1.
Time standard
–
A time standard is a specification for measuring time, either the rate at which time passes, or points in time, or both. In modern times, several time specifications have been recognized as standards. An example of a kind of time standard can be a time scale, a standard for civil time can specify both time intervals and time-of-day. Standardized time measurements are made using a clock to count periods of some cyclic change, historically, time standards were often based on the Earths rotational period. From the late 17th century to the 19th century it was assumed that the Earths daily rotational rate was constant. Time standards based on Earth rotation were replaced for use from 1952 onwards by an ephemeris time standard based on the Earths orbital period. Various types of second and day are used as the time interval for most time scales. Other intervals of time are usually defined in terms of these two, apparent solar time is based on the solar day, which is the period between one solar noon and the next. A solar day is approximately 24 hours of mean time, as the variation accumulates over a few weeks, there are differences as large as 16 minutes between apparent solar time and mean solar time. However, these cancel out over a year. There are also other perturbations such as Earths wobble, but these are less than a second per year, sidereal time is time by the stars. A sidereal rotation is the time it takes the Earth to make one revolution with respect to the stars and it is well known that observations of the Sun pose substantial obstacles to the achievement of accuracy in measurement. Mean solar time was originally apparent solar time corrected by the equation of time, Greenwich Mean Time was originally mean time deduced from meridian observations made at the Royal Greenwich Observatory. The principal meridian of that observatory was chosen in 1884 by the International Meridian Conference to be the Prime Meridian. GMT either by name or as mean time at Greenwich used to be an international time standard. The more current refined version of UT, UT1, is still in reality mean time at Greenwich, Greenwich Mean Time is still the legal time in the UK. But Coordinated Universal Time is in actual use in the UK. Universal Time is mean solar time at 0° longitude, some implementations are UT0 is the time of a particular place of observation
2.
Greenwich Mean Time
–
Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London. GMT was formerly used as the civil time standard, now superseded in that function by Coordinated Universal Time. Today GMT is considered equivalent to UTC for UK civil purposes and for navigation is considered equivalent to UT1, consequently, the term GMT should not be used for precise purposes. Due to Earths uneven speed in its orbit and its axial tilt, noon GMT is rarely the exact moment the sun crosses the Greenwich meridian. This event may occur up to 16 minutes before or after noon GMT, noon GMT is the annual average moment of this event, which accounts for the word mean in Greenwich Mean Time. Originally, astronomers considered a GMT day to start at noon while for almost everyone else it started at midnight, to avoid confusion, the name Universal Time was introduced to denote GMT as counted from midnight. Astronomers preferred the old convention to simplify their observational data, so each night was logged under a single calendar date. Today Universal Time usually refers to UTC or UT1, in some countries Greenwich Mean Time is the legal time in the winter and the population uses the term. For an explanation of why this is, see GMT in legislation below, synchronisation of the chronometer on GMT did not affect shipboard time, which was still solar time. Most time zones were based upon GMT, as an offset of a number of hours ahead of GMT or behind GMT and it was gradually adopted for other purposes, but a legal case in 1858 held local mean time to be the official time. On 14 May 1880, a signed by Clerk to Justices appeared in The Times, stating that Greenwich time is now kept almost throughout England. For example, our polling booths were opened, say, at 813 and closed at 413 PM. This was changed later in 1880, GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time, hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the time ball at the observatory redundant in the process. The daily rotation of the Earth is irregular and constantly slows, on 1 January 1972, GMT was superseded as the international civil time standard by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world. Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by the centre of the instrument at the Observatory at Greenwich. Nevertheless, the line in the old observatorys courtyard today differs no more than a few metres from that line which is now the prime meridian of the world. Historically GMT has been used two different conventions for numbering hours. The long-standing astronomical convention dating from the work of Ptolemy, was to refer to noon as zero hours and this contrasted with the civil convention of referring to midnight as zero hours dating from the Romans
3.
