Greek mythology is the body of myths told by the ancient Greeks. These stories concern the origin and the nature of the world, the lives and activities of deities and mythological creatures, the origins and significance of the ancient Greeks' own cult and ritual practices. Modern scholars study the myths in an attempt to shed light on the religious and political institutions of ancient Greece and its civilization, to gain understanding of the nature of myth-making itself; the Greek myths were propagated in an oral-poetic tradition most by Minoan and Mycenaean singers starting in the 18th century BC. Two poems by Homer's near contemporary Hesiod, the Theogony and the Works and Days, contain accounts of the genesis of the world, the succession of divine rulers, the succession of human ages, the origin of human woes, the origin of sacrificial practices. Myths are preserved in the Homeric Hymns, in fragments of epic poems of the Epic Cycle, in lyric poems, in the works of the tragedians and comedians of the fifth century BC, in writings of scholars and poets of the Hellenistic Age, in texts from the time of the Roman Empire by writers such as Plutarch and Pausanias.
Aside from this narrative deposit in ancient Greek literature, pictorial representations of gods and mythic episodes featured prominently in ancient vase-paintings and the decoration of votive gifts and many other artifacts. Geometric designs on pottery of the eighth century BC depict scenes from the Trojan cycle as well as the adventures of Heracles. In the succeeding Archaic and Hellenistic periods and various other mythological scenes appear, supplementing the existing literary evidence. Greek mythology has had an extensive influence on the culture and literature of Western civilization and remains part of Western heritage and language. Poets and artists from ancient times to the present have derived inspiration from Greek mythology and have discovered contemporary significance and relevance in the themes. Greek mythology is known today from Greek literature and representations on visual media dating from the Geometric period from c. 900 BC to c. 800 BC onward. In fact and archaeological sources integrate, sometimes mutually supportive and sometimes in conflict.
Mythical narration plays an important role in nearly every genre of Greek literature. The only general mythographical handbook to survive from Greek antiquity was the Library of Pseudo-Apollodorus; this work attempts to reconcile the contradictory tales of the poets and provides a grand summary of traditional Greek mythology and heroic legends. Apollodorus of Athens wrote on many of these topics, his writings may have formed the basis for the collection. Among the earliest literary sources are the Iliad and the Odyssey. Other poets completed the "epic cycle", but these and lesser poems now are lost entirely. Despite their traditional name, the "Homeric Hymns" have no direct connection with Homer, they are choral hymns from the earlier part of the so-called Lyric age. Hesiod, a possible contemporary with Homer, offers in his Theogony the fullest account of the earliest Greek myths, dealing with the creation of the world. Hesiod's Works and Days, a didactic poem about farming life includes the myths of Prometheus and the Five Ages.
The poet gives advice on the best way to succeed in a dangerous world, rendered yet more dangerous by its gods. Lyrical poets took their subjects from myth, but their treatment became less narrative and more allusive. Greek lyric poets, including Pindar and Simonides, bucolic poets such as Theocritus and Bion, relate individual mythological incidents. Additionally, myth was central to classical Athenian drama; the tragic playwrights Aeschylus and Euripides took most of their plots from myths of the age of heroes and the Trojan War. Many of the great tragic stories took on their classic form in these tragedies; the comic playwright Aristophanes used myths, in The Birds and The Frogs. Historians Herodotus and Diodorus Siculus, geographers Pausanias and Strabo, who traveled throughout the Greek world and noted the stories they heard, supplied numerous local myths and legends giving little-known alternative versions. Herodotus in particular, searched the various traditions presented him and found the historical or mythological roots in the confrontation between Greece and the East.
