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Definition Of Planet
The definition of ''planet'', since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term (), 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different objects, from the Sun and the Moon to satellites and asteroids. In modern astronomy, there are two primary conceptions of a 'planet'. Disregarding the often inconsistent technical details, they are whether an astronomical body ''dynamically dominates'' its region (that is, whether it controls the fate of other smaller bodies in its vicinity) or whether it is in ''hydrostatic equilibrium'' (that is, whether it looks round). These may be characterized as the dynamical dominance definition and the geophysical definition. The issue of a clear definition for ''planet'' came to a head in January 2005 with the discovery of the trans-Neptunian object Eris, a bo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Voyager 2 Neptune And Triton
Voyager may refer to: Computing and communications * LG Voyager, a mobile phone model manufactured by LG Electronics * NCR Voyager, a computer platform produced by NCR Corporation * Voyager (computer worm), a computer worm affecting Oracle databases * Voyager (library program), the integrated library system from Ex Libris Group * Voyager (web browser), a web browser for Amiga computers * HP Voyager series, code name for a Hewlett-Packard series of handheld programmable calculators Transport Air * Airbus Voyager, Royal Air Force version of the Airbus A330 MRTT * Frequent flyer program of South African Airways * Egvoyager Voyager 203, an Italian ultralight aircraft * Raj Hamsa Voyager, an Indian ultralight trike design * Rutan Voyager, the first airplane to fly around the world nonstop without refuelling Land * Bombardier Voyager, a high-speed train operated in the United Kingdom ** Bombardier ''Voyager'' (British Rail Class 220), a non-tilting train built 2000–2001 ** Bom ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Prague
Prague ( ; cs, Praha ; german: Prag, ; la, Praga) is the capital and largest city in the Czech Republic, and the historical capital of Bohemia. On the Vltava river, Prague is home to about 1.3 million people. The city has a temperate oceanic climate, with relatively warm summers and chilly winters. Prague is a political, cultural, and economic hub of central Europe, with a rich history and Romanesque, Gothic, Renaissance and Baroque architectures. It was the capital of the Kingdom of Bohemia and residence of several Holy Roman Emperors, most notably Charles IV (r. 1346–1378). It was an important city to the Habsburg monarchy and Austro-Hungarian Empire. The city played major roles in the Bohemian and the Protestant Reformations, the Thirty Years' War and in 20th-century history as the capital of Czechoslovakia between the World Wars and the post-war Communist era. Prague is home to a number of well-known cultural attractions, many of which survived the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; however, with its larger volume, Saturn is over 95 times more massive. Saturn's interior is most likely composed of a core of iron–nickel and rock (silicon and oxygen compounds). Its core is surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and finally, a gaseous outer layer. Saturn has a pale yellow hue due to ammonia crystals in its upper atmosphere. An electrical current within the metallic hydrogen layer is thought to give rise to Saturn's planetary magnetic field, which is weaker than Earth's, but which has a magnetic moment 580 times that of Earth due to Saturn's larger size. Saturn's magnetic field strength is around one-twentieth of Jupiter's. The outer atmosphere is g ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth the mass of the Sun. Jupiter is the List of brightest natural objects in the sky, third brightest natural object in the Earth's night sky after the Moon and Venus, and it has been observed since Pre-history, prehistoric times. It was named after the Jupiter (mythology), Roman god Jupiter, the king of the gods. Jupiter is primarily composed of hydrogen, but helium constitutes one-quarter of its mass and one-tenth of its volume. It probably has a rocky core of heavier elements, but, like the other giant planets in the Solar System, it lacks a well-defined solid surface. The ongoing contraction of Jupiter's interior generates more heat than it receives from the Sun. Because of its rapid rotation, the planet' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terrestrial planet with a thin atmosphere (less than 1% that of Earth's), and has a crust primarily composed of elements similar to Earth's crust, as well as a core made of iron and nickel. Mars has surface features such as impact craters, valleys, dunes and polar ice caps. It has two small and irregularly shaped moons, Phobos (moon), Phobos and Deimos (moon), Deimos. Some of the most notable surface features on Mars include Olympus Mons, the largest volcano and List of tallest mountains in the Solar System, highest known mountain in the Solar System and Valles Marineris, one of the largest canyons in the Solar System. The North Polar Basin (Mars), Borealis basin in the Northern Hemisphere covers approximately 40% of the planet and may be a la ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never far from the Sun, either as morning star or evening star. Aside from the Sun and Moon, Venus is the brightest natural object in Earth's sky, capable of casting visible shadows on Earth at dark conditions and being visible to the naked eye in broad daylight. Venus is the second largest terrestrial object of the Solar System. It has a surface gravity slightly lower than on Earth and has a very weak induced magnetosphere. The atmosphere of Venus, mainly consists of carbon dioxide, and is the densest and hottest of the four terrestrial planets at the surface. With an atmospheric pressure at the planet's surface of about 92 times the sea level pressure of Earth and a mean temperature of , the carbon dioxide gas at Venus's surface is in the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mercury (planet)
Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman god ' ( Mercury), god of commerce, messenger of the gods, and mediator between gods and mortals, corresponding to the Greek god Hermes (). Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet, and its apparent distance from the Sun as viewed from Earth never exceeds 28°. This proximity to the Sun means the planet can only be seen near the western horizon after sunset or the eastern horizon before sunrise, usually in twilight. At this time, it may appear as a bright star-like object, but is more difficult to observe than Venus. From Earth, the planet telescopically displays the complete range of phases, similar to Venus and the Moon, which recurs over its synodic period of approximately 116 days. The synodic proximity of Mercury to Earth makes Mercury most ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fixed Stars
In astronomy, fixed stars ( la, stellae fixae) is a term to name the full set of glowing points, astronomical objects actually and mainly stars, that appear not to move relative to one another against the darkness of the night sky in the background, unlike the few lights visible at naked eye, including comets, which appear to move slowly between those "fixed" stars along days, weeks or months on the foreground. Generally speaking, the fixed stars comprise all visible stars other than the Sun, plus the faint strip of the Milky Way. Due to their star-like appareance when sighted at naked eye, individual nebulae and other deep-sky objects also are counted among the fixed stars, despite the fact they are not true stars. The term is actually a misnomer due to the fact that those celestial objects are not really fixed with respect to one another; this is only a perceptual illusion, as stars and other extrasolar astronomical objects are so far away from Earth that human vision is unab ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heliocentrism
Heliocentrism (also known as the Heliocentric model) is the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. The notion that the Earth revolves around the Sun had been proposed as early as the third century BC by Aristarchus of Samos, who had been influenced by a concept presented by Philolaus of Croton (c. 470 – 385 BC). In the 5th century BC the Greek Philosophers Philolaus and Hicetas had the thought on different occasions that our Earth was spherical and revolving around a "mystical" central fire, and that this fire regulated the universe. In medieval Europe, however, Aristarchus' heliocentrism attracted little attention—possibly because of the loss of scientific works of the Hellenistic period. It was not until the sixteenth century that a mathematical model of a heliocentric system was presented by the Renaissance mathema ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Aristarchus Of Samos
Aristarchus of Samos (; grc-gre, Ἀρίσταρχος ὁ Σάμιος, ''Aristarkhos ho Samios''; ) was an ancient Greek astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ... and mathematician who presented the first known Heliocentrism, heliocentric model that placed the Sun at the center of the known universe, with the Earth revolving around the Sun once a year and rotating about its axis once a day. He was a student of Strato of Lampsacus, who was the third head of the Peripatetic school, Peripatetic School in Ancient Greece, Greece. According to Ptolemy, during Aristarchus' time there, he observed the summer solstice of 280 BCE. Along with his contributions to the heliocentric model, as reported by Vitruvius, he created two separate sundials: one that is a flat dis ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geocentric Model
In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded description of the Universe with Earth at the center. Under most geocentric models, the Sun, Moon, stars, and planets all orbit Earth. The geocentric model was the predominant description of the cosmos in many European ancient civilizations, such as those of Aristotle in Classical Greece and Ptolemy in Roman Egypt. Two observations supported the idea that Earth was the center of the Universe: * First, from anywhere on Earth, the Sun appears to revolve around Earth once per day. While the Moon and the planets have their own motions, they also appear to revolve around Earth about once per day. The stars appeared to be fixed on a celestial sphere rotating once each day about an axis through the geographic poles of Earth. * Second, Earth seems to be unmoving from the perspective of an earthbound observer; it feels solid, stable, and stationary. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ancient Greece
Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of culturally and linguistically related city-states and other territories. Most of these regions were officially unified only once, for 13 years, under Alexander the Great's empire from 336 to 323 BC (though this excludes a number of Greek city-states free from Alexander's jurisdiction in the western Mediterranean, around the Black Sea, Cyprus, and Cyrenaica). In Western history, the era of classical antiquity was immediately followed by the Early Middle Ages and the Byzantine period. Roughly three centuries after the Late Bronze Age collapse of Mycenaean Greece, Greek urban poleis began to form in the 8th century BC, ushering in the Archaic period and the colonization of the Mediterranean Basin. This was followed by the age of Classical G ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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