Mean solar time
–
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of time is the day. Two types of time are apparent solar time and mean solar time. Fix a tall pole vertically in the ground, at some instant on any day the shadow will point exactly north or south. That instant is local apparent noon,12,00 local apparent time, about 24 hours later the shadow will again point north/south, the Sun seeming to have covered a 360-degree arc around the Earths axis. When the Sun has covered exactly 15 degrees, local apparent time is 13,00 exactly, after 15 more degrees it will be 14,00 exactly. As explained in the equation of time article, this is due to the eccentricity of the Earths orbit, the effect of this is that a clock running at a constant rate – e. g. This is mean solar time, which is not perfectly constant from one century to the next but is close enough for most purposes. Currently a mean solar day is about 86,400.002 SI seconds, the two kinds of solar time are among the three kinds of time reckoning that were employed by astronomers until the 1950s. By the 1950s it had become clear that the Earths rotation rate was not constant, so astronomers developed ephemeris time, the apparent sun is the true sun as seen by an observer on Earth. Apparent solar time or true solar time is based on the apparent motion of the actual Sun and it is based on the apparent solar day, the interval between two successive returns of the Sun to the local meridian. Solar time can be measured by a sundial. The equivalent on other planets is termed local true solar time, the length of a solar day varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean. The effect has two main causes, first, Earths orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun and slower when it is farthest from the Sun. Second, due to Earths axial tilt, the Suns annual motion is along a circle that is tilted to Earths celestial equator. In June and December when the sun is farthest from the equator a given shift along the ecliptic corresponds to a large shift at the equator. So apparent solar days are shorter in March and September than in June or December and these lengths will change slightly in a few years and significantly in thousands of years. Mean solar time is the angle of the mean Sun plus 12 hours
4.
Prime meridian (Greenwich)
–
A prime meridian, based at the Royal Observatory, Greenwich, in London, was established by Sir George Airy in 1851. By 1884, over two-thirds of all ships and tonnage used it as the meridian on their charts. In October of that year, at the behest of U. S. President Chester A. Arthur,41 delegates from 25 nations met in Washington, United States, for the International Meridian Conference. This conference selected the meridian passing through Greenwich as the prime meridian due to its popularity. However, France abstained from the vote, and French maps continued to use the Paris meridian for several decades, the prime meridian passes through the Airy transit circle of the Greenwich observatory. The actual reason for the discrepancy is that the strip is indeed at astronomical longitude zero degrees, zero minutes. Before the establishment of a meridian, most maritime countries established their own prime meridian. In 1721, Great Britain established its own meridian passing through a transit circle at the newly established Royal Observatory at Greenwich. The meridian was moved around 10 metres or so east on three occasions as transit circles with newer and better instruments were built, on each occasion next door to the existing one and this was to allow uninterrupted observation during each new construction. The final meridian was established as a line from the north pole to the south pole passing through the Airy transit circle. This became Great Britains meridian in 1851, for all practical purposes of the period, the changes as the meridian was moved went unnoticed. Transit instruments are installed to be perpendicular to the local level, in 1884, the International Meridian Conference took place to establish an internationally recognised single meridian. The meridian chosen was that which passed through the Airy transit circle at Greenwich, at around the time of this conference, scientists were making measurements to determine the deflection of the vertical on a large scale. The downward extended plumb lines dont even all intersect the axis of the Earth. To make computations feasible, scientists defined ellipsoids of revolution, a given ellipsoid would be a compromise for measurements in a given area. When the Airy transit circle was built, a basin was used to align the telescope to the perpendicular. While the local vertical defined at the Airy transit circle still points to the celestial meridian. As a result of this, the ITRF zero meridian, defined by a passing through the Earths rotation axis, is 102 metres to the east of the prime meridian
5.
Coordinated Universal Time
–
Coordinated Universal Time, abbreviated to UTC, is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean time at 0° longitude. It is one of closely related successors to Greenwich Mean Time. For most purposes, UTC is considered interchangeable with GMT, the first Coordinated Universal Time was informally adopted on 1 January 1960. This change also adopted leap seconds to simplify future adjustments, a number of proposals have been made to replace UTC with a new system that would eliminate leap seconds, but no consensus has yet been reached. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of universal time, see the Current number of leap seconds section for the number of leap seconds inserted to date. The official abbreviation for Coordinated Universal Time is UTC and this abbreviation arose from a desire by the International Telecommunication Union and the International Astronomical Union to use the same abbreviation in all languages. English speakers originally proposed CUT, while French speakers proposed TUC, the compromise that emerged was UTC, which conforms to the pattern for the abbreviations of the variants of Universal Time. Time zones around the world are expressed using positive or negative offsets from UTC, the westernmost time zone uses UTC−12, being twelve hours behind UTC, the easternmost time zone, theoretically, uses UTC+12, being twelve hours ahead of UTC. In 1995, the nation of Kiribati moved those of its atolls in the Line Islands from UTC-10 to UTC+14 so that the country would all be on the same day. UTC is used in internet and World Wide Web standards. The Network Time Protocol, designed to synchronise the clocks of computers over the internet, computer servers, online services and other entities that rely on having a universally accepted time use UTC as it is more specific than GMT. If only limited precision is needed, clients can obtain the current UTC from a number of official internet UTC servers, for sub-microsecond precision, clients can obtain the time from satellite signals. UTC is also the standard used in aviation, e. g. for flight plans. Weather forecasts and maps all use UTC to avoid confusion about time zones, the International Space Station also uses UTC as a time standard. Amateur radio operators often schedule their radio contacts in UTC, because transmissions on some frequencies can be picked up by many time zones, UTC is also used in digital tachographs used on large goods vehicles under EU and AETR rules. UTC divides time into days, hours, minutes and seconds, days are conventionally identified using the Gregorian calendar, but Julian day numbers can also be used. Each day contains 24 hours and each hour contains 60 minutes, the number of seconds in a minute is usually 60, but with an occasional leap second, it may be 61 or 59 instead
6.
