1.
Atacama Large Millimeter Array
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The Atacama Large Millimeter Array is an astronomical interferometer of radio telescopes in the Atacama desert of northern Chile. ALMA is a partnership among Europe, the United States, Canada, Japan, South Korea, Taiwan. Costing about US$1.4 billion, it is the most expensive ground-based telescope in operation, ALMA began scientific observations in the second half of 2011 and the first images were released to the press on 3 October 2011. The array has been operational since March 2013. The initial ALMA array is composed of 66 high-precision antennas, the high sensitivity is mainly achieved through the large numbers of antenna dishes that will make up the array. The telescopes were provided by the European, North American and East Asian partners of ALMA, the American and European partners each provided twenty-five 12-meter diameter antennas, that compose the main array. The participating East Asian countries are contributing 16 antennas in the form of the Atacama Compact Array, by using smaller antennas than the main ALMA array, larger fields of view can be imaged at a given frequency using ACA. Placing the antennas closer together enables the imaging of sources of larger angular extent, the ACA works together with the main array in order to enhance the latters wide-field imaging capability. ALMA has its roots in three astronomical projects — the Millimeter Array of the United States, the Large Southern Array of Europe. The merged array combined the sensitivity of the LSA with the frequency coverage, ESO and NRAO worked together in technical, science, and management groups to define and organize a joint project between the two observatories with participation by Canada and Spain. A groundbreaking ceremony was held on November 6,2003 and the ALMA logo was unveiled and this was mainly for political reasons. Although very different approaches have been chosen by the providers, each of the antenna designs appears to be able to meet ALMAs stringent requirements, ALMA is the largest and most expensive ground-based astronomical project, costing between US$1.4 and 1.5 billion. S. Japanese, and Canadian companies and universities, three prototype antennas have undergone evaluation at the Very Large Array since 2002. The first antenna was delivered in 2008, the last in 2011, the solution chosen is to use two custom 28-wheel self-loading heavy haulers. The vehicles were made by Scheuerle Fahrzeugfabrik in Germany and are 10 m wide,20 m long and 6 m high and they are powered by twin turbocharged 500 kW Diesel engines. The transporters, which feature a drivers seat designed to accommodate an oxygen tank to aid breathing the thin high-altitude air, the first vehicle was completed and tested in July 2007. Both transporters were delivered to the ALMA Operations Support Facility in Chile on 15 February 2008, on 7 July 2008, an ALMA transporter moved an antenna for the first time, from inside the antenna assembly building to a pad outside the building for testing. During Autumn 2009, the first three antennas were transported one-by-one to the Array Operations Site, linking three antennas allows corrections of errors that can arise when only two antennas are used, thus paving the way for precise, high-resolution imaging
2.
Submillimeter Array
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The Submillimeter Array consists of eight 6-meter diameter radio telescopes arranged as an interferometer for submillimeter wavelength observations. All three of these observatories are located at Mauna Kea Observatory on Mauna Kea, Hawaii, the baseline lengths presently in use range from 16 to 508 meters, and up to 783 meters for eSMA operations. Although the array is capable of operating both day and night, most of the observations take place at nighttime when the phase stability is best. The SMA is jointly operated by the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy, the SMA is a multi-purpose instrument which can be used to observe diverse celestial phenomena. The SMA excels at observations of dust and gas with only a few tens of kelvins above absolute zero. Commonly observed classes of objects include star-forming molecular clouds in our own and other galaxies, highly redshifted galaxies, evolved stars, occasionally, bodies in the Solar System, such as planets, asteroids, comets and moons, are observed. The SMA has been used to discover that Pluto is 10 K cooler than expected and it was the first radio telescope to resolve Pluto and Charon as separate objects. The SMA is a part of the Event Horizon Telescope, which observes nearby supermassive black holes with a resolution comparable to the size of the objects event horizon. 4″N 155°28′40. 7″W
3.
Mauna Kea Observatories
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The facilities are located in a 525-acre special land use zone known as the Astronomy Precinct, which is located within the 11, 228-acre Mauna Kea Science Reserve. The Astronomy Precinct was established in 1967 and is located on land protected by the Historical Preservation Act for its significance to Hawaiian culture. After studying photos for NASAs Apollo program that contained greater detail than any ground based telescope, while he first began looking in Chile, he also made the decision to perform tests in the Hawaiian Islands. Tests on Mauis Haleakalā were promising, but the mountain was too low in the inversion layer, on the Big Island of Hawaiʻi, Mauna Kea is considered the highest island mountain in the world. While the summit is covered with snow, the air is extremely dry. Kuiper began looking into the possibility of an observatory on Mauna Kea, after testing, he discovered the low humidity was perfect for infrared signals. He persuaded Hawaiʻi Governor John A. Burns to bulldoze a road to the summit where he built a small telescope on Puʻu Poliʻahu. The peak was the second highest on the mountain with the highest peak being holy ground, next, Kuiper tried enlisting NASA to fund a larger facility with a large telescope, housing and other needed structures. NASA, in turn decided to make the open to competition. Professor of physics, John Jefferies of the University of Hawaii placed a bid on behalf of the university, Jefferies had gained his reputation through observations at Sacramento Peak Observatory. The proposal was for a telescope to serve both the needs of NASA and the university. Kuiper would abandon his site over the competition and begin work in Arizona on a different NASA project, after considerable testing by Jefferies team, the best locations were determined to be near the summit at the top of the cinder cones. Testing also determined Mauna Kea to be superb for nighttime viewing due to many factors, including the air, constant trade winds. Jefferies would build a 2.24 meter telescope with the State of Hawaiʻi agreeing to build a reliable, building began in 1967 and first light was seen in 1970. Other groups began requesting subleases on the newly accessible mountaintop, by 1970, two 24 in telescopes had been constructed by the United States Air Force and Lowell Observatory. In 1973, Canada and France agreed to build the 3.6 m CFHT on Mauna Kea, however, local organizations started to raise concerns about the environmental impact of the observatory. This led the Department of Land and Natural Resources to prepare a management plan, drafted in 1977. In January 1982, the University of Hawaiʻi Board of Regents approved a plan to support the development of scientific facilities at the site
4.
Astronomical interferometer
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The advantage of this technique is that it can theoretically produce images with the angular resolution of a huge telescope with an aperture equal to the separation between the component telescopes. The main drawback is that it does not collect as much light as the complete instruments mirror, thus it is mainly useful for fine resolution of more luminous astronomical objects, such as close binary stars. Another drawback is that the angular size of a detectable emission source is limited by the minimum gap between detectors in the collector array. Interferometry is most widely used in astronomy, in which signals from separate radio telescopes are combined. A mathematical signal processing technique called aperture synthesis is used to combine the signals to create high-resolution images. Practical infrared and optical astronomical interferometers have only recently been developed, at optical wavelengths, aperture synthesis allows the atmospheric seeing resolution limit to be overcome, allowing the angular resolution to reach the diffraction limit of the optics. Astronomical interferometers can produce higher resolution images than any other type of telescope. At radio wavelengths, image resolutions of a few micro-arcseconds have been obtained, one simple layout of an astronomical interferometer is a parabolic arrangement of mirror pieces, giving a partially complete reflecting telescope but with a sparse or dilute aperture. Instead, most existing arrays use a geometry, and Labeyries hypertelescope will use a spherical geometry. One of the first uses of optical interferometry was applied by the Michelson stellar interferometer on the Mount Wilson Observatorys reflector telescope to measure the diameters of stars, the red giant star Betelgeuse was the first to have its diameter determined in this way on December 13,1920. In the 1940s radio interferometry was used to perform the first high resolution radio astronomy observations, optical/infrared interferometry was extended to measurements using separated telescopes by Johnson, Betz and Townes in the infrared and by Labeyrie in the visible. Similar techniques have now been applied at other astronomical telescope arrays, including the Keck Interferometer, software packages such as BSMEM or MIRA are used to convert the measured visibility amplitudes and closure phases into astronomical images. If completed, the MRO Interferometer with up to ten movable telescopes will produce among the first higher fidelity images from a long baseline interferometer, astronomical interferometry is principally conducted using Michelson interferometers. The principal operational interferometric observatories which use this type of instrumentation include VLTI, NPOI, engineers at the European Southern Observatory ESO designed the Very Large Telescope VLT so that it can also be used as an interferometer. Along with the four 8. 2-metre unit telescopes, four mobile 1. 8-metre auxiliary telescopes were included in the overall VLT concept to form the Very Large Telescope Interferometer. The ATs can move between 30 different stations, and at present, the telescopes can form groups of two or three for interferometry, when using interferometry, a complex system of mirrors brings the light from the different telescopes to the astronomical instruments where it is combined and processed. This is technically demanding as the light paths must be equal to within 1/1000 mm over distances of a few hundred metres. For the Unit Telescopes, this gives an equivalent mirror diameter of up to 130 metres, and this is up to 25 times better than the resolution of a single VLT unit telescope
5.
Radio astronomy
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Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an object was in 1932. Subsequent observations have identified a number of different sources of radio emission and these include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy. Before Jansky observed the Milky Way in the 1930s, physicists speculated that radio waves could be observed from astronomical sources, in the 1860s, James Clerk Maxwells equations had shown that electromagnetic radiation is associated with electricity and magnetism, and could exist at any wavelength. These attempts were unable to detect any emission due to limitations of the instruments. The discovery of the radio reflecting ionosphere in 1902, led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, Karl Jansky made the discovery of the first astronomical radio source serendipitously in the early 1930s. As an engineer with Bell Telephone Laboratories, he was investigating static that interfered with short wave transatlantic voice transmissions, using a large directional antenna, Jansky noticed that his analog pen-and-paper recording system kept recording a repeating signal of unknown origin. Since the signal peaked about every 24 hours, Jansky originally suspected the source of the interference was the Sun crossing the view of his directional antenna. Continued analysis showed that the source was not following the 24-hour daily cycle of the Sun exactly and he concluded that since the Sun were not large emitters of radio noise, the strange radio interference may be generated by interstellar gas and dust in the galaxy. Jansky announced his discovery in 1933 and he wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy. His pioneering efforts in the field of astronomy have been recognized by the naming of the fundamental unit of flux density. Grote Reber was inspired by Janskys work, and built a radio telescope 9m in diameter in his backyard in 1937. He began by repeating Janskys observations, and then conducted the first sky survey in the radio frequencies, on February 27,1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun. Later that year George Clark Southworth, at Bell Labs like Jansky, both researchers were bound by wartime security surrounding radar, so Reber, who was not, published his 1944 findings first. Several other people independently discovered solar radiowaves, including E. Schott in Denmark, at Cambridge University, where ionospheric research had taken place during World War II, J. A. This early research soon branched out into the observation of celestial radio sources. Martin Ryle and Antony Hewish at the Cavendish Astrophysics Group developed the technique of Earth-rotation aperture synthesis, the radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s
6.