Herodotus attempted to reconcile the blending of differing cultural concepts. The poetry of the Hellenistic and Roman ages was composed as a literary rather than cultic exercise, it contains many important details that would otherwise be lost. This category includes the works of: The Roman poets Ovid, Valerius Flaccus and Virgil with Servius's commentary; the Greek poets of the Late Antique period: Nonnus, Antoninus Liberalis, Quintus Smyrnaeus. The Greek poets of the Hellenistic period: Apollonius of Rhodes, Pseudo-Eratosthenes, Parthenius. Prose writers from the same periods who make reference to myths includ
The term apsis refers to an extreme point in the orbit of an object. It denotes either the respective distance of the bodies; the word comes via Latin from Greek, there denoting a whole orbit, is cognate with apse. Except for the theoretical possibility of one common circular orbit for two bodies of equal mass at diametral positions, there are two apsides for any elliptic orbit, named with the prefixes peri- and ap-/apo-, added in reference to the body being orbited. All periodic orbits are, according to Newton's Laws of motion, ellipses: either the two individual ellipses of both bodies, with the center of mass of this two-body system at the one common focus of the ellipses, or the orbital ellipses, with one body taken as fixed at one focus, the other body orbiting this focus. All these ellipses share a straight line, the line of apsides, that contains their major axes, the foci, the vertices, thus the periapsis and the apoapsis; the major axis of the orbital ellipse is the distance of the apsides, when taken as points on the orbit, or their sum, when taken as distances.
The major axes of the individual ellipses around the barycenter the contributions to the major axis of the orbital ellipses are inverse proportional to the masses of the bodies, i.e. a bigger mass implies a smaller axis/contribution. Only when one mass is sufficiently larger than the other, the individual ellipse of the smaller body around the barycenter comprises the individual ellipse of the larger body as shown in the second figure. For remarkable asymmetry, the barycenter of the two bodies may lie well within the bigger body, e.g. the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface. If the smaller mass is negligible compared to the larger the orbital parameters are independent of the smaller mass. For general orbits, the terms periapsis and apoapsis are used. Pericenter and apocenter are equivalent alternatives, referring explicitly to the respective points on the orbits, whereas periapsis and apoapsis may refer to the smallest and largest distances of the orbiter and its host.
For a body orbiting the Sun, the point of least distance is the perihelion, the point of greatest distance is the aphelion. The terms become apastron when discussing orbits around other stars. For any satellite of Earth, including the Moon, the point of least distance is the perigee and greatest distance the apogee, from Ancient Greek Γῆ, "land" or "earth". For objects in lunar orbit, the point of least distance is sometimes called the pericynthion and the greatest distance the apocynthion. Perilune and apolune are used. In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and orbiting body. However, in the case of a spacecraft, the terms are used to refer to the orbital altitude of the spacecraft above the surface of the central body; these formulae characterize the pericenter and apocenter of an orbit: Pericenter Maximum speed, v per = μ a, at minimum distance, r per = a. Apocenter Minimum speed, v ap = μ a, at maximum distance, r ap = a.
While, in accordance with Kepler's laws of planetary motion and the conservation of energy, these two quantities are constant for a given orbit: Specific relative angular momentum h = μ a Specific orbital energy ε = − μ 2 a where: a is the semi-major axis: a = r per + r ap 2 μ is the standard gravitational parameter e is the eccentricity, defined as e = r ap − r per r ap + r per = 1 − 2 r ap r per + 1 Note t
Imaging radar is an application of radar, used to create two-dimensional images of landscapes. Imaging radar provides its light to illuminate an area on the ground and take a picture at radio wavelengths, it uses an antenna and digital computer storage to record its images. In a radar image, one can see only the energy, reflected back towards the radar antenna; the radar moves along a flight path and the area illuminated by the radar, or footprint, is moved along the surface in a swath, building the image as it does so. Digital radar images are composed of many dots; each pixel in the radar image represents the radar backscatter for that area on the ground: brighter areas represent high backscatter, darker areas represents low backscatter. The traditional application of radar is to display the position and motion of highly reflective objects by sending out a radiowave signal, detecting the direction and delay of the reflected signal. Imaging radar on the other hand attempts to form an image of one object by furthermore registering the intensity of the reflected signal to determine the amount of scattering.
The registered electromagnetic scattering is mapped onto a two-dimensional plane, with points with a higher reflectivity getting assigned a brighter color, thus creating an image. Several techniques have evolved to do this, they take advantage of the Doppler effect caused by the rotation or other motion of the object and by the changing view of the object brought about by the relative motion between the object and the back-scatter, perceived by the radar of the object flying over the earth. Through recent improvements of the techniques, radar imaging is getting more accurate. Imaging radar has been used to map the Earth, other planets, other celestial objects and to categorize targets for military systems. An imaging radar is a kind of radar equipment. A typical radar technology includes emitting radio waves, receiving their reflection, using this information to generate data. For an imaging radar, the returning waves are used to create an image; when the radio waves reflect off objects, this will make some changes in the radio waves and can provide data about the objects, including how far the waves traveled and what kind of objects they encountered.