Star
–
A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun, many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, indeed, most are invisible from Earth even through the most powerful telescopes. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the stars lifetime, near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity, and many properties of a star by observing its motion through space, its luminosity. The total mass of a star is the factor that determines its evolution. Other characteristics of a star, including diameter and temperature, change over its life, while the environment affects its rotation. A plot of the temperature of stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that allows the age and evolutionary state of that star to be determined. A stars life begins with the collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, the remainder of the stars interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The stars internal pressure prevents it from collapsing further under its own gravity, a star with mass greater than 0.4 times the Suns will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core, as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a remnant, a white dwarf. Binary and multi-star systems consist of two or more stars that are bound and generally move around each other in stable orbits. When two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, historically, stars have been important to civilizations throughout the world
7.
Quasar
–
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
8.
Solar time
–
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of time is the day. Two types of time are apparent solar time and mean solar time. Fix a tall pole vertically in the ground, at some instant on any day the shadow will point exactly north or south. That instant is local apparent noon,12,00 local apparent time, about 24 hours later the shadow will again point north/south, the Sun seeming to have covered a 360-degree arc around the Earths axis. When the Sun has covered exactly 15 degrees, local apparent time is 13,00 exactly, after 15 more degrees it will be 14,00 exactly. As explained in the equation of time article, this is due to the eccentricity of the Earths orbit, the effect of this is that a clock running at a constant rate – e. g. This is mean solar time, which is not perfectly constant from one century to the next but is close enough for most purposes. Currently a mean solar day is about 86,400.002 SI seconds, the two kinds of solar time are among the three kinds of time reckoning that were employed by astronomers until the 1950s. By the 1950s it had become clear that the Earths rotation rate was not constant, so astronomers developed ephemeris time, the apparent sun is the true sun as seen by an observer on Earth. Apparent solar time or true solar time is based on the apparent motion of the actual Sun and it is based on the apparent solar day, the interval between two successive returns of the Sun to the local meridian. Solar time can be measured by a sundial. The equivalent on other planets is termed local true solar time, the length of a solar day varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean. The effect has two main causes, first, Earths orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun and slower when it is farthest from the Sun. Second, due to Earths axial tilt, the Suns annual motion is along a circle that is tilted to Earths celestial equator. In June and December when the sun is farthest from the equator a given shift along the ecliptic corresponds to a large shift at the equator. So apparent solar days are shorter in March and September than in June or December and these lengths will change slightly in a few years and significantly in thousands of years. Mean solar time is the angle of the mean Sun plus 12 hours
9.
