Theology is the critical study of the nature of the divine. It is taught as an academic discipline in universities and seminaries. Theology is the study of deities or their scriptures in order to discover what they have revealed about themselves, it occupies itself with the unique content of analyzing the supernatural, but especially with epistemology, asks and seeks to answer the question of revelation. Revelation pertains to the acceptance of God, gods, or deities, as not only transcendent or above the natural world, but willing and able to interact with the natural world and, in particular, to reveal themselves to humankind. While theology has turned into a secular field, religious adherents still consider theology to be a discipline that helps them live and understand concepts such as life and love and that helps them lead lives of obedience to the deities they follow or worship. Theology is derived from the Greek theologia, which derived from Τheos, meaning "God", -logia, meaning "utterances, sayings, or oracles" which had passed into Latin as theologia and into French as théologie.
The English equivalent "theology" had evolved by 1362. The sense the word has in English depends in large part on the sense the Latin and Greek equivalents had acquired in patristic and medieval Christian usage, although the English term has now spread beyond Christian contexts. Augustine of Hippo defined the Latin equivalent, theologia, as "reasoning or discussion concerning the Deity"; the term can, however, be used for a variety of fields of study. Theology begins with the assumption that the divine exists in some form, such as in physical, mental, or social realities, that evidence for and about it may be found via personal spiritual experiences or historical records of such experiences as documented by others; the study of these assumptions is not part of theology proper but is found in the philosophy of religion, through the psychology of religion and neurotheology. Theology aims to structure and understand these experiences and concepts, to use them to derive normative prescriptions for how to live our lives.
Theologians use various forms of analysis and argument to help understand, test, defend or promote any myriad of religious topics. As in philosophy of ethics and case law, arguments assume the existence of resolved questions, develop by making analogies from them to draw new inferences in new situations; the study of theology may help a theologian more understand their own religious tradition, another religious tradition, or it may enable them to explore the nature of divinity without reference to any specific tradition. Theology may be used to propagate, reform, or justify a religious tradition or it may be used to compare, challenge, or oppose a religious tradition or world-view. Theology might help a theologian address some present situation or need through a religious tradition, or to explore possible ways of interpreting the world. Greek theologia was used with the meaning "discourse on god" in the fourth century BC by Plato in The Republic, Book ii, Ch. 18. Aristotle divided theoretical philosophy into mathematike and theologike, with the last corresponding to metaphysics, for Aristotle, included discourse on the nature of the divine.
Drawing on Greek Stoic sources, the Latin writer Varro distinguished three forms of such discourse: mythical and civil. Theologos related to theologia, appears once in some biblical manuscripts, in the heading to the Book of Revelation: apokalypsis ioannoy toy theologoy, "the revelation of John the theologos". There, the word refers not to John the "theologian" in the modern English sense of the word but—using a different sense of the root logos, meaning not "rational discourse" but "word" or "message"—one who speaks the words of God, logoi toy theoy; some Latin Christian authors, such as Tertullian and Augustine, followed Varro's threefold usage, though Augustine used the term more to mean'reasoning or discussion concerning the deity'In patristic Greek Christian sources, theologia could refer narrowly to devout and inspired knowledge of, teaching about, the essential nature of God. The Latin author Boethius, writing in the early 6th century, used theologia to denote a subdivision of philosophy as a subject of academic study, dealing with the motionless, incorporeal reality.
Boethius' definition influenced medieval Latin usage. In scholastic Latin sources, the term came to denote the rational study of the doctrines of the Christian religion, or the academic discipline which investigated the coherence and implications of the language and claims of the Bible and of the theological tradition. In the Renaissance with Florentine Platonist apologists of Dante's poetics, the distinction between "poetic theology" and "revealed" or Biblical theology serves as steppingstone for a revival of philosophy as independent of theological authority, it is in this last sense, theology as an academic discipline involving rational study of Christian teaching
Physics is the natural science that studies matter, its motion, behavior through space and time, that studies the related entities of energy and force. Physics is one of the most fundamental scientific disciplines, its main goal is to understand how the universe behaves. Physics is one of the oldest academic disciplines and, through its inclusion of astronomy the oldest. Over much of the past two millennia, chemistry and certain branches of mathematics, were a part of natural philosophy, but during the scientific revolution in the 17th century these natural sciences emerged as unique research endeavors in their own right. Physics intersects with many interdisciplinary areas of research, such as biophysics and quantum chemistry, the boundaries of physics which are not rigidly defined. New ideas in physics explain the fundamental mechanisms studied by other sciences and suggest new avenues of research in academic disciplines such as mathematics and philosophy. Advances in physics enable advances in new technologies.