Quasar
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A quasar is an active galactic nucleus of very high luminosity. A quasar consists of a black hole surrounded by an orbiting accretion disk of gas. As gas in the accretion disk falls toward the black hole, quasars emit energy across the electromagnetic spectrum and can be observed at radio, infrared, visible, ultraviolet, and X-ray wavelengths. The most powerful quasars have luminosities exceeding 1041 W, thousands of greater than the luminosity of a large galaxy such as the Milky Way. Quasars are found over a broad range of distances. The peak epoch of quasar activity in the Universe corresponds to redshifts around 2, as of 2011, the most distant known quasar is at redshift z=7.085, light observed from this quasar was emitted when the Universe was only 770 million years old. Because quasars are distant objects, any light which reaches the Earth is redshifted due to the expansion of space. In early optical images, quasars appeared as point sources, indistinguishable from stars, with infrared telescopes and the Hubble Space Telescope, the host galaxies surrounding the quasars have been detected in some cases. These galaxies are normally too dim to be seen against the glare of the quasar, most quasars, with the exception of 3C273 whose average apparent magnitude is 12.9, cannot be seen with small telescopes. The luminosity of some quasars changes rapidly in the optical range, because these changes occur very rapidly they define an upper limit on the volume of a quasar, quasars are not much larger than the Solar System. This implies a high power density. The mechanism of brightness changes probably involves relativistic beaming of astrophysical jets pointed nearly directly toward Earth, the highest redshift quasar known is ULAS J1120+0641, with a redshift of 7.085, which corresponds to a comoving distance of approximately 29 billion light-years from Earth. Since light cannot escape the black holes, the energy is actually generated outside the event horizon by gravitational stresses. Central masses of 105 to 109 solar masses have been measured in quasars by using reverberation mapping. The matter accreting onto the hole is unlikely to fall directly in. Quasars may also be ignited or re-ignited when normal galaxies merge, in fact, it has been suggested that a quasar could form as the Andromeda Galaxy collides with our own Milky Way galaxy in approximately 3–5 billion years. More than 200,000 quasars are known, most from the Sloan Digital Sky Survey, all observed quasar spectra have redshifts between 0.056 and 7.085. Applying Hubbles law to these redshifts, it can be shown that they are between 600 million and 28.85 billion light-years away
7.
Atomic clock
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The principle of operation of an atomic clock is not based on nuclear physics, but rather on atomic physics, it uses the microwave signal that electrons in atoms emit when they change energy levels. Early atomic clocks were based on masers at room temperature, currently, the most accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity. An example of this is the NIST-F1 atomic clock, one of the primary time. The accuracy of an atomic clock depends on two factors, the first factor is temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times. The second factor is the frequency and intrinsic width of the electronic transition, higher frequencies and narrow lines increase the precision. National standards agencies in many countries maintain a network of atomic clocks which are intercompared and these clocks collectively define a continuous and stable time scale, International Atomic Time. For civil time, another time scale is disseminated, Coordinated Universal Time, UTC is derived from TAI, but approximately synchronised, by using leap seconds, to UT1, which is based on actual rotation of the Earth with respect to the solar time. The idea of using atomic transitions to measure time was suggested by Lord Kelvin in 1879, magnetic resonance, developed in the 1930s by Isidor Rabi, became the practical method for doing this. In 1945, Rabi first publicly suggested that atomic beam magnetic resonance might be used as the basis of a clock, the first atomic clock was an ammonia maser device built in 1949 at the U. S. National Bureau of Standards. It was less accurate than existing quartz clocks, but served to demonstrate the concept, calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time. This led to the agreed definition of the latest SI second being based on atomic time. Equality of the ET second with the SI second has been verified to within 1 part in 1010, the SI second thus inherits the effect of decisions by the original designers of the ephemeris time scale, determining the length of the ET second. Since the beginning of development in the 1950s, atomic clocks have been based on the transitions in hydrogen-1, caesium-133. The first commercial atomic clock was the Atomichron, manufactured by the National Company, more than 50 were sold between 1956 and 1960. This bulky and expensive instrument was replaced by much smaller rack-mountable devices, such as the Hewlett-Packard model 5060 caesium frequency standard. In August 2004, NIST scientists demonstrated a chip-scale atomic clock, according to the researchers, the clock was believed to be one-hundredth the size of any other. It requires no more than 125 mW, making it suitable for battery-driven applications and this technology became available commercially in 2011. Ion trap experimental optical clocks are more precise than the current caesium standard, in March 2017, NASA plans to deploy the Deep Space Atomic Clock, a miniaturized, ultra-precise mercury-ion atomic clock, into outer space
8.
Hydrogen maser
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A hydrogen maser, also known as hydrogen frequency standard, is a specific type of maser that uses the intrinsic properties of the hydrogen atom to serve as a precision frequency reference. Both the proton and electron of a hydrogen atom have spins, the atom has a higher energy if both are spinning in the same direction, and a lower energy if they spin in opposite directions. The amount of energy needed to reverse the spin of the electron is equivalent to a photon at the frequency of 1,420,405,751.786 hertz, hydrogen masers are very complex devices and sell for as much as US$235,000. They are made in two types, active and passive, in both types, a small storage bottle of molecular hydrogen, H2, leaks a controlled amount of gas into a discharge bulb. The molecules are dissociated in the bulb into individual hydrogen atoms by an arc. This atomic hydrogen passes through a collimator and a state selector. The atoms are thereby selected for the state and passed on to a storage bulb. The storage bulb is roughly 20 cm high and 10 cm in diameter, a durable Teflon & bulb coating technology allows for over 20-year lifetime. The storage bulb is in turn inside a cavity made from a precisely machined copper or silver-plated ceramic cylinder. This cavity is tuned to the 1,420 GHz resonance frequency of the atoms, a weak static magnetic field is applied parallel to the cavity axis by a solenoid to lift the degeneracy of the magnetic Zeeman sublevels. In the active hydrogen maser, the cavity oscillates by itself and this requires a higher hydrogen atom density and a higher quality factor for the cavity. However, with advanced microwave cavities made out of silver-plated ceramic, the active maser is more complex and more expensive but has better short-term and long-term frequency stabilities. In the passive hydrogen maser, the cavity is fed from an external 1,420 GHz frequency, the external frequency is tuned to produce a maximum output in the cavity. This allows the use of hydrogen atom density and lower cavity quality factor. Timeline of hydrogen technologies Bauch, A, atomic frequency standards, properties and applications. In Hänsch, T. W. Leschiutta, S. Wallard, A. J
9.
Interferometry
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Interferometry is a family of techniques in which waves, usually electromagnetic waves, are superimposed causing the phenomenon of interference in order to extract information. Interferometers are widely used in science and industry for the measurement of small displacements, refractive index changes, in an interferometer, light from a single source is split into two beams that travel different optical paths, then combined again to produce interference. The resulting interference fringes give information about the difference in path length, waves which are not completely in phase nor completely out of phase will have an intermediate intensity pattern, which can be used to determine their relative phase difference. Most interferometers use light or some form of electromagnetic wave. Typically a single incoming beam of coherent light will be split into two beams by a beam splitter. Each of these beams travels a different route, called a path, the path difference, the difference in the distance traveled by each beam, creates a phase difference between them. It is this phase difference that creates the interference pattern between the initially identical waves. If a single beam has been split along two paths, then the difference is diagnostic of anything that changes the phase along the paths. This could be a change in the path length itself or a change in the refractive index along the path. As seen in Fig. 2a and 2b, the observer has a view of mirror M1 seen through the beam splitter. The fringes can be interpreted as the result of interference between light coming from the two virtual images S1 and S2 of the original source S, the characteristics of the interference pattern depend on the nature of the light source and the precise orientation of the mirrors and beam splitter. In Fig. 2a, the elements are oriented so that S1 and S2 are in line with the observer. Use of white light will result in a pattern of colored fringes, the central fringe representing equal path length may be light or dark depending on the number of phase inversions experienced by the two beams as they traverse the optical system. Interferometers and interferometric techniques may be categorized by a variety of criteria, In homodyne detection, the phase difference between the two beams results in a change in the intensity of the light on the detector. The resulting intensity of the light after mixing of two beams is measured, or the pattern of interference fringes is viewed or recorded. Most of the interferometers discussed in this article fall into this category, the heterodyne technique is used for shifting an input signal into a new frequency range as well as amplifying a weak input signal. A weak input signal of frequency f1 is mixed with a reference frequency f2 from a local oscillator. The nonlinear combination of the input signals creates two new signals, one at the sum f1 + f2 of the two frequencies, and the other at the difference f1 − f2 and these new frequencies are called heterodynes
10.