International Atomic Time
–
International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earths geoid. It is the basis for Coordinated Universal Time, which is used for civil timekeeping all over the Earths surface, and for Terrestrial Time, as of 31 December 2016 when another leap second was added, TAI is exactly 37 seconds ahead of UTC. The 37 seconds results from the difference of 10 seconds at the start of 1972. TAI may be reported using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth, specifically, both Julian Dates and the Gregorian calendar are used. TAI in this form was synchronised with Universal Time at the beginning of 1958, TAI as a time scale is a weighted average of the time kept by over 400 atomic clocks in over 50 national laboratories worldwide. The clocks are compared using GPS signals and two-way satellite time, due to the signal averaging it is an order of magnitude more stable than its best clock alone would be. The majority of the clocks are caesium clocks, the definition of the SI second is written in terms of caesium, the participating institutions each broadcast, in real time, a frequency signal with timecodes, which is their estimate of TAI. Time codes are published in the form of UTC, which differs from TAI by a well-known integer number of seconds. These time scales are denoted in the form UTC in the UTC form, the TAI form may be denoted TAI. The latter is not to be confused with TA, which denotes an independent atomic time scale, the clocks at different institutions are regularly compared against each other. The International Bureau of Weights and Measures, combines these measurements to retrospectively calculate the average that forms the most stable time scale possible. This combined time scale is published monthly in Circular T, and is the canonical TAI and this time scale is expressed in the form of tables of differences UTC-UTC for each participating institution k. Errors in publication may be corrected by issuing a revision of the faulty Circular T or by errata in a subsequent Circular T, aside from this, once published in Circular T the TAI scale is not revised. In hindsight it is possible to discover errors in TAI, doing so does not create another version of TAI, it is instead considered to be creating a better realisation of Terrestrial Time. Early atomic time scales consisted of quartz clocks with frequencies calibrated by an atomic clock. Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the National Physical Laboratory, the Greenwich Atomic scale began in 1955 at the Royal Greenwich Observatory. The International Time Bureau began a time scale, Tm or AM, in July 1955, both the BIH scale and A.1 were defined by an epoch at the beginning of 1958, it was set to read Julian Date 2436204.5 at the corresponding UT2 instant. The procedures used by the BIH evolved, and the name for the time scale changed, A3 in 1963 and this synchronisation was inevitably imperfect, depending as it did on the astronomical realisation of UT2
10.
Leap second
–
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time in order to keep its time of day close to the mean solar time, or UT1. Without such a correction, time reckoned by Earths rotation drifts away from atomic time because of irregularities in the Earths rate of rotation. Since this system of correction was implemented in 1972,27 leap seconds have been inserted, like most time standards, UTC defines a grouping of seconds into minutes, hours, days, months, and years. However, the duration of one solar day is now slightly longer than 24 hours because the rotation of the Earth has slowed down. The purpose of a second is to compensate for this drift. Specifically, a leap second is inserted between second 23,59,59 of a chosen UTC calendar date and second 00,00,00 of the following date. All leap seconds have been scheduled for either June 30 or December 31, the extra second is displayed on UTC clocks as 23,59,60. On clocks that display local time tied to UTC, the second may be inserted at the end of some other hour. A negative leap second would suppress second 23,59,59 of the last day of a chosen month, since the introduction of leap seconds, the mean solar day has outpaced UTC only for very brief periods, and has not triggered a negative leap second. Because the Earths rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced, with this definition, the second was proposed in 1874 as the base unit of time in the CGS system of units. Because the tropical year was considered more fundamental than the year, in 1955. That value agreed to 1 part in 1010 with the second then in use. It was also close to 1⁄86400 of the solar day as averaged between years 1750 and 1892. However, for the past several centuries, the length of the solar day has been increasing by about 1. 4–1.7 ms per century. By 1961, the solar day was already a millisecond or two longer than 86,400 SI seconds. Thus, from 1961 to 1971, the rate of clocks was constantly slowed to remain synchronised with GMT. During that period, therefore, the seconds of broadcast services were actually longer than the SI second. By then, the UTC clock was already 10 seconds behind TAI, which had been synchronized with UT1 in 1958, but had been counting true SI seconds since then
11.
Rail transport
–
Rail transport is a means of conveyance of passengers and goods on wheeled vehicles running on rails, also known as tracks. It is also referred to as train transport. In contrast to road transport, where vehicles run on a flat surface. Tracks usually consist of rails, installed on ties and ballast, on which the rolling stock, usually fitted with metal wheels. Other variations are possible, such as slab track, where the rails are fastened to a concrete foundation resting on a prepared subsurface. Rolling stock in a transport system generally encounters lower frictional resistance than road vehicles, so passenger. The operation is carried out by a company, providing transport between train stations or freight customer facilities. Power is provided by locomotives which either draw electric power from a railway system or produce their own power. Most tracks are accompanied by a signalling system, Railways are a safe land transport system when compared to other forms of transport. The oldest, man-hauled railways date back to the 6th century BC, with Periander, one of the Seven Sages of Greece, Rail transport blossomed after the British development of the steam locomotive as a viable source of power in the 19th centuries. With steam engines, one could construct mainline railways, which were a key component of the Industrial Revolution, also, railways reduced the costs of shipping, and allowed for fewer lost goods, compared with water transport, which faced occasional sinking of ships. The change from canals to railways allowed for markets in which prices varied very little from city to city. In the 1880s, electrified trains were introduced, and also the first tramways, starting during the 1940s, the non-electrified railways in most countries had their steam locomotives replaced by diesel-electric locomotives, with the process being almost complete by 2000. During the 1960s, electrified high-speed railway systems were introduced in Japan, other forms of guided ground transport outside the traditional railway definitions, such as monorail or maglev, have been tried but have seen limited use. The history of the growth, decline and restoration to use of transport can be divided up into several discrete periods defined by the principal means of motive power used. The earliest evidence of a railway was a 6-kilometre Diolkos wagonway, trucks pushed by slaves ran in grooves in limestone, which provided the track element. The Diolkos operated for over 600 years, Railways began reappearing in Europe after the Dark Ages. The earliest known record of a railway in Europe from this period is a window in the Minster of Freiburg im Breisgau in Germany
12.