Using the acquired data, a computer can create a 2-D image of the target. Imaging radar has several advantages, it can operate in the presence of obstacles that obscure the target, can penetrate ground, water, or walls. Applications include: surface topography & crustal change. Wall parameter estimation uses Utra Wide-Band radar systems; the handle M-sequence UWB radar with horn and circular antennas was used for data gathering and supporting the scanning method. 3-D measurements are supplied by amplitude-modulated laser radars—Erim sensor and Perceptron sensor. In terms of speed and reliability for median-range operations, 3-D measurements have superior performance. Current radar imaging techniques rely on synthetic aperture radar and inverse synthetic aperture radar imaging. Emerging technology utilizes monopulse radar 3-D imaging. Real aperture radar is a form of radar that transmits a narrow angle beam of pulse radio wave in the range direction at right angles to the flight direction and receives the backscattering from the targets which will be transformed to a radar image from the received signals.
The reflected pulse will be arranged in the order of return time from the targets, which corresponds to the range direction scanning. The resolution in the range direction depends on the pulse width; the resolution in the azimuth direction is identical to the multiplication of beam width and the distance to a target. The AVTIS radar is a 94 GHz real aperture 3D imaging radar, it uses Frequency-Modulated Continuous-Wave modulation and employs a mechanically scanned monostatic with sub-metre range resolution. Laser radar is a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light. Laser radar is used for multi-dimensional information gathering. In all information gathering modes, lasers that transmit in the eye-safe region are required as well as sensitive receivers at these wavelengths.3-D imaging requires the capacity to measure the range to the first scatter within every pixel. Hence, an array of range counters is needed. A monolithic approach to an array of range counters is being developed.
This technology must be coupled with sensitive detectors of eye-safe wavelengths. To measure Doppler information requires a different type of detection scheme than is used for spatial imaging; the returned laser energy must be mixed with a local oscillator in a heterodyne system to allow extraction of the Doppler shift. 3-D, Multi-wave and Multi-band, Imaging radar works in one of the two modes - as an arbitrary frequency, arbitrary wave radar, as a C-band analog and digital mode radar. The system architecture of 3-D, Multi-wave and Multi-band, Imaging radar is shown in the figure. Synthetic-aperture radar is a form of radar which moves a real aperture or antenna through a series of positions along the objects to provide distinctive long-term coherent-signal variations; this can be used to obtain higher resolution. SARs produce a two-dimensional image. One dimension in the image is called range and is a measure of the "line-of-sight" distance from the radar to t
The mile is an English unit of length of linear measure equal to 5,280 feet, or 1,760 yards, standardised as 1,609.344 metres by international agreement in 1959. With qualifiers, "mile" is used to describe or translate a wide range of units derived from or equivalent to the Roman mile, such as the nautical mile, the Italian mile, the Chinese mile; the Romans divided their mile into 5,000 roman feet but the greater importance of furlongs in pre-modern England meant that the statute mile was made equivalent to 8 furlongs or 5,280 feet in 1593. This form of the mile spread to the British-colonized nations some of which continue to employ the mile; the US Geological Survey now employs the metre for official purposes but legacy data from its 1927 geodetic datum has meant that a separate US survey mile continues to see some use. While most countries replaced the mile with the kilometre when switching to the International System of Units, the international mile continues to be used in some countries, such as Liberia, the United Kingdom, the United States, a number of countries with fewer than one million inhabitants, most of which are UK or US territories, or have close historical ties with the UK or US.