For example, advances in the understanding of electromagnetism and nuclear physics led directly to the development of new products that have transformed modern-day society, such as television, domestic appliances, nuclear weapons. Astronomy is one of the oldest natural sciences. Early civilizations dating back to beyond 3000 BCE, such as the Sumerians, ancient Egyptians, the Indus Valley Civilization, had a predictive knowledge and a basic understanding of the motions of the Sun and stars; the stars and planets were worshipped, believed to represent gods. While the explanations for the observed positions of the stars were unscientific and lacking in evidence, these early observations laid the foundation for astronomy, as the stars were found to traverse great circles across the sky, which however did not explain the positions of the planets. According to Asger Aaboe, the origins of Western astronomy can be found in Mesopotamia, all Western efforts in the exact sciences are descended from late Babylonian astronomy.
Egyptian astronomers left monuments showing knowledge of the constellations and the motions of the celestial bodies, while Greek poet Homer wrote of various celestial objects in his Iliad and Odyssey. Natural philosophy has its origins in Greece during the Archaic period, when pre-Socratic philosophers like Thales rejected non-naturalistic explanations for natural phenomena and proclaimed that every event had a natural cause, they proposed ideas verified by reason and observation, many of their hypotheses proved successful in experiment. The Western Roman Empire fell in the fifth century, this resulted in a decline in intellectual pursuits in the western part of Europe. By contrast, the Eastern Roman Empire resisted the attacks from the barbarians, continued to advance various fields of learning, including physics. In the sixth century Isidore of Miletus created an important compilation of Archimedes' works that are copied in the Archimedes Palimpsest. In sixth century Europe John Philoponus, a Byzantine scholar, questioned Aristotle's teaching of physics and noting its flaws.
He introduced the theory of impetus. Aristotle's physics was not scrutinized until John Philoponus appeared, unlike Aristotle who based his physics on verbal argument, Philoponus relied on observation. On Aristotle's physics John Philoponus wrote: “But this is erroneous, our view may be corroborated by actual observation more than by any sort of verbal argument. For if you let fall from the same height two weights of which one is many times as heavy as the other, you will see that the ratio of the times required for the motion does not depend on the ratio of the weights, but that the difference in time is a small one, and so, if the difference in the weights is not considerable, that is, of one is, let us say, double the other, there will be no difference, or else an imperceptible difference, in time, though the difference in weight is by no means negligible, with one body weighing twice as much as the other”John Philoponus' criticism of Aristotelian principles of physics served as an inspiration for Galileo Galilei ten centuries during the Scientific Revolution.
Galileo cited Philoponus in his works when arguing that Aristotelian physics was flawed. In the 1300s Jean Buridan, a teacher in the faculty of arts at the University of Paris, developed the concept of impetus, it was a step toward the modern ideas of momentum. Islamic scholarship inherited Aristotelian physics from the Greeks and during the Islamic Golden Age developed it further placing emphasis on observation and a priori reasoning, developing early forms of the scientific method; the most notable innovations were in the field of optics and vision, which came from the works of many scientists like Ibn Sahl, Al-Kindi, Ibn al-Haytham, Al-Farisi and Avicenna. The most notable work was The Book of Optics, written by Ibn al-Haytham, in which he conclusively disproved the ancient Greek idea about vision, but came up with a new theory. In the book, he presented a study of the phenomenon of the camera obscura (his thousand-year-old
Julius Wilhelm Richard Dedekind was a German mathematician who made important contributions to abstract algebra, axiomatic foundation for the natural numbers, algebraic number theory and the definition of the real numbers. Dedekind's father was Julius Levin Ulrich Dedekind, an administrator of Collegium Carolinum in Braunschweig. Dedekind had three older siblings; as an adult, he never used the names Julius Wilhelm. He was born, lived most of his life, died in Braunschweig, he first attended the Collegium Carolinum in 1848 before transferring to the University of Göttingen in 1850. There, Dedekind was taught number theory by professor Moritz Stern. Gauss was still teaching, although at an elementary level, Dedekind became his last student. Dedekind received his doctorate in 1852, for a thesis titled Über die Theorie der Eulerschen Integrale; this thesis did not display the talent evident by Dedekind's subsequent publications. At that time, the University of Berlin, not Göttingen, was the main facility for mathematical research in Germany.