Coaxial cable
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Coaxial cable, or coax, is a type of cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an outer sheath or jacket. The term coaxial comes from the conductor and the outer shield sharing a geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, Coaxial cable is used as a transmission line for radio frequency signals. Its applications include feedlines connecting radio transmitters and receivers with their antennas, computer network connections, digital audio and this allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other types of transmission lines. Coaxial cable also provides protection of the signal from external electromagnetic interference, the cable is protected by an outer insulating jacket. Normally, the shield is kept at ground potential and a signal carrying voltage is applied to the center conductor, the advantage of coaxial design is that electric and magnetic fields are restricted to the dielectric with little leakage outside the shield. Conversely, electric and magnetic fields outside the cable are largely kept from interfering with signals inside the cable, larger diameter cables and cables with multiple shields have less leakage. Common applications of coaxial cable include video and CATV distribution, RF and microwave transmission, the characteristic impedance of the cable is determined by the dielectric constant of the inner insulator and the radii of the inner and outer conductors. A controlled cable characteristic impedance is important because the source and load impedance should be matched to ensure maximum power transfer, other important properties of coaxial cable include attenuation as a function of frequency, voltage handling capability, and shield quality. Coaxial cable design choices affect physical size, frequency performance, attenuation, power handling capabilities, flexibility, strength, the inner conductor might be solid or stranded, stranded is more flexible. To get better performance, the inner conductor may be silver-plated. Copper-plated steel wire is used as an inner conductor for cable used in the cable TV industry. The insulator surrounding the conductor may be solid plastic, a foam plastic. The properties of control some electrical properties of the cable. A common choice is a solid polyethylene insulator, used in lower-loss cables, solid Teflon is also used as an insulator. Some coaxial lines use air and have spacers to keep the conductor from touching the shield. Many conventional coaxial cables use braided copper wire forming the shield and this allows the cable to be flexible, but it also means there are gaps in the shield layer, and the inner dimension of the shield varies slightly because the braid cannot be flat
11.
Waveguide
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A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting expansion to one dimension or two. This is an effect to waves of water constrained within a canal. Without the physical constraint of a waveguide, waves are decreasing according to the square law as they expand into three dimensional space. There are different types of waveguides for each type of wave, the original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves. The geometry of a waveguide reflects its function, slab waveguides confine energy in one dimension, fiber or channel waveguides in two dimensions. The frequency of the wave also dictates the shape of a waveguide. As a rule of thumb, the width of a waveguide needs to be of the order of magnitude as the wavelength of the guided wave. Some naturally occurring structures can act as waveguides. The SOFAR channel layer in the ocean can guide the sound of whale song across enormous distances, waves propagate in all directions in open space as spherical waves. The power of the falls with the distance R from the source as the square of the distance. A waveguide confines the wave to propagate in one dimension, so that, under ideal conditions, due to total reflection at the walls, waves are confined to the interior of a waveguide. The first structure for guiding waves was proposed by J. J. Thomson in 1893, the first mathematical analysis of electromagnetic waves in a metal cylinder was performed by Lord Rayleigh in 1897. For sound waves, Lord Rayleigh published a mathematical analysis of propagation modes in his seminal work. The study of dielectric waveguides began as early as the 1920s, by people, most famous of which are Rayleigh, Sommerfeld. Optical fiber began to receive attention in the 1960s due to its importance to the communications industry. The development of radio communication initially occurred at the lower frequencies because these could be easily propagated over large distances. The long wavelengths made these frequencies unsuitable for use in hollow metal waveguides because of the large diameter tubes required. Consequently, research into hollow metal waveguides stalled and the work of Lord Rayleigh was forgotten for a time and had to be rediscovered by others, practical investigations resumed in the 1930s by George C
12.
Optical fiber
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An optical fiber or optical fibre is a flexible, transparent fiber made by drawing glass or plastic to a diameter slightly thicker than that of a human hair. Fibers are also used for illumination, and are wrapped in bundles so that they may be used to carry images, thus allowing viewing in confined spaces, as in the case of a fiberscope. Specially designed fibers are used for a variety of other applications, some of them being fiber optic sensors. Optical fibers typically include a transparent core surrounded by a transparent cladding material with an index of refraction. Light is kept in the core by the phenomenon of internal reflection which causes the fiber to act as a waveguide. Fibers that support many propagation paths or transverse modes are called multi-mode fibers, multi-mode fibers generally have a wider core diameter and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 1,000 meters, being able to join optical fibers with low loss is important in fiber optic communication. This is more complex than joining electrical wire or cable and involves careful cleaving of the fibers, precise alignment of the cores. For applications that demand a permanent connection a fusion splice is common, in this technique, an electric arc is used to melt the ends of the fibers together. Another common technique is a splice, where the ends of the fibers are held in contact by mechanical force. Temporary or semi-permanent connections are made by means of specialized optical fiber connectors, the field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics. The term was coined by Indian physicist Narinder Singh Kapany who is acknowledged as the father of fiber optics. Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by Daniel Colladon, John Tyndall included a demonstration of it in his public lectures in London,12 years later. When the ray passes from water to air it is bent from the perpendicular. If the angle which the ray in water encloses with the perpendicular to the surface be greater than 48 degrees, the angle which marks the limit where total reflection begins is called the limiting angle of the medium. For water this angle is 48°27′, for flint glass it is 38°41′, unpigmented human hairs have also been shown to act as an optical fiber. Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century, image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s. The principle was first used for medical examinations by Heinrich Lamm in the following decade
13.
Transmission line
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This article covers two-conductor transmission line such as parallel line, coaxial cable, stripline, and microstrip. Ordinary electrical cables suffice to carry low frequency alternating current, such as power, which reverses direction 100 to 120 times per second. However, they cannot be used to carry currents in the frequency range, above about 30 kHz, because the energy tends to radiate off the cable as radio waves. Radio frequency currents tend to reflect from discontinuities in the cable such as connectors and joints. These reflections act as bottlenecks, preventing the signal power from reaching the destination, Transmission lines use specialized construction, and impedance matching, to carry electromagnetic signals with minimal reflections and power losses. Types of transmission line include parallel line, coaxial cable, and planar transmission lines such as stripline, the higher the frequency of electromagnetic waves moving through a given cable or medium, the shorter the wavelength of the waves. Transmission lines become necessary when the length of the cable is longer than a significant fraction of the transmitted frequencys wavelength. At microwave frequencies and above, power losses in transmission lines become excessive, and waveguides are used instead, some sources define waveguides as a type of transmission line, however, this article will not include them. At even higher frequencies, in the terahertz, infrared and light range, waveguides in turn become lossy, mathematical analysis of the behaviour of electrical transmission lines grew out of the work of James Clerk Maxwell, Lord Kelvin and Oliver Heaviside. In 1855 Lord Kelvin formulated a model of the current in a submarine cable. The model correctly predicted the performance of the 1858 trans-Atlantic submarine telegraph cable. In 1885 Heaviside published the first papers that described his analysis of propagation in cables, in many electric circuits, the length of the wires connecting the components can for the most part be ignored. That is, the voltage on the wire at a time can be assumed to be the same at all points. Stated another way, the length of the wire is important when the signal frequency components with corresponding wavelengths comparable to or less than the length of the wire. A common rule of thumb is that the cable or wire should be treated as a line if the length is greater than 1/10 of the wavelength. If the transmission line is uniform along its length, then its behaviour is described by a single parameter called the characteristic impedance. This is the ratio of the voltage of a given wave to the complex current of the same wave at any point on the line. Typical values of Z0 are 50 or 75 ohms for a cable, about 100 ohms for a twisted pair of wires
14.
Astrometry
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Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of the Solar System and our galaxy, the history of astrometry is linked to the history of star catalogues, which gave astronomers reference points for objects in the sky so they could track their movements. This can be dated back to Hipparchus, who around 190 BC used the catalogue of his predecessors Timocharis, in doing so, he also developed the brightness scale still in use today. Hipparchus compiled a catalogue with at least 850 stars and their positions, hipparchuss successor, Ptolemy, included a catalogue of 1,022 stars in his work the Almagest, giving their location, coordinates, and brightness. Ibn Yunus observed more than 10,000 entries for the Suns position for years using a large astrolabe with a diameter of nearly 1.4 metres. In the 15th century, the Timurid astronomer Ulugh Beg compiled the Zij-i-Sultani, like the earlier catalogs of Hipparchus and Ptolemy, Ulugh Begs catalogue is estimated to have been precise to within approximately 20 minutes of arc. In the 16th century, Tycho Brahe used improved instruments, including large mural instruments, to measure star positions more accurately than previously, Taqi al-Din measured the right ascension of the stars at the Istanbul observatory of Taqi al-Din using the observational clock he invented. When telescopes became commonplace, setting circles sped measurements James Bradley first tried to measure stellar parallaxes in 1729, the stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light and the nutation of the Earths axis. His cataloguing of 3222 stars was refined in 1807 by Friedrich Bessel and he made the first measurement of stellar parallax,0.3 arcsec for the binary star 61 Cygni. Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, astrographs using astronomical photographic plates sped the process in the early 20th century. Automated plate-measuring machines and more sophisticated technology of the 1960s allowed more efficient compilation of star catalogues. In the 1980s, charge-coupled devices replaced photographic plates and reduced optical uncertainties to one milliarcsecond and this technology made astrometry less expensive, opening the field to an amateur audience. In 1989, the European Space Agencys Hipparcos satellite took astrometry into orbit, operated from 1989 to 1993, Hipparcos measured large and small angles on the sky with much greater precision than any previous optical telescopes. During its 4-year run, the positions, parallaxes, and proper motions of 118,218 stars were determined with a degree of accuracy. A new Tycho catalog drew together a database of 1,058,332 to within 20-30 mas, additional catalogues were compiled for the 23,882 double/multiple stars and 11,597 variable stars also analyzed during the Hipparcos mission. Today, the catalogue most often used is USNO-B1.0, during the past 50 years,7,435 Schmidt camera plates were used to complete several sky surveys that make the data in USNO-B1.0 accurate to within 0.2 arcsec. In observational astronomy, astrometric techniques help identify stellar objects by their unique motions and it is instrumental for keeping time, in that UTC is basically the atomic time synchronized to Earths rotation by means of exact observations. Astrometry is an important step in the distance ladder because it establishes parallax distance estimates for stars in the Milky Way
15.
Plate tectonics
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The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s, the lithosphere, which is the rigid outermost shell of a planet, is broken up into tectonic plates. The Earths lithosphere is composed of seven or eight major plates, where the plates meet, their relative motion determines the type of boundary, convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The relative movement of the plates typically ranges from zero to 100 mm annually, tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle, the material lost is balanced by the formation of new crust along divergent margins by seafloor spreading. In this way, the surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle, earlier theories, since disproven, proposed gradual shrinking or gradual expansion of the globe. Tectonic plates are able to move because the Earths lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the ridge and drag, with downward suction. Another explanation lies in the different forces generated by forces of the Sun. The relative importance of each of these factors and their relationship to other is unclear. The outer layers of the Earth are divided into the lithosphere and asthenosphere and this is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter, in terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year. The driving mechanism behind this movement is described below, tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material, oceanic crust and continental crust. Average oceanic lithosphere is typically 100 km thick, its thickness is a function of its age, as passes, it conductively cools
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Geodesy
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Geodesists also study geodynamical phenomena such as crustal motion, tides, and polar motion. For this they design global and national networks, using space and terrestrial techniques while relying on datums. Geodesy — from the Ancient Greek word γεωδαισία geodaisia — is primarily concerned with positioning within the temporally varying gravity field, such geodetic operations are also applied to other astronomical bodies in the solar system. It is also the science of measuring and understanding the earths geometric shape, orientation in space and this applies to the solid surface, the liquid surface and the Earths atmosphere. For this reason, the study of the Earths gravity field is called physical geodesy by some, the geoid is essentially the figure of the Earth abstracted from its topographical features. It is an idealized surface of sea water, the mean sea level surface in the absence of currents, air pressure variations etc. The geoid, unlike the ellipsoid, is irregular and too complicated to serve as the computational surface on which to solve geometrical problems like point positioning. The geometrical separation between the geoid and the ellipsoid is called the geoidal undulation. It varies globally between ±110 m, when referred to the GRS80 ellipsoid, a reference ellipsoid, customarily chosen to be the same size as the geoid, is described by its semi-major axis a and flattening f. The quantity f = a − b/a, where b is the axis, is a purely geometrical one. The mechanical ellipticity of the Earth can be determined to high precision by observation of satellite orbit perturbations and its relationship with the geometrical flattening is indirect. The relationship depends on the density distribution, or, in simplest terms. The 1980 Geodetic Reference System posited a 6,378,137 m semi-major axis and this system was adopted at the XVII General Assembly of the International Union of Geodesy and Geophysics. It is essentially the basis for geodetic positioning by the Global Positioning System and is also in widespread use outside the geodetic community. The locations of points in space are most conveniently described by three cartesian or rectangular coordinates, X, Y and Z. Since the advent of satellite positioning, such systems are typically geocentric. The X-axis lies within the Greenwich observatorys meridian plane, the coordinate transformation between these two systems is described to good approximation by sidereal time, which takes into account variations in the Earths axial rotation. A more accurate description also takes polar motion into account, a closely monitored by geodesists
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Gravity
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Gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward one another, including planets, stars and galaxies. Since energy and mass are equivalent, all forms of energy, including light, on Earth, gravity gives weight to physical objects and causes the ocean tides. Gravity has a range, although its effects become increasingly weaker on farther objects. The most extreme example of this curvature of spacetime is a hole, from which nothing can escape once past its event horizon. More gravity results in time dilation, where time lapses more slowly at a lower gravitational potential. Gravity is the weakest of the four fundamental interactions of nature, the gravitational attraction is approximately 1038 times weaker than the strong force,1036 times weaker than the electromagnetic force and 1029 times weaker than the weak force. As a consequence, gravity has an influence on the behavior of subatomic particles. On the other hand, gravity is the dominant interaction at the macroscopic scale, for this reason, in part, pursuit of a theory of everything, the merging of the general theory of relativity and quantum mechanics into quantum gravity, has become an area of research. While the modern European thinkers are credited with development of gravitational theory, some of the earliest descriptions came from early mathematician-astronomers, such as Aryabhata, who had identified the force of gravity to explain why objects do not fall out when the Earth rotates. Later, the works of Brahmagupta referred to the presence of force, described it as an attractive force. Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and this was a major departure from Aristotles belief that heavier objects have a higher gravitational acceleration. Galileo postulated air resistance as the reason that objects with less mass may fall slower in an atmosphere, galileos work set the stage for the formulation of Newtons theory of gravity. In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. Newtons theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted for by the actions of the other planets. Calculations by both John Couch Adams and Urbain Le Verrier predicted the position of the planet. A discrepancy in Mercurys orbit pointed out flaws in Newtons theory, the issue was resolved in 1915 by Albert Einsteins new theory of general relativity, which accounted for the small discrepancy in Mercurys orbit. The simplest way to test the equivalence principle is to drop two objects of different masses or compositions in a vacuum and see whether they hit the ground at the same time. Such experiments demonstrate that all objects fall at the rate when other forces are negligible
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Sun
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The Sun is the star at the center of the Solar System. It is a 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 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 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 a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
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Moon
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The Moon is an astronomical body that orbits planet Earth, being Earths only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, following Jupiters satellite Io, the Moon is second-densest satellite among those whose densities are known. The average distance of the Moon from the Earth is 384,400 km, the Moon is thought to have formed about 4.51 billion years ago, not long after Earth. It is the second-brightest regularly visible celestial object in Earths sky, after the Sun and its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its cycle of phases have made the Moon an important cultural influence since ancient times on language, calendars, art. The Moons gravitational influence produces the ocean tides, body tides, and this matching of apparent visual size will not continue in the far future. The Moons linear distance from Earth is currently increasing at a rate of 3.82 ±0.07 centimetres per year, since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft. The usual English proper name for Earths natural satellite is the Moon, the noun moon is derived from moone, which developed from mone, which is derived from Old English mōna, which ultimately stems from Proto-Germanic *mǣnōn, like all Germanic language cognates. Occasionally, the name Luna is used, in literature, especially science fiction, Luna is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of our moon. The principal modern English adjective pertaining to the Moon is lunar, a less common adjective is selenic, derived from the Ancient Greek Selene, from which is derived the prefix seleno-. Both the Greek Selene and the Roman goddess Diana were alternatively called Cynthia, the names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana is connected to dies meaning day, several mechanisms have been proposed for the Moons formation 4.51 billion years ago, and some 60 million years after the origin of the Solar System. These hypotheses also cannot account for the angular momentum of the Earth–Moon system. This hypothesis, although not perfect, perhaps best explains the evidence, eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists, You have eighteen months. Go back to your Apollo data, go back to computer, do whatever you have to. Dont come to our conference unless you have something to say about the Moons birth, at the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular. Afterward there were only two groups, the giant impact camp and the agnostics. Giant impacts are thought to have been common in the early Solar System, computer simulations of a giant impact have produced results that are consistent with the mass of the lunar core and the present angular momentum of the Earth–Moon system
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Huygens (spacecraft)
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Huygens was an atmospheric entry probe that landed successfully on Saturns moon Titan in 2005. The probe was named after the Dutch 17th-century astronomer Christiaan Huygens, the combined Cassini–Huygens spacecraft was launched from Earth on October 15,1997. Huygens separated from the Cassini orbiter on December 25,2004 and this was the first and, so far, only landing ever accomplished in the outer Solar System. It touched down on land, although the possibility that it would touch down in an ocean was also taken into account in its design, the probe was designed to gather data for a few hours in the atmosphere, and possibly a short time at the surface. It continued to send data for about 90 minutes after touchdown and it remains the most distant landing of any human-made craft. Huygens was designed to enter and brake in Titans atmosphere and parachute a fully instrumented robotic laboratory to the surface, based on pictures taken by Cassini at 1,200 km above Titan, the landing site appeared to be a shoreline. Assuming the landing site could be non-solid, Huygens was designed to survive the impact, splash down on a surface on Titan. If that occurred it was expected to be the first time a human-made probe would land in an extraterrestrial ocean, the spacecraft had no more than three hours of battery life, most of which was planned to be used during the descent. Engineers expected to get at most only 30 minutes of data from the surface, the Huygens probe system consists of the 318 kg probe itself, which descended to Titan, and the probe support equipment, which remained attached to the orbiting spacecraft. Huygens heat shield was 2.7 m in diameter, after ejecting the shield, the probe was 1.3 m in diameter. The probe remained dormant throughout the 6. 7-year interplanetary cruise and these checkouts followed preprogrammed descent scenario sequences as closely as possible, and the results were relayed to Earth for examination by system and payload experts. Prior to the separation from the orbiter on December 25,2004. The main mission phase was a descent through Titans atmosphere. The probes radio link was activated early in the descent phase, and the orbiter listened to the probe for the next 3 hours, including the descent phase, and the first thirty minutes after touchdown. Not long after the end of this three-hour communication window, Cassinis high-gain antenna was turned away from Titan, very large radio telescopes on Earth were also listening to Huygens 10-watt transmission using the technique of very long baseline interferometry and aperture synthesis mode. At 11,25 CET on January 14, the Robert C. Byrd Green Bank Telescope in West Virginia detected the carrier signal from Huygens, the GBT continued to detect the carrier signal well after Cassini stopped listening to the incoming data stream. Instead, wide-band recordings of the signal were made throughout the three-hour descent. A position of Huygens landing site on Titan was found with precision using the Doppler data at a distance from Earth of about 1.2 billion kilometers
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Titan (moon)
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Titan is the largest moon of Saturn. It is the only known to have a dense atmosphere. Titan is the sixth ellipsoidal moon from Saturn, frequently described as a planet-like moon, Titan is 50% larger than Earths Moon, and it is 80% more massive. It is the second-largest moon in the Solar System, after Jupiters moon Ganymede, and is larger than the smallest planet, Mercury, discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite. Titan orbits Saturn at 20 Saturn radii, from Titans surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth. Titan is primarily composed of ice and rocky material. The geologically young surface is smooth, with few impact craters, although mountains. The atmosphere of Titan is largely nitrogen, minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas, and deltas, Huygens was inspired by Galileos discovery of Jupiters four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his brother Constantijn Huygens, Jr. began building telescopes around 1650 and it was the sixth moon to be discovered. He named it Saturni Luna, publishing in the 1655 tract De Saturni Luna Observatio Nova, after Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V. Other early epithets for Titan include Saturns ordinary satellite, Titan is officially numbered Saturn VI because after the 1789 discoveries the numbering scheme was frozen to avoid causing any more confusion. Numerous small moons have been discovered closer to Saturn since then and he suggested the names of the mythological Titans, brothers and sisters of Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, Titan orbits Saturn once every 15 days and 22 hours. Because of this, there is a point on its surface. Longitudes on Titan are measured westward, starting from the passing through this point. Its orbital eccentricity is 0.0288, and the plane is inclined 0.348 degrees relative to the Saturnian equator. Viewed from Earth, Titan reaches a distance of about 20 Saturn radii from Saturn
22.
Europe
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Europe is a continent that comprises the westernmost part of Eurasia. Europe is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, yet the non-oceanic borders of Europe—a concept dating back to classical antiquity—are arbitrary. Europe covers about 10,180,000 square kilometres, or 2% of the Earths surface, politically, Europe is divided into about fifty sovereign states of which the Russian Federation is the largest and most populous, spanning 39% of the continent and comprising 15% of its population. Europe had a population of about 740 million as of 2015. Further from the sea, seasonal differences are more noticeable than close to the coast, Europe, in particular ancient Greece, was the birthplace of Western civilization. The fall of the Western Roman Empire, during the period, marked the end of ancient history. Renaissance humanism, exploration, art, and science led to the modern era, from the Age of Discovery onwards, Europe played a predominant role in global affairs. Between the 16th and 20th centuries, European powers controlled at times the Americas, most of Africa, Oceania. The Industrial Revolution, which began in Great Britain at the end of the 18th century, gave rise to economic, cultural, and social change in Western Europe. During the Cold War, Europe was divided along the Iron Curtain between NATO in the west and the Warsaw Pact in the east, until the revolutions of 1989 and fall of the Berlin Wall. In 1955, the Council of Europe was formed following a speech by Sir Winston Churchill and it includes all states except for Belarus, Kazakhstan and Vatican City. Further European integration by some states led to the formation of the European Union, the EU originated in Western Europe but has been expanding eastward since the fall of the Soviet Union in 1991. The European Anthem is Ode to Joy and states celebrate peace, in classical Greek mythology, Europa is the name of either a Phoenician princess or of a queen of Crete. The name contains the elements εὐρύς, wide, broad and ὤψ eye, broad has been an epithet of Earth herself in the reconstructed Proto-Indo-European religion and the poetry devoted to it. For the second part also the divine attributes of grey-eyed Athena or ox-eyed Hera. The same naming motive according to cartographic convention appears in Greek Ανατολή, Martin Litchfield West stated that phonologically, the match between Europas name and any form of the Semitic word is very poor. Next to these there is also a Proto-Indo-European root *h1regʷos, meaning darkness. Most major world languages use words derived from Eurṓpē or Europa to refer to the continent, in some Turkic languages the originally Persian name Frangistan is used casually in referring to much of Europe, besides official names such as Avrupa or Evropa
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Canada
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Canada is a country in the northern half of North America. Canadas border with the United States is the worlds longest binational land border, the majority of the country has a cold or severely cold winter climate, but southerly areas are warm in summer. Canada is sparsely populated, the majority of its territory being dominated by forest and tundra. It is highly urbanized with 82 per cent of the 35.15 million people concentrated in large and medium-sized cities, One third of the population lives in the three largest cities, Toronto, Montreal and Vancouver. Its capital is Ottawa, and other urban areas include Calgary, Edmonton, Quebec City, Winnipeg. Various aboriginal peoples had inhabited what is now Canada for thousands of years prior to European colonization. Pursuant to the British North America Act, on July 1,1867, the colonies of Canada, New Brunswick and this began an accretion of provinces and territories to the mostly self-governing Dominion to the present ten provinces and three territories forming modern Canada. With the Constitution Act 1982, Canada took over authority, removing the last remaining ties of legal dependence on the Parliament of the United Kingdom. Canada is a parliamentary democracy and a constitutional monarchy, with Queen Elizabeth II being the head of state. The country is officially bilingual at the federal level and it is one of the worlds most ethnically diverse and multicultural nations, the product of large-scale immigration from many other countries. Its advanced economy is the eleventh largest in the world, relying chiefly upon its abundant natural resources, Canadas long and complex relationship with the United States has had a significant impact on its economy and culture. Canada is a country and has the tenth highest nominal per capita income globally as well as the ninth highest ranking in the Human Development Index. It ranks among the highest in international measurements of government transparency, civil liberties, quality of life, economic freedom, Canada is an influential nation in the world, primarily due to its inclusive values, years of prosperity and stability, stable economy, and efficient military. While a variety of theories have been postulated for the origins of Canada. In 1535, indigenous inhabitants of the present-day Quebec City region used the word to direct French explorer Jacques Cartier to the village of Stadacona, from the 16th to the early 18th century Canada referred to the part of New France that lay along the St. Lawrence River. In 1791, the area became two British colonies called Upper Canada and Lower Canada collectively named The Canadas, until their union as the British Province of Canada in 1841. Upon Confederation in 1867, Canada was adopted as the name for the new country at the London Conference. The transition away from the use of Dominion was formally reflected in 1982 with the passage of the Canada Act, later that year, the name of national holiday was changed from Dominion Day to Canada Day
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United States
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Forty-eight of the fifty states and the federal district are contiguous and located in North America between Canada and Mexico. The state of Alaska is in the northwest corner of North America, bordered by Canada to the east, the state of Hawaii is an archipelago in the mid-Pacific Ocean. The U. S. territories are scattered about the Pacific Ocean, the geography, climate and wildlife of the country are extremely diverse. At 3.8 million square miles and with over 324 million people, the United States is the worlds third- or fourth-largest country by area, third-largest by land area. It is one of the worlds most ethnically diverse and multicultural nations, paleo-Indians migrated from Asia to the North American mainland at least 15,000 years ago. European colonization began in the 16th century, the United States emerged from 13 British colonies along the East Coast. Numerous disputes between Great Britain and the following the Seven Years War led to the American Revolution. On July 4,1776, during the course of the American Revolutionary War, the war ended in 1783 with recognition of the independence of the United States by Great Britain, representing the first successful war of independence against a European power. The current constitution was adopted in 1788, after the Articles of Confederation, the first ten amendments, collectively named the Bill of Rights, were ratified in 1791 and designed to guarantee many fundamental civil liberties. During the second half of the 19th century, the American Civil War led to the end of slavery in the country. By the end of century, the United States extended into the Pacific Ocean. The Spanish–American War and World War I confirmed the status as a global military power. The end of the Cold War and the dissolution of the Soviet Union in 1991 left the United States as the sole superpower. The U. S. is a member of the United Nations, World Bank, International Monetary Fund, Organization of American States. The United States is a developed country, with the worlds largest economy by nominal GDP. It ranks highly in several measures of performance, including average wage, human development, per capita GDP. While the U. S. economy is considered post-industrial, characterized by the dominance of services and knowledge economy, the United States is a prominent political and cultural force internationally, and a leader in scientific research and technological innovations. In 1507, the German cartographer Martin Waldseemüller produced a map on which he named the lands of the Western Hemisphere America after the Italian explorer and cartographer Amerigo Vespucci
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Russia
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Russia, also officially the Russian Federation, is a country in Eurasia. The European western part of the country is more populated and urbanised than the eastern. Russias capital Moscow is one of the largest cities in the world, other urban centers include Saint Petersburg, Novosibirsk, Yekaterinburg, Nizhny Novgorod. Extending across the entirety of Northern Asia and much of Eastern Europe, Russia spans eleven time zones and incorporates a range of environments. It shares maritime borders with Japan by the Sea of Okhotsk, the East Slavs emerged as a recognizable group in Europe between the 3rd and 8th centuries AD. Founded and ruled by a Varangian warrior elite and their descendants, in 988 it adopted Orthodox Christianity from the Byzantine Empire, beginning the synthesis of Byzantine and Slavic cultures that defined Russian culture for the next millennium. Rus ultimately disintegrated into a number of states, most of the Rus lands were overrun by the Mongol invasion. The Soviet Union played a role in the Allied victory in World War II. The Soviet era saw some of the most significant technological achievements of the 20th century, including the worlds first human-made satellite and the launching of the first humans in space. By the end of 1990, the Soviet Union had the second largest economy, largest standing military in the world. It is governed as a federal semi-presidential republic, the Russian economy ranks as the twelfth largest by nominal GDP and sixth largest by purchasing power parity in 2015. Russias extensive mineral and energy resources are the largest such reserves in the world, making it one of the producers of oil. The country is one of the five recognized nuclear weapons states and possesses the largest stockpile of weapons of mass destruction, Russia is a great power as well as a regional power and has been characterised as a potential superpower. The name Russia is derived from Rus, a state populated mostly by the East Slavs. However, this name became more prominent in the later history, and the country typically was called by its inhabitants Русская Земля. In order to distinguish this state from other states derived from it, it is denoted as Kievan Rus by modern historiography, an old Latin version of the name Rus was Ruthenia, mostly applied to the western and southern regions of Rus that were adjacent to Catholic Europe. The current name of the country, Россия, comes from the Byzantine Greek designation of the Kievan Rus, the standard way to refer to citizens of Russia is Russians in English and rossiyane in Russian. There are two Russian words which are translated into English as Russians
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South Korea
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South Korea, officially the Republic of Korea, is a sovereign state in East Asia, constituting the southern part of the Korean Peninsula. The earliest Korean pottery dates to 8000 BC, with three kingdoms flourishing in the 1st century BC and its rich and vibrant culture left 19 UNESCO Intangible Cultural Heritages of Humanity, the third largest in the world, along with 12 World Heritage Sites. Annexed into Imperial Japan in 1910, Korea was divided after its surrender in 1945, peace has since mostly continued with the two agreeing to work peacefully for reunification and the South solidifying peace as a regional power with the worlds 10th largest defence budget. South Koreas tiger economy soared at an average of 10% for over 30 years in a period of rapid transformation called the Miracle on the Han River. A long legacy of openness and focus on innovation made it successful, today, it is the worlds fifth largest exporter with the G20s largest budget surplus and highest credit rating of any country in East Asia. It has free trade agreements with 75% of the economy and is the only G20 nation trading freely with China, the US. Since 1988, its constitution guarantees a liberal democracy with high government transparency, high personal freedoms led to the rise of a globally influential pop culture such as K-pop and K-drama, a phenomenon called the Korean Wave, known for its distinctive fashionable and trendy style. Home of the UN Green Climate Fund and GGGI, South Korea is a leader in low carbon growth, committed to helping developing countries as a major DAC. It is the third least ignorant country in the Index of Ignorance, ranking eighth highest for peaceful tolerance. It is the worlds largest spender on R&D per GDP, leading the OECD in graduates in science, the name Korea derives from the name Goryeo. The name Goryeo itself was first used by the ancient kingdom of Goguryeo in the 5th century as a form of its name. The 10th-century kingdom of Goryeo succeeded Goguryeo, and thus inherited its name, the modern spelling of Korea first appeared in the late 17th century in the travel writings of the Dutch East India Companys Hendrick Hamel. After Goryeo was replaced by Joseon in 1392, Joseon became the name for the entire territory. The new official name has its origin in the ancient country of Gojoseon, in 1897, the Joseon dynasty changed the official name of the country from Joseon to Daehan Jeguk. The name Daehan, which means great Han literally, derives from Samhan, however, the name Joseon was still widely used by Koreans to refer to their country, though it was no longer the official name. Under Japanese rule, the two names Han and Joseon coexisted, there were several groups who fought for independence, the most notable being the Provisional Government of the Republic of Korea. Following the surrender of Japan, in 1945, the Republic of Korea was adopted as the name for the new country. Since the government only controlled the part of the Korean Peninsula
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Japan
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Japan is a sovereign island nation in Eastern Asia. Located in the Pacific Ocean, it lies off the eastern coast of the Asia Mainland and stretches from the Sea of Okhotsk in the north to the East China Sea, the kanji that make up Japans name mean sun origin. 日 can be read as ni and means sun while 本 can be read as hon, or pon, Japan is often referred to by the famous epithet Land of the Rising Sun in reference to its Japanese name. Japan is an archipelago consisting of about 6,852 islands. The four largest are Honshu, Hokkaido, Kyushu and Shikoku, the country is divided into 47 prefectures in eight regions. Hokkaido being the northernmost prefecture and Okinawa being the southernmost one, the population of 127 million is the worlds tenth largest. Japanese people make up 98. 5% of Japans total population, approximately 9.1 million people live in the city of Tokyo, the capital of Japan. Archaeological research indicates that Japan was inhabited as early as the Upper Paleolithic period, the first written mention of Japan is in Chinese history texts from the 1st century AD. Influence from other regions, mainly China, followed by periods of isolation, from the 12th century until 1868, Japan was ruled by successive feudal military shoguns who ruled in the name of the Emperor. Japan entered into a period of isolation in the early 17th century. The Second Sino-Japanese War of 1937 expanded into part of World War II in 1941, which came to an end in 1945 following the bombings of Hiroshima and Nagasaki. Japan is a member of the UN, the OECD, the G7, the G8, the country has the worlds third-largest economy by nominal GDP and the worlds fourth-largest economy by purchasing power parity. It is also the worlds fourth-largest exporter and fourth-largest importer, although Japan has officially renounced its right to declare war, it maintains a modern military with the worlds eighth-largest military budget, used for self-defense and peacekeeping roles. Japan is a country with a very high standard of living. Its population enjoys the highest life expectancy and the third lowest infant mortality rate in the world, in ancient China, Japan was called Wo 倭. It was mentioned in the third century Chinese historical text Records of the Three Kingdoms in the section for the Wei kingdom, Wa became disliked because it has the connotation of the character 矮, meaning dwarf. The 倭 kanji has been replaced with the homophone Wa, meaning harmony, the Japanese word for Japan is 日本, which is pronounced Nippon or Nihon and literally means the origin of the sun. The earliest record of the name Nihon appears in the Chinese historical records of the Tang dynasty, at the start of the seventh century, a delegation from Japan introduced their country as Nihon
28.
Mexico
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Mexico, officially the United Mexican States, is a federal republic in the southern half of North America. It is bordered to the north by the United States, to the south and west by the Pacific Ocean, to the southeast by Guatemala, Belize, and the Caribbean Sea, and to the east by the Gulf of Mexico. Covering almost two million square kilometers, Mexico is the sixth largest country in the Americas by total area, Mexico is a federation comprising 31 states and a federal district that is also its capital and most populous city. Other metropolises include Guadalajara, Monterrey, Puebla, Toluca, Tijuana, pre-Columbian Mexico was home to many advanced Mesoamerican civilizations, such as the Olmec, Toltec, Teotihuacan, Zapotec, Maya and Aztec before first contact with Europeans. In 1521, the Spanish Empire conquered and colonized the territory from its base in Mexico-Tenochtitlan, Three centuries later, this territory became Mexico following recognition in 1821 after the colonys Mexican War of Independence. The tumultuous post-independence period was characterized by instability and many political changes. The Mexican–American War led to the cession of the extensive northern borderlands, one-third of its territory. The Pastry War, the Franco-Mexican War, a civil war, the dictatorship was overthrown in the Mexican Revolution of 1910, which culminated with the promulgation of the 1917 Constitution and the emergence of the countrys current political system. Mexico has the fifteenth largest nominal GDP and the eleventh largest by purchasing power parity, the Mexican economy is strongly linked to those of its North American Free Trade Agreement partners, especially the United States. Mexico was the first Latin American member of the Organisation for Economic Co-operation and Development and it is classified as an upper-middle income country by the World Bank and a newly industrialized country by several analysts. By 2050, Mexico could become the fifth or seventh largest economy. The country is considered both a power and middle power, and is often identified as an emerging global power. Due to its culture and history, Mexico ranks first in the Americas. Mexico is a country, ranking fourth in the world by biodiversity. In 2015 it was the 9th most visited country in the world, Mexico is a member of the United Nations, the World Trade Organization, the G8+5, the G20, the Uniting for Consensus and the Pacific Alliance. Mēxihco is the Nahuatl term for the heartland of the Aztec Empire, namely, the Valley of Mexico, and its people, the Mexica and this became the future State of Mexico as a division of New Spain prior to independence. It is generally considered to be a toponym for the valley became the primary ethnonym for the Aztec Triple Alliance as a result. After New Spain won independence from Spain, representatives decided to name the new country after its capital and this was founded in 1524 on top of the ancient Mexica capital of Mexico-Tenochtitlan
29.
Australia
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Australia, officially the Commonwealth of Australia, is a country comprising the mainland of the Australian continent, the island of Tasmania and numerous smaller islands. It is the worlds sixth-largest country by total area, the neighbouring countries are Papua New Guinea, Indonesia and East Timor to the north, the Solomon Islands and Vanuatu to the north-east, and New Zealand to the south-east. Australias capital is Canberra, and its largest urban area is Sydney, for about 50,000 years before the first British settlement in the late 18th century, Australia was inhabited by indigenous Australians, who spoke languages classifiable into roughly 250 groups. The population grew steadily in subsequent decades, and by the 1850s most of the continent had been explored, on 1 January 1901, the six colonies federated, forming the Commonwealth of Australia. Australia has since maintained a liberal democratic political system that functions as a federal parliamentary constitutional monarchy comprising six states. The population of 24 million is highly urbanised and heavily concentrated on the eastern seaboard, Australia has the worlds 13th-largest economy and ninth-highest per capita income. With the second-highest human development index globally, the country highly in quality of life, health, education, economic freedom. The name Australia is derived from the Latin Terra Australis a name used for putative lands in the southern hemisphere since ancient times, the Dutch adjectival form Australische was used in a Dutch book in Batavia in 1638, to refer to the newly discovered lands to the south. On 12 December 1817, Macquarie recommended to the Colonial Office that it be formally adopted, in 1824, the Admiralty agreed that the continent should be known officially as Australia. The first official published use of the term Australia came with the 1830 publication of The Australia Directory and these first inhabitants may have been ancestors of modern Indigenous Australians. The Torres Strait Islanders, ethnically Melanesian, were originally horticulturists, the northern coasts and waters of Australia were visited sporadically by fishermen from Maritime Southeast Asia. The first recorded European sighting of the Australian mainland, and the first recorded European landfall on the Australian continent, are attributed to the Dutch. The first ship and crew to chart the Australian coast and meet with Aboriginal people was the Duyfken captained by Dutch navigator, Willem Janszoon. He sighted the coast of Cape York Peninsula in early 1606, the Dutch charted the whole of the western and northern coastlines and named the island continent New Holland during the 17th century, but made no attempt at settlement. William Dampier, an English explorer and privateer, landed on the north-west coast of New Holland in 1688, in 1770, James Cook sailed along and mapped the east coast, which he named New South Wales and claimed for Great Britain. The first settlement led to the foundation of Sydney, and the exploration, a British settlement was established in Van Diemens Land, now known as Tasmania, in 1803, and it became a separate colony in 1825. The United Kingdom formally claimed the part of Western Australia in 1828. Separate colonies were carved from parts of New South Wales, South Australia in 1836, Victoria in 1851, the Northern Territory was founded in 1911 when it was excised from South Australia
30.