Clock
–
A clock is an instrument to measure, keep, and indicate time. The word clock is derived from the Celtic words clagan and clocca meaning bell, a silent instrument missing such a striking mechanism has traditionally been known as a timepiece. In general usage today a clock refers to any device for measuring and displaying the time, Watches and other timepieces that can be carried on ones person are often distinguished from clocks. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units, the day, the lunar month. Devices operating on several physical processes have been used over the millennia, a sundial shows the time by displaying the position of a shadow on a flat surface. There is a range of duration timers, an example being the hourglass. Water clocks, along with the sundials, are possibly the oldest time-measuring instruments, spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries, clockmaking flourished, the next development in accuracy occurred after 1656 with the invention of the pendulum clock. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation, the electric clock was patented in 1840. The development of electronics in the 20th century led to clocks with no clockwork parts at all, the timekeeping element in every modern clock is a harmonic oscillator, a physical object that vibrates or oscillates at a particular frequency. This object can be a pendulum, a fork, a quartz crystal. Analog clocks usually indicate time using angles, Digital clocks display a numeric representation of time. Two numeric display formats are used on digital clocks, 24-hour notation. Most digital clocks use electronic mechanisms and LCD, LED, or VFD displays, for convenience, distance, telephony or blindness, auditory clocks present the time as sounds. There are also clocks for the blind that have displays that can be read by using the sense of touch, some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them. The evolution of the technology of clocks continues today, the study of timekeeping is known as horology. The apparent position of the Sun in the sky moves over the course of a day, shadows cast by stationary objects move correspondingly, so their positions can be used to indicate the time of day. A sundial shows the time by displaying the position of a shadow on a flat surface, sundials can be horizontal, vertical, or in other orientations
13.
Train
–
A train is a form of rail transport consisting of a series of vehicles that usually runs along a rail track to transport cargo or passengers. Motive power is provided by a locomotive or individual motors in self-propelled multiple units. Although historically steam propulsion dominated, the most common forms are diesel and electric locomotives. Other energy sources include horses, engine or water-driven rope or wire winch, gravity, pneumatics, batteries, the word train comes from the Old French trahiner, from the Latin trahere pull, draw. There are various types of trains that are designed for particular purposes, a train may consist of a combination of one or more locomotives and attached railroad cars, or a self-propelled multiple unit. The first trains were rope-hauled, gravity powered or pulled by horses, from the early 19th century almost all were powered by steam locomotives. A passenger train is one which includes passenger-carrying vehicles which can often be very long, one notable and growing long-distance train category is high-speed rail. In order to much faster operation over 500 km/h, innovative Maglev technology has been researched for years. In most countries, such as the United Kingdom, the distinction between a tramway and a railway is precise and defined in law, a freight train uses freight cars to transport goods or materials. Freight and passengers may be carried in the train in a mixed consist. Rail cars and machinery used for maintenance and repair of tracks, etc. are termed maintenance of way equipment, similarly, dedicated trains may be used to provide support services to stations along a train line, such as garbage or revenue collection. There are various types of trains that are designed for particular purposes, a train can consist of a combination of one or more locomotives and attached railroad cars, or a self-propelled multiple unit. Trains can also be hauled by horses, pulled by a cable, special kinds of trains running on corresponding special railways are atmospheric railways, monorails, high-speed railways, maglev, rubber-tired underground, funicular and cog railways. A passenger train may consist of one or several locomotives and coaches, alternatively, a train may consist entirely of passenger carrying coaches, some or all of which are powered as a multiple unit. In many parts of the world, particularly the Far East and Europe, freight trains are composed of wagons or trucks rather than carriages, though some parcel and mail trains are outwardly more like passenger trains. Trains can also be mixed, comprising both passenger accommodation and freight vehicles, special trains are also used for track maintenance, in some places, this is called maintenance of way. A train with a locomotive attached at each end is described as top and tailed, where a second locomotive is attached temporarily to assist a train up steep banks or grades it is referred to as banking in the UK, or helper service in North America. Recently, many loaded trains in the United States have been made up one or more locomotives in the middle or at the rear of the train
14.
