sub-brown dwarfs are theoretically possible; Oph was initially thought to be a binary system of a brown dwarf of 14 Jupiter masses and a sub-brown dwarf of 7 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions.
In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. Some large satellites moons are of similar size or larger than the planet Mercury , e. Jupiter's Galilean moons and Titan. Proponents of the geophysical definition of planets argue that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet. The term satellite planet is sometimes used for planet-sized satellites.
Rogue planets in stellar clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between and 10 5 AU. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system. Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are also commonly observed in extrasolar planets.
According to current definitions, all planets must revolve around stars; thus, any potential " rogue planets " are excluded. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates counter-clockwise as seen from above the Sun's north pole. At least one extrasolar planet, WASPb , has been found to orbit in the opposite direction to its star's rotation. No planet's orbit is perfectly circular, and hence the distance of each varies over the course of its year. The closest approach to its star is called its periastron perihelion in the Solar System , whereas its farthest separation from the star is called its apastron aphelion.
As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory. Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa.
Each planet therefore has seasons, changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest.
The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.
The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole , the exceptions being Venus  and Uranus,  which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.
The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet. However, for "hot" Jupiters , their proximity to their stars means that they are tidally locked i. This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night. The defining dynamic characteristic of a planet is that it has cleared its neighborhood.
A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU 's official definition of a planet in August, This criterion excludes such planetary bodies as Pluto , Eris and Ceres from full-fledged planethood, making them instead dwarf planets.
A planet's size is defined at least by an average radius e. Derived quantities include the flattening, surface area, and volume. Knowing further the rotation rate and mass, allows the calculation of normal gravity. A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.
Mass is also the prime attribute by which planets are distinguished from stars. While the lower stellar mass limit is estimated to be around 75 times that of Jupiter M J , the upper planetary mass limit for planethood is only roughly 13 M J for objects with solar-type isotopic abundance , beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. Its mass is roughly half that of the planet Mercury. Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface.
Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle that either is or was a fluid. The terrestrial planets are sealed within hard crusts ,  but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel , and mantles of silicates. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen. All of the Solar System planets except Mercury  have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium , whereas the smaller planets lose these gases into space.
Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes , on Earth , planet-wide dust storms on Mars , a greater-than-Earth-sized anticyclone on Jupiter called the Great Red Spot , and holes in the atmosphere on Neptune. Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. One important characteristic of the planets is their intrinsic magnetic moments , which in turn give rise to magnetospheres.
The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet. Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.
Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System so strong in fact that it poses a serious health risk to future manned missions to its moons. The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet. In , a team of astronomers in Hawaii observed an extrasolar planet around the star HD , which appeared to be creating a sunspot on the surface of its parent star.
Several planets or dwarf planets in the Solar System such as Neptune and Pluto have orbital periods that are in resonance with each other or with smaller bodies this is also common in satellite systems. All except Mercury and Venus have natural satellites , often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life especially Europa. The four giant planets are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny ' moonlets ' whose gravity shapes and maintains their structure.
Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit and were torn apart by tidal forces. No secondary characteristics have been observed around extrasolar planets. The sub-brown dwarf Cha , which has been described as a rogue planet , is believed to be orbited by a tiny protoplanetary disc  and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses. From Wikipedia, the free encyclopedia. Redirected from Planets. This article is about the astronomical object.
For planets in astrology, see Planets in astrology. For other uses, see Planet disambiguation. Class of astronomical body directly orbiting a star or stellar remnant. Further information: History of astronomy , Definition of planet , and Timeline of Solar System astronomy. Main article: Babylonian astronomy. See also: Greek astronomy. Main articles: Indian astronomy and Hindu cosmology. Main articles: Astronomy in the medieval Islamic world and Cosmology in medieval Islam. See also: Heliocentrism. Main article: IAU definition of planet. See also: List of former planets.
See also: Weekday names and Naked-eye planet. Main article: Nebular hypothesis. Supernova remnant ejecta producing planet-forming material. Solar System — sizes but not distances are to scale. The Sun and the eight planets of the Solar System. The inner planets , Mercury , Venus , Earth , and Mars. Main article: Solar System. See also: List of gravitationally rounded objects of the Solar System. Main article: Exoplanet. Main article: Geophysical definition of planet. Main article: Dwarf planet. Main article: Rogue planet. See also: Five-planet Nice model.