The mile was abbreviated m. in the past but is now sometimes written as mi to avoid confusion with the SI metre. However, derived units, such as miles per hour or miles per gallon, continue to be universally abbreviated as mph and mpg, respectively; the modern English word mile derives from Middle English myl and Old English mīl, cognate with all other Germanic terms for "miles". These derived from apocopated forms of the Latin mīlia or mīllia, the plural of mīle or mīlle "thousand" but used as a clipped form of mīlle passus or passuum, the Roman mile of one thousand paces; the present international mile is what is understood by the unqualified term "mile". When this distance needs to be distinguished from the nautical mile, the international mile may be described as a "land mile" or "statute mile". In British English, the "statute mile" may refer to the present international miles or to any other form of English mile since the 1593 Act of Parliament, which set it as a distance of 1,760 yards.
Under American law, the "statute mile" refers to the US survey mile. Foreign and historical units translated into English as miles employ a qualifier to describe the kind of mile being used but this may be omitted if it is obvious from the context, such as a discussion of the 2nd-century Antonine Itinerary describing its distances in terms of "miles" rather than "Roman miles"; the mile has been variously abbreviated—with and without a trailing period—as m, M, ml, mi. The American National Institute of Standards and Technology now uses and recommends mi in order to avoid confusion with the SI metre and millilitre. However, derived units such as miles per hour or miles per gallon continue to be abbreviated as mph and mpg rather than mi/h and mi/gal. In the United Kingdom road signs use m as the abbreviation for mile though height and width restrictions use m as the abbreviation for the metre, which may be displayed alongside feet and inches; the BBC style holds that "There is no acceptable abbreviation for'miles'" and so it should be spelt out when used in describing areas.
The Roman mile consisted of a thousand paces as measured by every other step—as in the total distance of the left foot hitting the ground 1,000 times. The ancient Romans, marching their armies through uncharted territory, would push a carved stick in the ground after each 1,000 paces. Well-fed and harshly driven Roman legionaries in good weather thus created longer miles; the distance was indirectly standardised by Agrippa's establishment of a standard Roman foot in 29 BC, the definition of a pace as 5 feet. An Imperial Roman mile thus denoted 5,000 Roman feet. Surveyors and specialized equipment such as the decempeda and dioptra spread its use. In modern times, Agrippa's Imperial Roman mile was empirically estimated to have been about 1,617 yards in length. In Hellenic areas of the Empire, the Roman mile was used beside the native Greek units as equivalent to 8 stadia of 600 Greek feet; the mílion continued to be used as a Byzantine unit and was used as the name of the zero mile marker for the Byzantine Empire, the Milion, located at the head of the Mese near Hagia Sophia.
The Roman mile spread throughout Europe, with its local variations giving rise to the different units below. Arising from the Roman mile is the "milestone". All roads radiated out from the Roman Forum throughout the Empire – 50,000 miles of stone-paved roads. At every mile was placed a shaped stone, on, carved a Roman numeral, indicating the number of miles from the center of Rome – the Forum. Hence, one always knew; the Italian mile was traditionally considered a direct continuation of the Roman mile, equal to 1000 paces, although its actual value over time or between regions could vary greatly. It was used in international contexts from the Middle Ages into the 17th century and is thus known as the "geographical mile", although the geographical mile is now a separate standard unit; the Arabic mile was not the common Arabic unit of length. The Arabic mile was, used by medieval geographers and scientists and constituted a kind of precursor to the nautical or geographical mile, it extended the Roman mile to fit an astronomical approximatio
Washington State University
Washington State University is a public research university in Pullman, Washington. Founded in 1890, WSU is a land-grant university with programs in a broad range of academic disciplines. With an undergraduate enrollment of 24,470 and a total enrollment of 29,686, it is the second largest institution of higher education in Washington state behind the University of Washington; the university operates campuses across Washington known as WSU Spokane, WSU Tri-Cities, WSU Vancouver, all founded in 1989. In 2012, WSU launched an Internet-based Global Campus, which includes its online degree program, WSU Online. In 2015, WSU expanded to a sixth campus, known as WSU Everett; these campuses award bachelor's and master's degrees. Freshmen and sophomores were first admitted to the Vancouver campus in 2006 and to the Tri-Cities campus in 2007. Enrollment for the four campuses and WSU Online exceeds 29,686 students; this includes 1,751 international students. WSU's athletic teams are called the Cougars and the school colors are crimson and gray.