Marine chronometer
–
A marine chronometer is a timepiece that is precise and accurate enough to be used as a portable time standard, it can therefore be used to determine longitude by means of celestial navigation. Timepieces made in Switzerland may display the word chronometer only if certified by the COSC, to determine a position on the Earths surface, it is necessary and sufficient to know the latitude, longitude, and altitude. Altitude considerations can, of course, be ignored for vessels operating at sea level, until the mid-1750s, accurate navigation at sea out of sight of land was an unsolved problem due to the difficulty in calculating longitude. Navigators could determine their latitude by measuring the angle at noon or, in the Northern Hemisphere. To find their longitude, however, they needed a standard that would work aboard a ship. Observation of regular celestial motions, such as Galileos method based on observing Jupiters natural satellites, was not possible at sea due to the ships motion. The lunar distances method, initially proposed by Johannes Werner in 1514, was developed in parallel with the marine chronometer, the Dutch scientist Gemma Frisius was the first to propose the use of a chronometer to determine longitude in 1530. The purpose of a chronometer is to measure accurately the time of a fixed location. This is particularly important for navigation, knowing GMT at local noon allows a navigator to use the time difference between the ships position and the Greenwich Meridian to determine the ships longitude. The creation of a timepiece which would work reliably at sea was difficult, christiaan Huygens, following his invention of the pendulum clock in 1656, made the first attempt at a marine chronometer in 1673 in France, under the sponsorship of Jean-Baptiste Colbert. He obtained a patent for his invention from Colbert, but his clock remained imprecise at sea, the first published use of the term was in 1684 in Arcanum Navarchicum, a theoretical work by Kiel professor Matthias Wasmuth. This was followed by a theoretical description of a chronometer in works published by English scientist William Derham in 1713. Attempts to construct a marine chronometer were begun by Jeremy Thacker in England in 1714. In 1714, the British government offered a prize for a method of determining longitude at sea. His first two sea timepieces H1 and H2 used this system, but he realised that they had a sensitivity to centrifugal force. However, H3s circular balances still proved too inaccurate and he abandoned the large machines. Harrison solved the problems with his much smaller H4 chronometer design in 1761. H4 looked much like a large five-inch diameter pocket watch, in 1761, Harrison submitted H4 for the £20,000 longitude prize
15.
Telegraphy
–
Telegraphy is the long-distance transmission of textual or symbolic messages without the physical exchange of an object bearing the message. Thus semaphore is a method of telegraphy, whereas pigeon post is not, telegraphy requires that the method used for encoding the message be known to both sender and receiver. Such methods are designed according to the limits of the medium used. The use of signals, beacons, reflected light signals. In the 19th century, the harnessing of electricity led to the invention of electrical telegraphy, the advent of radio in the early 20th century brought about radiotelegraphy and other forms of wireless telegraphy. The word telegraph was first coined by the French inventor of the Semaphore line, Claude Chappe, a telegraph is a device for transmitting and receiving messages over long distances, i. e. for telegraphy. The word telegraph alone now generally refers to an electrical telegraph, Wireless telegraphy is also known as CW, for continuous wave, as opposed to the earlier radio technique of using a spark gap. Contrary to the definition used by Chappe, Morse argued that the term telegraph can strictly be applied only to systems that transmit. This is to be distinguished from semaphore, which transmits messages. Smoke signals, for instance, are to be considered semaphore, according to Morse, telegraph dates only from 1832 when Pavel Schilling invented one of the earliest electrical telegraphs. A telegraph message sent by a telegraph operator or telegrapher using Morse code was known as a telegram. A cablegram was a sent by a submarine telegraph cable. Later, a Telex was a sent by a Telex network. A wire picture or wire photo was a picture that was sent from a remote location by a facsimile telegraph. A diplomatic telegram, also known as a cable, is the term given to a confidential communication between a diplomatic mission and the foreign ministry of its parent country. These continue to be called telegrams or cables regardless of the used for transmission. Commercial electrical telegraphs were introduced from 1837, the first telegraphs came in the form of optical telegraph, including the use of smoke signals, beacons, or reflected light, which have existed since ancient times. Early proposals for a telegraph system were made to the Royal Society by Robert Hooke in 1684 and were first implemented on an experimental level by Sir Richard Lovell Edgeworth in 1767
16.
Sandford Fleming
–
Sir Sandford Fleming, KCMG was a Scottish Canadian engineer and inventor. Born and raised in Scotland, he emigrated to colonial Canada at the age of 18, in 1827, Fleming was born in Kirkcaldy, Fife, Scotland to Andrew and Elizabeth Fleming. At the age of 14 he was apprenticed as a surveyor and in 1845, at the age of 18, he emigrated with his older brother David to colonial Canada. Their route took them through many cities of the Canadian colonies, Quebec City, Montreal and he qualified as a surveyor in Canada in 1849. In 1849 he created the Royal Canadian Institute with several friends, although initially intended as a professional institute for surveyors and engineers it became a more general scientific society. In 1851 he designed the Threepenny Beaver, the first Canadian postage stamp, throughout this time he was fully employed as a surveyor, mostly for the Grand Trunk Railway. Fleming served in the 10th Battalion Volunteer Rifles of Canada and was appointed to the rank of captain on January 1,1862 and he retired from the militia in 1865. As soon as he arrived in Peterborough, Ontario in 1845, Fleming became friendly with the family of his wife, the Halls. However, it was not until an accident almost ten years later that the young people’s love for each other was revealed. A year after this incident, in January 1855, Sandford married Ann Jane Hall and they were to have nine children of whom two died young. The oldest son, Frank Andrew, accompanied Fleming in his great Western expedition of 1872, the Fleming and Hall families saw each other often. Starting as assistant engineer in 1852, Fleming replaced Cumberland in 1855 but was in turn ousted by him in 1862, in 1863 he became the chief government surveyor of Nova Scotia charged with the construction of a line from Truro to Pictou. In 1862 he placed before the government a plan for a railway connecting the Atlantic and Pacific oceans. In 1867 he was appointed engineer-in-chief of the Intercolonial Railway which became a federal project and his insistence on building the bridges of iron and stone instead of wood was controversial at the time, but was soon vindicated by their resistance to fire. By 1871, the strategy of a connection was being used to bring British Columbia into federation. One of his companions, George Monro Grant wrote an account of the trip, which became a best-seller. By 1880, with 600 miles completed, a change of government brought a desire for a company to own the whole project and Fleming was dismissed by Sir Charles Tupper. It was the hardest blow of Flemings life, though he obtained a promise of monopoly, later revoked, on his next project, nevertheless, in 1884 he became a director of the Canadian Pacific Railway and was present as the last spike was driven
17.
Royal Observatory, Greenwich
–
The Royal Observatory, Greenwich is an observatory situated on a hill in Greenwich Park, overlooking the River Thames. It played a role in the history of astronomy and navigation. The observatory was commissioned in 1675 by King Charles II, with the stone being laid on 10 August. The site was chosen by Sir Christopher Wren and he appointed John Flamsteed as the first Astronomer Royal. The building was completed in the summer of 1676, the building was often called Flamsteed House, in reference to its first occupant. The scientific work of the observatory was relocated elsewhere in stages in the first half of the 20th century,1675 –22 June, Royal Observatory founded. 1675 –10 August, construction began,1714 Longitude Act established the Board of Longitude and Longitude rewards. The Astronomer Royal was, until the Board was dissolved in 1828,1767 Astronomer Royal Nevil Maskelyne began publication of the Nautical Almanac, based on observations made at the Observatory. 1833 Daily time signals began, marked by dropping a Time ball,1899 The New Physical Observatory was completed. 1924 Hourly time signals from the Royal Observatory were first broadcast on 5 February,1948 Office of the Astronomer Royal was moved to Herstmonceux. 1957 Royal Observatory completed its move to Herstmonceux, becoming the Royal Greenwich Observatory, the Greenwich site is renamed the Old Royal Observatory. Greenwich site is returned to its name, the Royal Observatory. The Ordnance Office was given responsibility for building the Observatory, with Moore providing the key instruments, Moore donated two clocks, built by Thomas Tompion, which were installed in the 20 foot high Octagon Room, the principal room of the building. They were of unusual design, each with a pendulum 13 feet in length mounted above the face, giving a period of four seconds. British astronomers have used the Royal Observatory as a basis for measurement. Four separate meridians have passed through the buildings, defined by successive instruments, subsequently, nations across the world used it as their standard for mapping and timekeeping. When the Airy circle became the reference for the meridian, the difference resulting from the change was considered enough to be neglected. When a new triangulation was done between 1936 and 1962, scientists determined that in the Ordnance Survey system the longitude of the international Greenwich meridian was not 0° and this old astronomical prime meridian has been replaced by a more precise prime meridian
18.
