History (from Greek ἱστορία, historia, meaning "inquiry; knowledge acquired by investigation") is the study of the past. Events before the invention of writing systems are considered prehistory. "History" is an umbrella term comprising past events as well as the memory, discovery, collection, organization, presentation, and interpretation of these events. Historians seek knowledge of the past using historical sources such as written documents, oral accounts, art and material artifacts, and ecological markers.History also includes the academic discipline which uses narrative to describe, examine, question, and analyze past events, and investigate their patterns of cause and effect. Historians often debate which narrative best explains an event, as well as the significance of different causes and effects. Historians also debate the nature of history as an end in itself, as well as its usefulness to give perspective on the problems of the present.Stories common to a particular culture, but not supported by external sources (such as the tales surrounding King Arthur), are usually classified as cultural heritage or legends. History differs from myth in that it is supported by evidence. However, ancient cultural influences have helped spawn variant interpretations of the nature of history which have evolved over the centuries and continue to change today. The modern study of history is wide-ranging, and includes the study of specific regions and the study of certain topical or thematic elements of historical investigation. History is often taught as part of primary and secondary education, and the academic study of history is a major discipline in university studies.
Herodotus, a 5th-century BC Greek historian, is often considered the "father of history" in the Western tradition, although he has also been criticized as the "father of lies". Along with his contemporary Thucydides, he helped form the foundations for the modern study of past events and societies. Their works continue to be read today, and the gap between the culture-focused Herodotus and the military-focused Thucydides remains a point of contention or approach in modern historical writing. In East Asia, a state chronicle, the Spring and Autumn Annals, was reputed to date from as early as 722 BC, although only 2nd-century BC texts have survived.Affect may refer to:
Affect (education)
Affect (linguistics), attitude or emotion that a speaker brings to an utterance
Affect (philosophy)
Affect (psychology), the experience of feeling or emotion
Affect display, signs of emotion, such as facial expression, vocalization, and posture
Affect theory
Affective science, the scientific study of emotion
Affective computing, an area of research in computer science aiming to understand the emotional state of users
Reduced affect display, a.k.a. emotional blunting or affective flattening, a reduction in emotional reactivity
Pseudobulbar affect, a.k.a. labile affect, the unstable display of emotion
Affect (rhetoric), the responsive, emotional feeling that precedes cognition
Affected accent; see Accent (sociolinguistics)
Affect (company), a defunct Japanese video game developerA planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Apart from Earth itself, five planets in the Solar System are often visible to the naked eye. These were regarded by many early cultures as divine, or 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 (IAU) officially 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 current definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as planets under the current definition of planet.
The planets were thought by Ptolemy to orbit Earth in deferent and 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, and 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, tectonics, and even hydrology.
Planets in the Solar System are divided into two main types: large low-density giant planets, and smaller rocky terrestrials. There are eight planets in the Solar System according to the IAU definition. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, the two exceptions being Mercury and Venus.
Several thousands of planets around other stars ("extrasolar planets" or "exoplanets") have been discovered in the Milky Way. As of 1 September 2021, 4,834 known extrasolar planets in 3,572 planetary systems (including 795 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 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 20 December 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.Prevention may refer to: