Asteroids are minor planets of the inner Solar System. Larger asteroids have been called planetoids; these terms have been applied to any astronomical object orbiting the Sun that did not resemble a planet-like disc and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered they were found to have volatile-rich surfaces similar to comets; as a result, they were distinguished from objects found in the main asteroid belt. In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There exist millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets; the vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, S-type. These were named after and are identified with carbon-rich and silicate compositions, respectively; the sizes of asteroids varies greatly. Asteroids are differentiated from meteoroids. In the case of comets, the difference is one of composition: while asteroids are composed of mineral and rock, comets are composed of dust and ice. Furthermore, asteroids formed closer to the sun; the difference between asteroids and meteoroids is one of size: meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter. Meteoroids can be composed of either cometary or asteroidal materials. Only one asteroid, 4 Vesta, which has a reflective surface, is visible to the naked eye, this only in dark skies when it is favorably positioned. Small asteroids passing close to Earth may be visible to the naked eye for a short time; as of October 2017, the Minor Planet Center had data on 745,000 objects in the inner and outer Solar System, of which 504,000 had enough information to be given numbered designations.
The United Nations declared 30 June as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, Russian Federation, on 30 June 1908. In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit, but we're not 100 percent sure when." In 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched; the first asteroid to be discovered, was considered to be a new planet.
This was followed by the discovery of other similar bodies, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid", coined in Greek as ἀστεροειδής, or asteroeidēs, meaning'star-like, star-shaped', derived from the Ancient Greek ἀστήρ astēr'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" were still used interchangeably. Overview of discovery timeline: 10 by 1849 1 Ceres, 1801 2 Pallas – 1802 3 Juno – 1804 4 Vesta – 1807 5 Astraea – 1845 in 1846, planet Neptune was discovered 6 Hebe – July 1847 7 Iris – August 1847 8 Flora – October 1847 9 Metis – 25 April 1848 10 Hygiea – 12 April 1849 tenth asteroid discovered 100 asteroids by 1868 1,000 by 1921 10,000 by 1989 100,000 by 2005 ~700,000 by 2015 Asteroid discovery methods have improved over the past two centuries.
In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would be spotted; the expected motion of the missing planet was about 30 seconds of arc per hour discernible by observers. The first object, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily, he discovered a new star-like object in Taurus and followed the displacement of this object during several nights. That year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, found to be between the planets Mars and Jupiter.
Piazzi named it after Ceres, the Roman goddess of agriculture. Three other asteroids (2 Pallas, 3 Juno, 4 Ves
The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process, it is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Three quarters of the Sun's mass consists of hydrogen; the Sun is a G-type main-sequence star based on its spectral class. As such, it is informally and not accurately referred to as a yellow dwarf, it formed 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System; the central mass became so hot and dense that it initiated nuclear fusion in its core. It is thought that all stars form by this process.
The Sun is middle-aged. It fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result; this energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand to become a red giant, it is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, render Earth uninhabitable. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, no longer produce energy by fusion, but still glow and give off heat from its previous fusion; the enormous effect of the Sun on Earth has been recognized since prehistoric times, the Sun has been regarded by some cultures as a deity.
The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of, the predominant calendar in use today. The English proper name Sun may be related to south. Cognates to English sun appear in other Germanic languages, including Old Frisian sunne, Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, Gothic sunnō. All Germanic terms for the Sun stem from Proto-Germanic *sunnōn; the Latin name for the Sun, Sol, is not used in everyday English. Sol is used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars; the related word solar is the usual adjectival term used for the Sun, in terms such as solar day, solar eclipse, Solar System. A mean Earth solar day is 24 hours, whereas a mean Martian'sol' is 24 hours, 39 minutes, 35.244 seconds. The English weekday name Sunday stems from Old English and is a result of a Germanic interpretation of Latin dies solis, itself a translation of the Greek ἡμέρα ἡλίου.
The Sun is a G-type main-sequence star. The Sun has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. The Sun is heavy-element-rich, star; the formation of the Sun may have been triggered by shockwaves from more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars; the heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star. The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is 1 astronomical unit, though the distance varies as Earth moves from perihelion in January to aphelion in July.
At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports all life on Earth by photosynthesis, drives Earth's climate and weather; the Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere. For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometres; the tidal effect of the planets is weak and does not affect the shape of the Sun. The Sun rotates faster at its equator than at its poles; this differential rotation is caused by convective motion
Rochester, New York
Rochester is a city on the southern shore of Lake Ontario in western New York. With a population of 208,046 residents, Rochester is the seat of Monroe County and the third most populous city in New York state, after New York City and Buffalo; the metropolitan area has a population of just over 1 million people. It is about 73 miles east of Buffalo and 87 miles west of Syracuse. Rochester was one of America's first boomtowns due to the fertile Genesee River Valley, which gave rise to numerous flour mills, as a manufacturing hub. Several of the region's universities have renowned research programs. Rochester is the site of many important innovations in consumer products; the Rochester area has been the birthplace to Kodak, Western Union, French's, Bausch & Lomb and Xerox, which conduct extensive research and manufacturing of industrial and consumer products. Until 2010, the Rochester metropolitan area was the second-largest regional economy in New York State, after the New York City metropolitan area.
Rochester's GMP has since ranked just below Buffalo, New York, while exceeding it in per-capita income. The 25th edition of the Places Rated Almanac rated Rochester as the "most livable city" in 2007, among 379 U. S. metropolitan areas. In 2010 Forbes rated Rochester as the third-best place to raise a family in the United States. In 2012 Kiplinger rated Rochester as the fifth-best city in the United States for families, citing low cost of living, top public schools, a low jobless rate. Rochester is a Global city with Sufficiency status; the Seneca tribe of Native Americans lived in and around Rochester until they lost their claim to most of this land in the Treaty of Big Tree in 1797. Settlement before the Seneca tribe is unknown. Development of Rochester followed the American Revolution, forced cession of their territory by the Iroquois after the defeat of Great Britain. Allied with the British, four major Iroquois tribes were forced out of New York; as a reward for their loyalty to the British Crown, they were given a large land grant on the Grand River in Canada.
Rochester was founded shortly after the American Revolution by a wave of English-Puritan descended immigrants from New England who were looking for new agricultural land. They would be the dominant cultural group in Rochester for over a century. On November 8, 1803, Col. Nathaniel Rochester, Maj. Charles Carroll, Col. William Fitzhugh, Jr. all of Hagerstown, purchased a 100-acre tract from the state in Western New York along the Genesee River. They chose the site because its three cataracts on the Genesee offered great potential for water power. Beginning in 1811, with a population of 15, the three founders surveyed the land and laid out streets and tracts. In 1817, the Brown brothers and other landowners joined their lands with the Hundred Acre Tract to form the village of Rochesterville. By 1821, Rochesterville was the seat of Monroe County. In 1823, Rochesterville consisted of 1,012 acres and 2,500 residents, the Village of Rochesterville became known as Rochester. In 1823, the Erie Canal aqueduct over the Genesee River was completed, the Erie Canal east to the Hudson River was opened.
In the early 20th century, after the advent of railroads, the presence of the canal in the center city was an obstacle. By 1830, Rochester's population was 9,200 and in 1834, it was re-chartered as a city. Rochester was first known as "the Young Lion of the West", as the "Flour City". By 1838, Rochester was the largest flour-producing city in the United States. Having doubled its population in only 10 years, Rochester became America's first "boomtown". In 1830-31, Rochester experienced one of the nation's biggest Protestant revivalist movements, led by Charles Finney; the revival has been noted as inspiring other revivals of the Second Great Awakening. A leading pastor in New York, converted in the Rochester meetings gave the following account of the effects of Finney's meetings in that city: "The whole community was stirred. Religion was the topic of conversation in the house, in the office and on the street; the only theater in the city was converted into a livery stable. Grog shops were closed.
Nurseries ringed the city, the most famous of, started in 1840 by immigrants Georg Ellwanger from Germany and Patrick Barry from Ireland. In 1847, Frederick Douglass founded the abolitionist newspaper The North Star in Rochester. Douglass, a former slave and an antislavery speaker and writer, gained a circulation of over 4,000 readers in the United States and the Caribbean; the North Star served as a forum for abolitionist views. The Douglass home burnt down in 1872, but a marker for it is found in Highland Park off South Avenue. Susan B. Anthony, a national leader of the women's suffrage movement, was from Rochester; the Nineteenth Amendment to the United States Constitution, which guaranteed the right of women to vote in 1920, was known as the Susan B. Anthony Amendment because of her work toward its passage, which she did not live to see. Anthony's home is a National Historic Landmark known as the National Susan B. Anthony Museum and House. At the end of the 19th century, anarchi
Michigan is a state in the Great Lakes and Midwestern regions of the United States. The state's name, originates from the Ojibwe word mishigamaa, meaning "large water" or "large lake". With a population of about 10 million, Michigan is the tenth most populous of the 50 United States, with the 11th most extensive total area, is the largest state by total area east of the Mississippi River, its capital is Lansing, its largest city is Detroit. Metro Detroit is among the nation's largest metropolitan economies. Michigan is the only state to consist of two peninsulas; the Lower Peninsula is noted as shaped like a mitten. The Upper Peninsula is separated from the Lower Peninsula by the Straits of Mackinac, a five-mile channel that joins Lake Huron to Lake Michigan; the Mackinac Bridge connects the peninsulas. The state has the longest freshwater coastline of any political subdivision in the world, being bounded by four of the five Great Lakes, plus Lake Saint Clair; as a result, it is one of the leading U.
S. states for recreational boating. Michigan has 64,980 inland lakes and ponds. A person in the state is never more than six miles from a natural water source or more than 85 miles from a Great Lakes shoreline; the area was first occupied by a succession of Native American tribes over thousands of years. Inhabited by Natives, Métis, French explorers in the 17th century, it was claimed as part of New France colony. After France's defeat in the French and Indian War in 1762, the region came under British rule. Britain ceded this territory to the newly independent United States after Britain's defeat in the American Revolutionary War; the area was part of the larger Northwest Territory until 1800, when western Michigan became part of the Indiana Territory. Michigan Territory was formed in 1805, but some of the northern border with Canada was not agreed upon until after the War of 1812. Michigan was admitted into the Union in 1837 as a free one, it soon became an important center of industry and trade in the Great Lakes region and a popular immigrant destination in the late 19th and early 20th centuries.
Although Michigan developed a diverse economy, it is known as the center of the U. S. automotive industry, which developed as a major economic force in the early 20th century. It is home to the country's three major automobile companies. While sparsely populated, the Upper Peninsula is important for tourism thanks to its abundance of natural resources, while the Lower Peninsula is a center of manufacturing, agriculture and high-tech industry; when the first European explorers arrived, the most populous tribes were Algonquian peoples, which include the Anishinaabe groups of Ojibwe, Odaawaa/Odawa, the Boodewaadamii/Bodéwadmi. The three nations co-existed peacefully as part of a loose confederation called the Council of Three Fires; the Ojibwe, whose numbers are estimated to have been between 25,000 and 35,000, were the largest. The Ojibwe were established in Michigan's Upper Peninsula and northern and central Michigan, inhabited Ontario and southern Manitoba, Canada; the Ottawa lived south of the Straits of Mackinac in northern and southern Michigan, but in southern Ontario, northern Ohio and eastern Wisconsin.
The Potawatomi were in southern and western Michigan, in addition to northern and central Indiana, northern Illinois, southern Wisconsin, southern Ontario. Other Algonquian tribes in Michigan, in the south and east, were the Mascouten, the Menominee, the Miami, the Sac, the Fox; the Wyandot were an Iroquoian-speaking people in this area. French voyageurs and coureurs des bois settled in Michigan in the 17th century; the first Europeans to reach what became Michigan were those of Étienne Brûlé's expedition in 1622. The first permanent European settlement was founded in 1668 on the site where Père Jacques Marquette established Sault Ste. Marie, Michigan as a base for Catholic missions. Missionaries in 1671–75 founded outlying stations at Saint Ignace and Marquette. Jesuit missionaries were well received by the area's Indian populations, with few difficulties or hostilities. In 1679, Robert Cavelier, Sieur de la Salle built Fort Miami at present-day St. Joseph. In 1691, the French established a trading post and Fort St. Joseph along the St. Joseph River at the present-day city of Niles.
In 1701, French explorer and army officer Antoine de la Mothe Cadillac founded Fort Pontchartrain du Détroit or "Fort Pontchartrain on-the-Strait" on the strait, known as the Detroit River, between lakes Saint Clair and Erie. Cadillac had convinced King Louis XIV's chief minister, Louis Phélypeaux, Comte de Pontchartrain, that a permanent community there would strengthen French control over the upper Great Lakes and discourage British aspirations; the hundred soldiers and workers who accompanied Cadillac built a fort enclosing one arpent and named it Fort Pontchartrain. Cadillac's wife, Marie Thérèse Guyon, soon moved to Detroit, becoming one of the first European women to settle in what was considered the wilderness of Michigan; the town became a major fur-trading and shipping post. The Église de Saint-Anne was founded the same year. While the original building does not survive, the congregation remains active. Cadillac departed to serve as the French governor of Louisiana from 1710 to 1716.
French attempts to consol
Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, the densest giant planet. Neptune is 17 times the mass of Earth more massive than its near-twin Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 AU. It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune's trident. Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier.
Its largest moon, was discovered shortly thereafter, though none of the planet's remaining known 13 moons were located telescopically until the 20th century. The planet's distance from Earth gives it a small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989; the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has allowed for additional detailed observations from afar. Like Jupiter and Saturn, Neptune's atmosphere is composed of hydrogen and helium, along with traces of hydrocarbons and nitrogen, though it contains a higher proportion of "ices" such as water and methane. However, similar to Uranus, its interior is composed of ices and rock. Traces of methane in the outermost regions in part account for the planet's blue appearance. In contrast to the hazy featureless atmosphere of Uranus, Neptune's atmosphere has active and visible weather patterns.
For example, at the time of the Voyager 2 flyby in 1989, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h; because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K. Temperatures at the planet's centre are 5,400 K. Neptune has a faint and fragmented ring system, discovered in 1984 later confirmed by Voyager 2; some of the earliest recorded observations made through a telescope, Galileo's drawings on 28 December 1612 and 27 January 1613 contain plotted points that match up with what is now known to be the position of Neptune. On both occasions, Galileo seems to have mistaken Neptune for a fixed star when it appeared close—in conjunction—to Jupiter in the night sky. At his first observation in December 1612, Neptune was stationary in the sky because it had just turned retrograde that day.
This apparent backward motion is created. Because Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope. In July 2009, University of Melbourne physicist David Jamieson announced new evidence suggesting that Galileo was at least aware that the "star" he had observed had moved relative to the fixed stars. In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbour Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesise that an unknown body was perturbing the orbit through gravitational interaction. In 1843, John Couch Adams began work on the orbit of Uranus using the data. Via Cambridge Observatory director James Challis, he requested extra data from Sir George Airy, the Astronomer Royal, who supplied it in February 1844. Adams produced several different estimates of a new planet. In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations but aroused no enthusiasm in his compatriots.
In June 1846, upon seeing Le Verrier's first published estimate of the planet's longitude and its similarity to Adams's estimate, Airy persuaded Challis to search for the planet. Challis vainly scoured the sky throughout September. Meanwhile, Le Verrier by letter urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory's refractor. Heinrich d'Arrest, a student at the observatory, suggested to Galle that they could compare a drawn chart of the sky in the region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. On the evening of 23 September 1846, the day Galle received the letter, he discovered Neptune within 1° of where Le Verrier had predicted it to be, about 12° from Adams' prediction. Challis realised that he had observed the planet twice, on 4 and 12 August, but did not recognise it as a planet because he lacked an up-to-date star map and was distracted by his concurrent work on comet observations.
In the wake of the discovery, there was much nationalistic rivalry between the French and the British over who deserved credit for the discovery. An international consen
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
Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 87.97 days is the shortest of all the planets in the Solar System. It is named after the messenger of the gods. Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet, never exceeds 28° away from the Sun when viewed from Earth; this proximity to the Sun means the planet can only be seen near the western or eastern horizon during the early evening or early morning. At this time it may appear as a bright star-like object, but is far more difficult to observe than Venus; the planet telescopically displays the complete range of phases, similar to Venus and the Moon, as it moves in its inner orbit relative to Earth, which reoccurs over the so-called synodic period every 116 days. Mercury is tidally locked with the Sun in a 3:2 spin-orbit resonance, rotates in a way, unique in the Solar System; as seen relative to the fixed stars, it rotates on its axis three times for every two revolutions it makes around the Sun.
As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two Mercurian years. Mercury's axis has the smallest tilt of any of the Solar System's planets, its orbital eccentricity is the largest of all known planets in the Solar System. Mercury's surface appears cratered and is similar in appearance to the Moon's, indicating that it has been geologically inactive for billions of years. Having no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K at night to 700 K during the day across the equatorial regions; the polar regions are below 180 K. The planet has no known natural satellites. Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; the BepiColombo spacecraft is planned to arrive at Mercury in 2025. Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, a solid inner core.
Mercury is one of four terrestrial planets in the Solar System, is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres. Mercury is smaller—albeit more massive—than the largest natural satellites in the Solar System and Titan. Mercury consists of 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm3, only less than Earth's density of 5.515 g/cm3. If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm3 versus Earth's 4.4 g/cm3. Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be rich in iron.
Geologists estimate. Research published in 2007 suggests. Surrounding the core is a 500–700 km mantle consisting of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury's crust is estimated to be 35 km thick.. One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length, it is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had solidified. Mercury's core has a higher iron content than that of any other major planet in the Solar System, several theories have been proposed to explain this; the most accepted theory is that Mercury had a metal–silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, a mass 2.25 times its current mass. Early in the Solar System's history, Mercury may have been struck by a planetesimal of 1/6 that mass and several thousand kilometers across.
The impact would have stripped away much of the original crust and mantle, leaving the core behind as a major component. A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of the Moon. Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized, it would have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and even as high as 10,000 K. Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by the solar wind. A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury; each hypothesis predicts a different surface composition, there are two space missions set to make observations.