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EVERY THING ABOUT PLANET


140px-Mercury in color - Prockter07 centered130px-Venus-real color126px-Africa and Europe from a Million Miles Away130px-Mars 23 aug 2003 hubble123px-Jupiter New Horizons138px-Uranus2148px-Jewel of the Solar System138px-Neptune Full

DEFINITION OF A PLANET

A planet (from Ancient Greek ἀστήρ πλανήτης(astēr planētēs), or πλάνηςἀστήρ(plánēs astēr), meaning "wandering star") is an astronomical object orbiting a star, brown dwarf, or stellar remnant that


*.is massive enough to be rounded by its own gravity,

*.is not massive enough to cause thermonuclear fusion, and

*.has cleared its neighbouring regionof planetesimals.


The term planet is ancient, with ties to history, science, mythology, and religion. Several planets in the Solar Systemcan be seen with 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 Astron ...(the first trans-Neptunian object discovered), that were once considered planets by the scientific community are no longer viewed as such.
The planets were thought by Ptolemyto orbit Earth in deferent an ..., tectonics, and even hydrology. Planets are generally divided into two main types: large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. 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.
More than a thousand planets around other stars (" extrasolar planets" or "exoplanets") have been discovered in the Milky Way: as of 25 August 2015, 1949 known extrasolar planets in 1233 planetary systems(including 487 multiple planetary systems), ranging in size from just above the size of the Moonto gas giants about twice as large as Jupiter. On December 20, 2011, the Kepler Space Telescopeteam reported the discovery of the first Earth-sizedextrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20.

A 2012 study, analyzing gravitational microlensingdata, 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.




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HISTORY OF A PLANET


History of a planet uzopedia 1The idea of planets has evolved over its history, from the divine wandering starsof 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, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars.

Ancient Greeks called these lights πλάνητες ἀστέρες(planētes asteres, "wandering stars") or simply πλανῆται(planētai, "wanderers"), from which today's word "planet" was derived. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universeand 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 apparently common-senseperceptions that Earth was solid and stable and that it was not moving but at rest.

Babylon

Main article: Babylonian astronomy

The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamiain 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 second millennium BC. The MUL.APINis a pair of cuneiformtablets dating from the 7th century BC that lays out the motions of the Sun, Moon and planets over the course of the year. The Babylonian astrologersalso laid the foundations of what would eventually become Western astrology. The Enuma anu enlil, written during the Neo-Assyrianperiod in the 7th century BC, comprises a list of omensand their relationships with various celestial phenomena including the motions of the planets. Venus, Mercuryand the outer planets Mars, Jupiterand Saturnwere all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescopein early modern times.

Greco-Roman astronomy

Ptolemy's 7 planetary spheres
1
Moon
2
Mercury
3
Venus
4
Sun
5
Mars
6
Jupiter
7
Saturn

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. Pythagorasor Parmenidesis 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). In the 3rd century BC, Aristarchus of Samosproposed a heliocentricsystem, 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 Almagestwritten by Ptolemyin 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.To the Greeks and Romans there were seven known planets, each presumed to be circling Earthaccording to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.

INDIA

In 499 CE, the Indian astronomer Aryabhatapropounded a planetary model that explicitly incorporated Earth's rotationabout 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.
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