Thus Dedekind went to Berlin for two years of study, where he and Bernhard Riemann were contemporaries. Dedekind returned to Göttingen to teach as a Privatdozent, giving courses on probability and geometry, he studied for a while with Peter Gustav Lejeune Dirichlet, they became good friends. Because of lingering weaknesses in his mathematical knowledge, he studied elliptic and abelian functions, yet he was the first at Göttingen to lecture concerning Galois theory. About this time, he became one of the first people to understand the importance of the notion of groups for algebra and arithmetic. In 1858, he began teaching at the Polytechnic school in Zürich; when the Collegium Carolinum was upgraded to a Technische Hochschule in 1862, Dedekind returned to his native Braunschweig, where he spent the rest of his life, teaching at the Institute. He retired in 1894, but continued to publish, he never married. Dedekind was elected to the Academies of Berlin and Rome, to the French Academy of Sciences.
He received honorary doctorates from the universities of Oslo and Braunschweig. While teaching calculus for the first time at the Polytechnic school, Dedekind developed the notion now known as a Dedekind cut, now a standard definition of the real numbers; the idea of a cut is that an irrational number divides the rational numbers into two classes, with all the numbers of one class being greater than all the numbers of the other class. For example, the square root of 2 defines all the nonnegative numbers whose squares are less than 2 and the negative numbers into the lesser class, the positive numbers whose squares are greater than 2 into the greater class; every location on the number line continuum contains an irrational number. Thus there are gaps, or discontinuities. Dedekind published his thoughts on irrational numbers and Dedekind cuts in his pamphlet "Stetigkeit und irrationale Zahlen". Dedekind's theorem states that if there existed a one-to-one correspondence between two sets the two sets were "similar".
He invoked similarity to give the first precise definition of an infinite set: a set is infinite when it is "similar to a proper part of itself," in modern terminology, is equinumerous to one of its proper subsets. Thus the set N of natural numbers can be shown to be similar to the subset of N whose members are the squares of every member of N,: N 1 2 3 4 5 6 7 8 9 10... ↓ N2 1 4 9 16 25 36 49 64 81 100... Dedekind edited the collected works of Lejeune Dirichlet and Riemann. Dedekind's study of Lejeune Dirichlet's work led him to his study of algebraic number fields and ideals. In 1863, he published Lejeune Dirichlet's lectures on number theory as Vorlesungen über Zahlentheorie about which it has been written that: Although the book is assuredly based on Dirichlet's lectures, although Dedekind himself referred to the book throughout his life as Dirichlet's, the book itself was written by Dedekind, for the most part after Dirichlet's death; the 1879 and 1894 editions of the Vorlesungen included supplements introducing the notion of an ideal, fundamental to ring theory.