Very Long Baseline Array
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These ten radio antennas work together as an array that forms the longest system in the world that uses very long baseline interferometry. The longest baseline available in this interferometer is about 8,611 kilometres, the construction of the VLBA began in February 1986 and it was completed in May 1993. The first astrometrical observation using all ten antennas was carried out on May 29,1993, the total cost of building the VLBA was about $85 million. Each receiver in the VLBA consists of a dish antenna 25 meters in diameter. This contains the electronics and machinery for the receiver, including low-noise electronics, digital computers, data storage units. Each of the antennas is about as tall as a building when the antenna is pointed straight up. The signals from each antenna are recorded on a bank of approximately one-terabyte hard disc drives, once the disc drives are loaded with information, they are carried to the Pete V. Domenici Science Operations Center at the NRAO in Socorro. There the information undergoes signal processing in a set of digital computers that carry out the interferometry. These computers also make corrections for the rotation of the Earth, the shifts in the crust of the Earth over time. The Very Long Baseline Array usually makes radio observations at wavelengths from three millimeters to 90 centimeters, or in words, at frequencies from 0.3 gigahertz to 96 gigahertz. Within this frequency range, the VLBA observes in eight different frequency bands that are useful for radio astronomy, the VLBA also makes observations in two narrow radio bands below one gigahertz that include spectral lines produced by bright maser emissions. These four additional sites are brought online for as much as 100 hours per four-month trimester, in this configuration, the entire array is known as the High-Sensitivity Array. These sites, with coordinates, are as follows, Distance between each VLBA baseline, The longest baseline in the array is 8,611 kilometres, minimum angular resolution, List of radio telescopes Official site
31.
Spektr-R
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Spektr-R is a Russian scientific satellite with a 10 m radio telescope on board. It was launched on 18 July 2011, by Zenit-3F launcher, from Baikonur Cosmodrome to perform research on the structure and dynamics of radio sources within, together with some of the largest ground-based radio telescopes, this telescope forms interferometric baselines extending up to 350,000 km. In comparison, the distance from Earth to the Moon is 384,400 km. The main scientific goal of the mission is the study of objects with an angular resolution up to a few millionths of an arcsecond. This is accomplished by using the satellite in conjunction with ground-based observatories, another purpose of the project was to develop an understanding of fundamental issues of astrophysics and cosmology. This included star formations, the structure of galaxies, interstellar space, black holes, Spektr-R is one of the instruments in the RadioAstron program, an international network of observatories led by the Astro Space Center of the Lebedev Physical Institute. The telescope is intended for radio-astrophysical observations of extragalactic objects with ultra-high resolution, as well as researching of characteristics of near-Earth, once in space, the flower-like main dish was to open its 27 petals within 30 minutes. At launch the mass of the spacecraft was 3,660 kg and it was launched from the Baikonur Cosmodrome on 18 July 2011 at 02,31 UTC by a Zenit-3F launch vehicle, which is composed of a Zenit-2M with a Fregat-SB upper stage. At the beginning of the 1980s, one of the USSRs leading developers of scientific space probes had completed a design of a revolutionary, new-generation spacecrafts, 1F. The main purpose of Spektr was to develop a platform that could be used for future deep-space missions. NPO Lavochkin, hoped to use the designs of the 1F as the design for space telescopes. In 1982, Lavochkin had completed technical blueprints for RadioAstron, a radio telescope. The expectation was that the 1F and 2F spacecraft would follow the expectations of the RadioAstron mission, early on, many criticized the 1F platform for its questionable astrophysics missions, even when compared to the older 4V spacecraft bus. Although the altitude control system of the 1F seemed to have little issues navigating planetary probes, to add to 1F’s technical issues, the spacecraft seemed to lack electrically driven fly-wheels, which critics believed would have increased its stabilization in space. This document outlined a new impetus for the development of satellites, the new technical proposals submitted in mid-1984 included a gamma-ray telescope designated to register radio waves in the millimetre range. Both of these satellites incorporated rotating solar panels, a highly sensitive star-tracking operating system, according to this plan, Oko-1 would eventually be replaced with scientific instruments where the satellite would be pointed towards space rather than Earth. The RadioAstron satellites main 10-metre radio telescope would communicate in four different bands of waves with the international ground telescopes. It can also locate sources from two frequencies simultaneously, the Spektr-R was also planned to include a secondary BMSV within the Plazma-F experiment, the goal of which was to measure the directions and intensity of solar wind
32.
Minute and second of arc
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A minute of arc, arcminute, arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn, a second of arc, arcsecond, or arc second is 1/60 of an arcminute, 1/3600 of a degree, 1/1296000 of a turn, and π/648000 of a radian. To express even smaller angles, standard SI prefixes can be employed, the number of square arcminutes in a complete sphere is 4 π2 =466560000 π ≈148510660 square arcminutes. The standard symbol for marking the arcminute is the prime, though a single quote is used where only ASCII characters are permitted. One arcminute is thus written 1′ and it is also abbreviated as arcmin or amin or, less commonly, the prime with a circumflex over it. The standard symbol for the arcsecond is the prime, though a double quote is commonly used where only ASCII characters are permitted. One arcsecond is thus written 1″ and it is also abbreviated as arcsec or asec. In celestial navigation, seconds of arc are used in calculations. This notation has been carried over into marine GPS receivers, which normally display latitude and longitude in the format by default. An arcsecond is approximately the angle subtended by a U. S. dime coin at a distance of 4 kilometres, a milliarcsecond is about the size of a dime atop the Eiffel Tower as seen from New York City. A microarcsecond is about the size of a period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth, since antiquity the arcminute and arcsecond have been used in astronomy. The principal exception is Right ascension in equatorial coordinates, which is measured in units of hours, minutes. These small angles may also be written in milliarcseconds, or thousandths of an arcsecond, the unit of distance, the parsec, named from the parallax of one arcsecond, was developed for such parallax measurements. It is the distance at which the radius of the Earths orbit would subtend an angle of one arcsecond. The ESA astrometric space probe Gaia is hoped to measure star positions to 20 microarcseconds when it begins producing catalog positions sometime after 2016, there are about 1.3 trillion µas in a turn. Currently the best catalog positions of stars actually measured are in terms of milliarcseconds, apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red supergiant with a diameter of 0.05 arcsecond. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond, space telescopes are not affected by the Earths atmosphere but are diffraction limited. For example, the Hubble space telescope can reach a size of stars down to about 0. 1″
33.
Supermassive black hole
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In the case of the Milky Way, the SMBH corresponds with the location of Sagittarius A*. Supermassive black holes have properties that distinguish them from lower-mass classifications, first, the average density of a supermassive black hole can be less than the density of water in the case of some supermassive black holes. This is because the Schwarzschild radius is proportional to mass. In addition, the forces in the vicinity of the event horizon are significantly weaker for massive black holes. Unlike with stellar mass black holes, one would not experience significant tidal force until very deep into the black hole, donald Lynden-Bell and Martin Rees hypothesized in 1971 that the center of the Milky Way galaxy would contain a supermassive black hole. Sagittarius A* was discovered and named on February 13 and 15,1974, by astronomers Bruce Balick and they discovered a radio source that emits synchrotron radiation, it was found to be dense and immobile because of its gravitation. This was, therefore, the first indication that a black hole exists in the center of the Milky Way. The origin of black holes remains an open field of research. Astrophysicists agree that once a black hole is in place in the center of a galaxy, it can grow by accretion of matter, there are, however, several hypotheses for the formation mechanisms and initial masses of the progenitors, or seeds, of supermassive black holes. The most obvious hypothesis is that the seeds are black holes of tens or perhaps hundreds of masses that are left behind by the explosions of massive stars. Yet another model involves a dense stellar cluster undergoing core-collapse as the heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. Finally, primordial black holes may have been produced directly from external pressure in the first moments after the Big Bang, formation of black holes from the deaths of the first stars has been extensively studied and corroborated by observations. The other models for black hole formation listed above are theoretical, the difficulty in forming a supermassive black hole resides in the need for enough matter to be in a small enough volume. This matter needs to have very little angular momentum in order for this to happen, normally, the process of accretion involves transporting a large initial endowment of angular momentum outwards, and this appears to be the limiting factor in black hole growth. This is a component of the theory of accretion disks. Gas accretion is the most efficient and also the most conspicuous way in black holes grow. The majority of the growth of supermassive black holes is thought to occur through episodes of rapid gas accretion. Observations reveal that quasars were more frequent when the Universe was younger, indicating that supermassive black holes formed
34.
Milky Way
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The Milky Way is the galaxy that contains our Solar System. The descriptive milky is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610, until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies, the Milky Way is a barred spiral galaxy with a diameter between 100,000 light-years and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars, there are probably at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the edge of one of the spiral-shaped concentrations of gas. The stars in the inner ≈10,000 light-years form a bulge, the very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. Stars and gases at a range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed dark matter, the rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself, the Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. The Milky Way can be seen as a band of white light some 30 degrees wide arcing across the sky. Dark regions within the band, such as the Great Rift, the area of the sky obscured by the Milky Way is called the Zone of Avoidance. The Milky Way has a low surface brightness. Its visibility can be reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen and it should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1
35.