Six men's and nine women's varsity teams compete in NCAA Division I in the Pac-12 Conference. Both men's and women's indoor track teams compete in the Mountain Pacific Sports Federation. Washington State College was established by the Washington Legislature on March 28, 1890, less than five months after statehood; the institution was one of the land-grant colleges created under the 1862 federal Morrill Act signed into law by President Abraham Lincoln. The federal land grants for the new institution included 90,000 acres of federal land for an agricultural college and 100,000 acres for a school of science. After an extended search for a location, the state's new land-grant college opened in Pullman on January 13, 1892; the year 1897 saw the first graduating class of women. The school changed its name from Washington Agricultural College and School of Science to State College of Washington in 1905, but was called Washington State College; the state legislature changed the name to Washington State University in 1959.
Enoch Albert Bryan, appointed July 22, 1893, was the first influential president of WSU. Bryan held graduate degrees from Harvard and Columbia and served as the president of Vincennes University in Indiana. Before Bryan's arrival, the fledgling university suffered through significant organizational instability. Bryan guided WSU toward respectability and is arguably the most influential figure in the university's history; the landmark clock tower in the center of campus is his namesake. WSU's role as a statewide institution became clear in 1894 with the launch of its first agricultural experiment station west of the Cascade Mountains near Puyallup. WSU has subsequently established extension offices and research centers in all regions of the state, with major research facilities in Prosser, Mount Vernon, Wenatchee. In 1989, WSU gained branch campuses in Spokane, the Tri-Cities, Vancouver. Overall, the federal government and the State of Washington have entrusted 190,000 acres of land to WSU for agricultural and scientific research throughout the Pacific Northwest.
Professional education began with the establishment of the School of Veterinary Science in 1899. The veterinary school was elevated to college status in 1916 and became the College of Veterinary Medicine in 1925. Graduate education began in the early years and, in 1902, the first master's degree was conferred, an M. S. in Botany. In 1917, the institution was organized into five colleges and four schools, with deans as administrative heads. In 1922 a graduate school was created. In 1929, the first Ph. D. degree was conferred, in bacteriology. The university offers bachelor's, master's and doctoral degrees in 200 fields of study through 65 departments and programs; these departments and programs are organized into 10 academic colleges as follows: College of Agricultural and Natural Resource Sciences College of Arts and Sciences Carson College of Business Edward R. Murrow College of Communication College of Education Voiland College of Engineering and Architecture Elson S. Floyd College of Medicine College of Nursing College of Pharmacy College of Veterinary MedicineIn addition, WSU has an all-university honors college, a graduate school, an online global campus, an accredited intensive English program for non-native speakers.
Washington State University is chartered by the State of Washington. A board of regents provides direction to the president. There are ten regents appointed by the governor; the tenth is a student regent appointed on an annual basis. A bill adding an eleventh regent, who would be a full-time or emeritus faculty member, stalled in the Washington legislature in 2018; the regents are Theodor P. Baseler, Brett Blankenship, Scott E. Carson, Marty Dickinson, Ron Sims, Jordan Frost, Lura J. Powell, Heather Redman, Lisa K. Schauer, Michael C. Worthy. Kirk Schulz serves as WSU's president and chief executive officer. Daniel Bernardo serves as provost and handles academics and faculty matters for WSU statewide; the former president, Elson Floyd the former president of University of Missouri System, succeeded V. Lane Rawlins on May 21, 2007, served until his death on June 20, 2015. Bernardo was dean of the WSU College of Agricultural and Natural Resource Sciences. WSU has had 11 presidents in its 125-year history: George W. Lilley, John W. Heston, Enoch A. Bryan, Ernest O. Holland, Wil
The Arecibo Observatory is a radio telescope in the municipality of Arecibo, Puerto Rico. This observatory is operated by University of Central Florida, Yang Enterprises and UMET, under cooperative agreement with the US National Science Foundation; the observatory is the sole facility of the National Astronomy and Ionosphere Center, the formal name of the observatory. From its construction in the 1960s until 2011, the observatory was managed by Cornell University. For more than 50 years, from its completion in 1963 until July 2016 when the Five hundred meter Aperture Spherical Telescope in China was completed, the Arecibo Observatory's 1,000-foot radio telescope was the world's largest single-aperture telescope, it is used in three major areas of research: radio astronomy, atmospheric science, radar astronomy. Scientists who want to use the observatory submit proposals that are evaluated by an independent scientific board; the observatory has appeared in film and television productions, gaining more recognition in 1999 when it began to collect data for the SETI@home project.
It has been listed on the US National Register of Historic Places starting in 2008. It was the featured listing in the US National Park Service's weekly list of October 3, 2008; the center was named an IEEE Milestone in 2001. It has a visitor center, open part-time. On September 21, 2017, high winds associated with Hurricane Maria caused the 430 MHz line feed to break and fall onto the primary dish, damaging about 30 out of 38,000 aluminum panels. Most Arecibo observations do not use the line feed but instead rely on the feeds and receivers located in the dome. Overall, the damage inflicted by Maria was minimal; the main collecting dish is 305 m in diameter, constructed inside the depression left by a karst sinkhole. The dish surface is made of 38,778 perforated aluminum panels, each about 3 by 6 feet, supported by a mesh of steel cables; the ground beneath supports shade-tolerant vegetation. The observatory has four radar transmitters, with effective isotropic radiated powers of 20 TW at 2380 MHz, 2.5 TW at 430 MHz, 300 MW at 47 MHz, 6 MW at 8 MHz.
The reflector is a spherical reflector, not a parabolic reflector. To aim the device, the receiver is moved to intercept signals reflected from different directions by the spherical dish surface of 270 m radius. A parabolic mirror would have varying astigmatism when the receiver is off the focal point, but the error of a spherical mirror is uniform in every direction; the receiver is on a 900-ton platform suspended 150 m above the dish by 18 cables running from three reinforced concrete towers, one 111 m high and the other two 81 m high, placing their tops at the same elevation. The platform has a rotating, bow-shaped track 93 m long, called the azimuth arm, carrying the receiving antennas and secondary and tertiary reflectors; this allows the telescope to observe any region of the sky in a forty-degree cone of visibility about the local zenith. Puerto Rico's location near the Northern Tropic allows Arecibo to view the planets in the Solar System over the Northern half of their orbit; the round trip light time to objects beyond Saturn is longer than the 2.6 hour time that the telescope can track a celestial position, preventing radar observations of more distant objects.
The origins of the observatory trace to late 1950s efforts to develop anti-ballistic missile defences as part of the newly formed ARPA's ABM umbrella-effort, Project Defender. At this early stage it was clear that the use of radar decoys would be a serious problem at the long ranges needed to attack a warhead, ranges on the order of 1,000 miles. Among the many Defender projects were several studies based on the concept that a re-entering nuclear warhead would cause unique physical signatures while still in the upper atmosphere, it was known that hot, high-speed objects caused ionization of the atmosphere that reflects radar waves, it appeared that a warhead's signature would be different enough from decoys that a detector could pick out the warhead directly, or alternately, provide added information that would allow operators to focus a conventional tracking radar on the single return from the warhead. Although the concept appeared to offer a solution to the tracking problem, there was no information on either the physics of re-entry or a strong understanding of the normal composition of the upper layers of the ionosphere.
ARPA began to address both simultaneously. To better understand the radar returns from a warhead, several radars were built on Kwajalein Atoll, while Arecibo started with the dual purpose of understanding the ionosphere's F-layer while producing a general-purpose scientific radio observatory; the observatory was built between mid-1960 and November 1963. William E. Gordon of Cornell University oversaw its design, who intended to use it to study the Earth's ionosphere, he was attracted to the sinkholes in the karst regions of Puerto Rico that offered perfect cavities for a large dish. A fixed parabolic reflector was envisioned, pointing in a fixed direction with a 150 m tower to hold equipment at the focus; this design would have limited its use in other research areas, such as radar astronomy, radio astronomy and atmospheric science, which require the ability to point at different positions in the sky and track those positions for an extended time as Earth rotates. Ward Low of the Advanced Research Projects Agency pointed out this flaw and put Gordon in touch with the Air Force Cambridge Research Laboratory in Boston, where one group headed by Phil Blacksmith was working
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