Dedekind defined an ideal as a subset of a set of numbers, composed of algebraic integers that satisfy polynomial equations with integer coefficients. The concept underwent further development in the hands of Hilbert and of Emmy Noether. Ideals generalize Ernst Eduard Kummer's ideal numbers, devised as part of Kummer's 1843 attempt to prove Fermat's Last Theorem. In an 1882 article and Heinrich Martin Weber applied ideals to Riemann surfaces, giving an algebraic proof of the Riemann–Roch theorem. In 1888, he published a short monograph titled Was sind und was sollen die Zahlen?, which included his definition of an infinite set. He proposed an axiomatic foundation for the natural numbers, whose primitive notions were the number one and the successor function; the next year, Giuseppe Peano, citing Dedekind, formulated an equivalent but simpler set of axioms, now the standard ones. Dedekind made other
Sicily is the largest island in the Mediterranean Sea and one of the 20 regions of Italy. It is one of the five Italian autonomous regions, in Southern Italy along with surrounding minor islands referred to as Regione Siciliana. Sicily is located in the central Mediterranean Sea, south of the Italian Peninsula, from which it is separated by the narrow Strait of Messina, its most prominent landmark is Mount Etna, the tallest active volcano in Europe, one of the most active in the world 3,329 m high. The island has a typical Mediterranean climate; the earliest archaeological evidence of human activity on the island dates from as early as 12,000 BC. By around 750 BC, Sicily had three Phoenician and a dozen Greek colonies and, for the next 600 years, it was the site of the Sicilian Wars and the Punic Wars. After the fall of the Roman Empire in the 5th century AD, Sicily was ruled during the Early Middle Ages by the Vandals, the Ostrogoths, the Byzantine Empire, the Emirate of Sicily; the Norman conquest of southern Italy led to the creation of the Kingdom of Sicily, subsequently ruled by the Hohenstaufen, the Capetian House of Anjou and the House of Habsburg.
It was unified under the House of Bourbon with the Kingdom of Naples as the Kingdom of the Two Sicilies. It became part of Italy in 1860 following the Expedition of the Thousand, a revolt led by Giuseppe Garibaldi during the Italian unification, a plebiscite. Sicily was given special status as an autonomous region on 15th May 1946, 18 days before the Italian constitutional referendum of 1946. Albeit, much of the autonomy still remains unapplied financial autonomy, because the autonomy-activating laws have been deferred to be approved by the parithetic committee, since 1946. Sicily has a rich and unique culture with regard to the arts, literature and architecture, it is home to important archaeological and ancient sites, such as the Necropolis of Pantalica, the Valley of the Temples and Selinunte. Sicily has a triangular shape, earning it the name Trinacria. To the east, it is separated from the Italian mainland by the Strait of Messina, about 3 km wide in the north, about 16 km wide in the southern part.
The northern and southern coasts are each about 280 km long measured as a straight line, while the eastern coast measures around 180 km. The total area of the island is 25,711 km2, while the Autonomous Region of Sicily has an area of 27,708 km2; the terrain of inland Sicily is hilly and is intensively cultivated wherever possible. Along the northern coast, the mountain ranges of Madonie, 2,000 m, Nebrodi, 1,800 m, Peloritani, 1,300 m, are an extension of the mainland Apennines; the cone of Mount Etna dominates the eastern coast. In the southeast lie the lower Hyblaean Mountains, 1,000 m; the mines of the Enna and Caltanissetta districts were part of a leading sulphur-producing area throughout the 19th century, but have declined since the 1950s. Sicily and its surrounding small islands have some active volcanoes. Mount Etna is the largest active volcano in Europe and still casts black ash over the island with its ever-present eruptions, it stands 3,329 metres high, though this varies with summit eruptions.
It is the highest mountain in Italy south of the Alps. Etna covers an area of 1,190 km2 with a basal circumference of 140 km; this makes it by far the largest of the three active volcanoes in Italy, being about two and a half times the height of the next largest, Mount Vesuvius. In Greek mythology, the deadly monster Typhon was trapped under the mountain by Zeus, the god of the sky. Mount Etna is regarded as a cultural symbol and icon of Sicily; the Aeolian Islands in the Tyrrhenian Sea, to the northeast of mainland Sicily form a volcanic complex, include Stromboli. The three volcanoes of Vulcano and Lipari are currently active, although the latter is dormant. Off the southern coast of Sicily, the underwater volcano of Ferdinandea, part of the larger Empedocles volcano, last erupted in 1831, it is located between the island of Pantelleria. The autonomous region includes several neighbouring islands: the Aegadian Islands, the Aeolian Islands and Lampedusa; the island is drained by several rivers, most of which flow through the central area and enter the sea at the south of the island.
The Salso flows through parts of Enna and Caltanissetta before entering the Mediterranean Sea at the port of Licata. To the east, the Alcantara flows through the province of Messina and enters the sea at Giardini Naxos, the Simeto, which flows into the Ionian Sea south of Catania. Other important rivers on the island are the Platani in the southwest. Sicily has a typical Mediterranean climate with mild and wet winters and hot, dry summers with changeable intermediate seasons. On the coasts the south-western, the climate is affected by the African currents and summers can be scorching. Sicily is seen as an island of warm winters but above all along the Tyrrhenian coast and in the inland areas, winters can be cold, with typical continental climate. Snow falls in abundance above 900–1000 metres, but stronger cold waves can carry it in the hills and in coastal cities on the northern coast of the island; the interi
Mausolus was a ruler of Caria, nominally a satrap of the Achaemenid Empire. He enjoyed the status of king or dynast by virtue of the powerful position created by his father Hecatomnus who had succeeded the assassinated Persian Satrap Tissaphernes in the Carian satrapy and founded the hereditary dynasty of the Hecatomnids. Mausolus was the eldest son of Hecatomnus, a native Carian who became the satrap of Caria when Tissaphernes died, around 395 BC. Mausolus participated in the Revolt of the Satraps, both on his nominal sovereign Artaxerxes Mnemon's side and against him. In 366 BC, Mausolus together with Autophradates of Lydia, at the request of Artaxerxes, led the siege of Adramyttium against Ariobarzanes, one of the members of the Great Satraps' Revolt, until Agesilaus, king of Sparta, negotiated the besiegers' retreat. Mausolus conquered a great part of Lycia circa 360 BC, putting an end to the line of dynasts that had ruled there, he invaded Ionia and several Greek islands. He moved his capital from the ancient seat of the Carian kings, to Halicarnassus.
Mausolus embraced Hellenic culture. He is best known for the monumental shrine, the Mausoleum at Halicarnassus and named for him by order of his widow Artemisia. Antipater of Sidon listed the Mausoleum as one of the Seven Wonders of the Ancient World; the architects Satyrus and Pythis, the sculptors Scopas of Paros, Leochares and Timotheus, finished the work after the death of Artemisia, some of them working purely for renown. The site and a few remains can still be seen in the Turkish town of Bodrum. Derived from his name, the term mausoleum has come to be used generically for any grand tomb. An inscription discovered at Milas, the ancient Mylasa, details the punishment of certain conspirators who had made an attempt upon his life at a festival in a temple at Labranda in 353 BC. Simon Hornblower: Mausolus, Clarendon Press, Oxford 1982 Livius, Maussolus by Jona Lendering Caria This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed.. "Mausolus". Encyclopædia Britannica.
17. Cambridge University Press. P. 917
An impact crater is an circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters range from small, bowl-shaped depressions to large, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth. Impact craters are the dominant geographic features on many solid Solar System objects including the Moon, Callisto and most small moons and asteroids. On other planets and moons that experience more active surface geological processes, such as Earth, Mars, Europa, Io and Titan, visible impact craters are less common because they become eroded, buried or transformed by tectonics over time. Where such processes have destroyed most of the original crater topography, the terms impact structure or astrobleme are more used.
In early literature, before the significance of impact cratering was recognised, the terms cryptoexplosion or cryptovolcanic structure were used to describe what are now recognised as impact-related features on Earth. The cratering records of old surfaces, such as Mercury, the Moon, the southern highlands of Mars, record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago. The rate of crater production on Earth has since been lower, but it is appreciable nonetheless; this indicates that there should be far more young craters on the planet than have been discovered so far. The cratering rate in the inner solar system fluctuates as a consequence of collisions in the asteroid belt that create a family of fragments that are sent cascading into the inner solar system. Formed in a collision 160 million years ago, the Baptistina family of asteroids is thought to have caused a large spike in the impact rate causing the Chicxulub impact that may have triggered the extinction of the non-avian dinosaurs 66 million years ago.