Messier 87
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Messier 87 is a supergiant elliptical galaxy in the constellation Virgo. It is one of the brightest radio sources in the sky, French astronomer Charles Messier discovered M87 in 1781, cataloguing it as a nebulous feature while searching for objects that would confuse comet hunters. The second brightest galaxy within the northern Virgo Cluster, M87 is located about 16.4 million parsecs from Earth, unlike a disk-shaped spiral galaxy, M87 has no distinctive dust lanes. Instead, it has an almost featureless, ellipsoidal shape typical of most giant elliptical galaxies, forming around one sixth of M87s mass, the stars in this galaxy have a nearly spherically symmetric distribution, their density decreasing with increasing distance from the core. At the core is a black hole, which forms the primary component of an active galactic nucleus. This object is a source of multiwavelength radiation, particularly radio waves. M87s galactic envelope extends out to a radius of about 150 kiloparsecs, between the stars is a diffuse interstellar medium of gas that has been chemically enriched by elements emitted from evolved stars. In 1781, French astronomer Charles Messier published a catalogue of 103 objects that had a nebulous appearance and this list was intended to identify objects that might be confused with comets. In subsequent use, each item in the catalogue was prefixed with an M. Thus, during the 1880s, this nebula was included in the New General Catalogue as NGC4486. This compilation of nebulae and star clusters was assembled by the Danish-Irish astronomer John Dreyer based primarily on the observations of English astronomer John Herschel, the ray appeared brightest at the inner end. In 1926 he produced a new categorization of nebulae, with M87 being classified as a type of elliptical extra-galactic nebula with no apparent elongation. By 1931, Hubble had identified M87 as a member of the Virgo Cluster, at the time it was the only known example of an elliptical nebula for which individual stars could be resolved. M87 continued to be called an extragalactic nebula for many years thereafter, in 1947, a prominent radio source was identified overlapping the location of M87, and this was labeled Virgo-A. This source was confirmed to be M87 by 1953, and the relativistic jet emerging from the core of the galaxy was suggested as the cause. This jet extended from the core at a angle of 260° to an angular distance of 20″ with an angular width of 2″. In 1969–70, a component of the radio emission was found to closely align with the optical source of the jet. In April 1965, the United States Naval Research Laboratory group launched an Aerobee 150 rocket equipped with a pair of Geiger counters and this flight discovered seven candidate X-ray sources, including the first extragalactic X-ray source, Virgo X-1 was designated as the first X-ray source detected in Virgo. A later Aerobee rocket launched from White Sands Missile Range on July 7,1967, subsequent X-ray observations by the HEAO1 and Einstein Observatory showed a complex source that included the active galactic nucleus of M87
36.
Maser
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A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, James P. Gordon, Townes, Nikolay Basov and Alexander Prokhorov were awarded the 1964 Nobel Prize in Physics for theoretical work leading to the maser. Masers are used as the device in atomic clocks, and as extremely low-noise microwave amplifiers in radio telescopes. Modern masers can be designed to generate electromagnetic waves at not only microwave frequencies but also radio, for this reason Charles Townes suggested replacing microwave with the word molecular as the first word in the acronym maser. The laser works by the principle as the maser. The maser was the forerunner of the laser, inspiring theoretical work by Townes, when the coherent optical oscillator was first imagined in 1957, it was originally called the optical maser. This was ultimately changed to laser for Light Amplification by Stimulated Emission of Radiation, Gordon Gould is credited with creating this acronym in 1957. Independently, Charles Hard Townes, James P. Gordon, and H. J. Zeiger built the first ammonia maser at Columbia University in 1953. This device used stimulated emission in a stream of energized ammonia molecules to produce amplification of microwaves at a frequency of about 24.0 gigahertz. Townes later worked with Arthur L. Schawlow to describe the principle of the maser, or laser. For their research in the field of stimulated emission, Townes, the maser is based on the principle of stimulated emission proposed by Albert Einstein in 1917. When atoms have been induced into an energy state, they can amplify radiation at a frequency particular to the element or molecule used as the masing medium. By putting such a medium in a resonant cavity, feedback is created that can produce coherent radiation. This development could lead to wide range of new applications for maser technology, including communications, Masers serve as high precision frequency references. These atomic frequency standards are one of the forms of atomic clocks. They are often used as low-noise microwave amplifiers in radio telescopes, as of 2012, the most important type of maser is the hydrogen maser which is currently used as an atomic frequency standard. Together with other kinds of clocks, these help make up the International Atomic Time standard. This is the time scale coordinated by the International Bureau of Weights
37.
Solar System
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The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
38.
Magnetic tape data storage
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Magnetic tape data storage is a system for storing digital information on magnetic tape using digital recording. Modern magnetic tape is most commonly packaged in cartridges and cassettes, the device that performs writing or reading of data is a tape drive. Autoloaders and tape libraries automate cartridge handling, for example, a common cassette-based format is Linear Tape-Open, which comes in a variety of densities and is manufactured by several companies. Sony announced in 2014 that they had developed a storage technology with the highest reported magnetic tape data density,148 Gbit/in². Initially, magnetic tape for storage was wound on 10. 5-inch reels. This de facto standard for computer systems persisted through the late 1980s. Tape cartridges and cassettes were available as early as the mid-1970s and were used with small computer systems. With the introduction of the IBM3480 cartridge in 1984, large computer systems started to move away from open reel tapes, magnetic tape was first used to record computer data in 1951 on the Eckert-Mauchly UNIVAC I. The UNISERVO drive recording medium was a metal strip of 0. 5-inch wide nickel-plated phosphor bronze. Recording density was 128 characters per inch on eight tracks at a speed of 100 in/s. Of the eight tracks, six were data, one was a parity track, making allowance for the empty space between tape blocks, the actual transfer rate was around 7,200 characters per second. A small reel of mylar tape provided separation from the metal tape, IBM computers from the 1950s used ferrous-oxide coated tape similar to that used in audio recording. IBMs technology soon became the de facto industry standard, magnetic tape dimensions were 0. 5-inch wide and wound on removable reels up to 10.5 inches in diameter. Different tape lengths were available with 1,200 feet and 2,400 feet on mil, during the 1980s, longer tape lengths such as 3,600 feet became available using a much thinner PET film. Most tape drives could support a maximum size of 10.5 inches. CDC used IBM compatible 1/2 inch magnetic tapes, but also offered a 1 inch wide variant, a so-called mini-reel was common for smaller data sets, such as for software distribution. These were 7-inch reels, often with no fixed length—the tape was sized to fit the amount of data recorded on it as a cost-saving measure. Early IBM tape drives, such as the IBM727 and IBM729, were mechanically sophisticated floor-standing drives that used vacuum columns to buffer long u-shaped loops of tape
39.
Hard disk drive
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The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a manner, meaning that individual blocks of data can be stored or retrieved in any order. HDDs are a type of storage, retaining stored data even when powered off. Introduced by IBM in 1956, HDDs became the dominant secondary storage device for computers by the early 1960s. Continuously improved, HDDs have maintained this position into the era of servers. More than 200 companies have produced HDDs historically, though after extensive industry consolidation most current units are manufactured by Seagate, Toshiba, as of 2016, HDD production is growing, although unit shipments and sales revenues are declining. While SSDs have higher cost per bit, SSDs are replacing HDDs where speed, power consumption, small size, the primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of 1000, the two most common form factors for modern HDDs are 3. 5-inch, for desktop computers, and 2. 5-inch, primarily for laptops. HDDs are connected to systems by standard interface cables such as PATA, SATA, Hard disk drives were introduced in 1956, as data storage for an IBM real-time transaction processing computer and were developed for use with general-purpose mainframe and minicomputers. The first IBM drive, the 350 RAMAC in 1956, was approximately the size of two medium-sized refrigerators and stored five million six-bit characters on a stack of 50 disks. In 1962 the IBM350 RAMAC disk storage unit was superseded by the IBM1301 disk storage unit, cylinder-mode read/write operations were supported, and the heads flew about 250 micro-inches above the platter surface. Motion of the head array depended upon a binary system of hydraulic actuators which assured repeatable positioning. The 1301 cabinet was about the size of three home refrigerators placed side by side, storing the equivalent of about 21 million eight-bit bytes, access time was about a quarter of a second. Also in 1962, IBM introduced the model 1311 disk drive, users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations, non-removable HDDs were called fixed disk drives. Some high-performance HDDs were manufactured with one head per track so that no time was lost physically moving the heads to a track, known as fixed-head or head-per-track disk drives they were very expensive and are no longer in production. In 1973, IBM introduced a new type of HDD code-named Winchester and its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to land on an area of the disk surface upon spin-down
40.
Radio telescope
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A radio telescope is a specialized antenna and radio receiver used to receive radio waves from astronomical radio sources in the sky in radio astronomy. Radio telescopes are typically large parabolic antennas similar to those employed in tracking and communicating with satellites and they may be used singly, or linked together electronically in an array. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night, Radio waves from space were first detected by engineer Karl Guthe Jansky in 1932 at Bell Telephone Laboratories in Holmdel, New Jersey using an antenna built to study noise in radio receivers. The first purpose-built radio telescope was a 9-meter parabolic dish constructed by radio amateur Grote Reber in his yard in Wheaton. The sky survey he did with it is considered the beginning of the field of radio astronomy. The first radio antenna used to identify an astronomical radio source was one built by Karl Guthe Jansky, Jansky was assigned the job of identifying sources of static that might interfere with radio telephone service. Janskys antenna was an array of dipoles and reflectors designed to receive short wave radio signals at a frequency of 20.5 MHz and it was mounted on a turntable that allowed it to rotate in any direction, earning it the name Janskys merry-go-round. It had a diameter of approximately 100 ft and stood 20 ft tall, by rotating the antenna, the direction of the received interfering radio source could be pinpointed. A small shed to the side of the antenna housed an analog recording system. Jansky finally determined that the faint hiss repeated on a cycle of 23 hours and 56 minutes and this period is the length of an astronomical sidereal day, the time it takes any fixed object located on the celestial sphere to come back to the same location in the sky. An amateur radio operator, Grote Reber, was one of the pioneers of what became known as radio astronomy and he built the first parabolic dish radio telescope, a 9 metres in diameter) in his back yard in Wheaton, Illinois in 1937. The range of frequencies in the spectrum that makes up the radio spectrum is very large. This means that the types of antennas that are used as radio telescopes vary widely in design, size, at wavelengths of 30 meters to 3 meters, they are generally either directional antenna arrays similar to TV antennas or large stationary reflectors with moveable focal points. Since the wavelengths being observed with these types of antennas are so long, at shorter wavelengths parabolic dish antennas predominate. The angular resolution of an antenna is determined by the ratio of the diameter of the dish to the wavelength of the radio waves being observed. This dictates the size a radio telescope needs for a useful resolution. Radio telescopes that operate at wavelengths of 3 meters to 30 cm are usually well over 100 meters in diameter, telescopes working at wavelengths shorter than 30 cm range in size from 3 to 90 meters in diameter. The Wilkinson Microwave Anisotropy Probe mapped the Cosmic microwave background radiation in 5 different frequency bands, centered on 23 GHz,33 GHz,41 GHz,61 GHz, the worlds largest filled-aperture radio telescope is the Five hundred meter Aperture Spherical Telescope completed in 2016 by China