Note that the rate of impact cratering in the outer Solar System could be different from the inner Solar System. Although Earth's active surface processes destroy the impact record, about 190 terrestrial impact craters have been identified; these range in diameter from a few tens of meters up to about 300 km, they range in age from recent times to more than two billion years, though most are less than 500 million years old because geological processes tend to obliterate older craters. They are selectively found in the stable interior regions of continents. Few undersea craters have been discovered because of the difficulty of surveying the sea floor, the rapid rate of change of the ocean bottom, the subduction of the ocean floor into Earth's interior by processes of plate tectonics. Impact craters are not to be confused with landforms that may appear similar, including calderas, glacial cirques, ring dikes, salt domes, others. Daniel M. Barringer, a mining engineer, was convinced that the crater he owned, Meteor Crater, was of cosmic origin.
Yet, most geologists at the time assumed. In the 1920s, the American geologist Walter H. Bucher studied a number of sites now recognized as impact craters in the United States, he concluded they had been created by some great explosive event, but believed that this force was volcanic in origin. However, in 1936, the geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and concluded that the craters that he studied were formed by impacts. Grove Karl Gilbert suggested in 1893. Ralph Baldwin in 1949 wrote that the Moon's craters were of impact origin. Around 1960, Gene Shoemaker revived the idea. According to David H. Levy, Gene "saw the craters on the Moon as logical impact sites that were formed not in eons, but explosively, in seconds." For his Ph. D. degree at Princeton, under the guidance of Harry Hammond Hess, Shoemaker studied the impact dynamics of Barringer Meteor Crater. Shoemaker noted Meteor Crater had the same form and structure as two explosion craters created from atomic bomb tests at the Nevada Test Site, notably Jangle U in 1951 and Teapot Ess in 1955.
In 1960, Edward C. T. Chao and Shoemaker identified at Meteor Crater, proving the crater was formed from an impact generating high temperatures and pressures, they followed this discovery with the identification of coesite within suevite at Nördlinger Ries, proving its impact origin. Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Astrophysical Observatory in Victoria, British Columbia and Wolf von Engelhardt of the University of Tübingen in Germany began a methodical search for impact craters. By 1970, they had tentatively identified more than 50. Although their work was controversial, the American Apollo Moon landings, which were in progress at the time, provided supportive evidence by recognizing the rate of impact cratering on the Moon; because the processes of erosion on the Moon are minimal, craters persist. Since the Earth could be expected to have the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters.
Impact cratering invo
A planet is an astronomical body orbiting a star or stellar remnant, massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, has cleared its neighbouring region of planetesimals. The term planet is ancient, with ties to history, science and religion. Five planets in the Solar System are visible to the naked eye; these were regarded by many early cultures as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union adopted a resolution defining planets within the Solar System; this definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas and Vesta, Pluto, that were once considered planets by the scientific community, are no longer viewed as such.
The planets were thought by Ptolemy to orbit Earth in epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular; as observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes and hydrology. Planets are divided into two main types: large low-density giant planets, smaller rocky terrestrials. There are eight planets in the Solar System.
In order of increasing distance from the Sun, they are the four terrestrials, Venus and Mars the four giant planets, Saturn and Neptune. Six of the planets are orbited by one or more natural satellites. Several thousands of planets around other stars have been discovered in the Milky Way; as of 1 April 2019, 4,023 known extrasolar planets in 3,005 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 100 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.e. in the circumstellar habitable zone. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. 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, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, 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. Ancient Greeks called these lights πλάνητες ἀστέρες or πλανῆται, from which today's word "planet" was derived. In ancient Greece, China and indeed all pre-modern civilizations, it was universally believed that Earth was the center of the Universe and that all the "planets" circled Earth.
The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the 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 dates as early as the second millennium BC. The MUL. APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun and planets over the course of the year; the Babylonian astrologers laid the foundations of what would become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.
Venus and the outer planets Mars and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times; the ancient Greeks did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5t