International Meridian Conference
–
The International Meridian Conference was a conference held in October 1884 in Washington, D. C. in the United States, to determine a prime meridian for international use. The conference was held at the request of U. S. President Chester A. Arthur, the subject to discuss was the choice of a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world. It resulted in selection of the Greenwich Meridian as the standard for zero degrees longitude. By the 1870s there was both to establish a prime meridian for worldwide navigation purposes and to unify local times for railway timetables. In Britain, the Great Western Railway had standardised time by 1840, the Post Office was by this time transmitting time signals from Greenwich by telegraph to most parts of the country to set the clocks. In the United States, the problems were more severe. This envisaged clocks showing 24-hour universal time with an extra dial having a local time rounded to the nearest hour and he also pointed out that many of the corrections for local mean time were greater than those involved in abandoning solar time. In 1878/9, he produced modified proposals using the Greenwich meridian, flemings two papers were considered so important that in June 1879 the British Government forwarded copies to eighteen foreign countries and to various scientific bodies in England. At the same time the American Metrological Society produced a Report on Standard Time by Cleveland Abbe, the Naval Observatory had also foiled the onward transmission of the Greenwich time signal from Harvard, where it was received via transatlantic cable and used to time a signal ball in Boston. The Seventh International Geodesic Conference in Rome in October 1883 then thrashed out most of the technical details and that from this meridian longitude shall be counted in two directions up to 180 degrees, east longitude being plus and west longitude minus. That the Conference expresses the hope that as soon as may be practicable the astronomical and nautical days will be arranged everywhere to begin at midnight, resolution 2, fixing the meridian at Greenwich, was passed 22–1, France and Brazil abstained. The French did not adopt the Greenwich meridian until 1911, resolution 4 expressly exempts standard time from the universal day. Thus the conference did not adopt any time zones, contrary to popular belief, regarding resolution 6, Great Britain had already shifted the beginning of the nautical day from noon, twelve hours before midnight, to midnight in 1805, during the Battle of Trafalgar. The astronomical day was shifted from noon, twelve hours after midnight, the following resolutions were not adopted, On 13 Oct, Resolved, That the initial meridian should have a character of absolute neutrality. It should be chosen exclusively so as to secure to science and to international commerce all possible advantages, - Twenty-one noes and three ayes. - France, Brazil, San Domingo voted in favor, in the end, the pragmatic argument for continuity with most nautical charts won the day and the French delegation abstained in the vote. There was discussion of setting zones as small as 10 minutes, most European countries aligned their clocks with Greenwich within ten years, Sweden and North America already having done so, and the trend continued. Greenwich Time and the discovery of the longitude, Proceedings of the 1884 International Meridian Conference - notes with links to scanned page images at Steve Allens site Fleming, Sandford
19.
Nautical chart
–
A nautical chart is a graphic representation of a maritime area and adjacent coastal regions. Nautical charts are essential tools for marine navigation, many countries require vessels, especially commercial ships, Nautical charting may take the form of charts printed on paper or computerized electronic navigational charts. With each daily download, critical data such as Local Notice to Mariners is added to the on-demand chart files so that these charts will be up to date at the time of printing, Nautical charts are based on hydrographic surveys. As surveying is laborious and time-consuming, hydrographic data for many areas of sea may be dated, depths are measured in a variety of ways. Historically the sounding line was used, in modern times, echo sounding is used for measuring the seabed in the open sea. When measuring the depth of water over an entire obstruction, such as a shipwreck. This ensures that difficult to find projections, such as masts, Nautical charts are issued by power of the national hydrographic offices in many countries. These charts are considered official in contrast to those made by commercial publishers, many hydrographic offices provide regular, sometimes weekly, manual updates of their charts through their sales agents. Individual hydrographic offices produce national chart series and international chart series, there are also commercially published charts, some of which may carry additional information of particular interest, e. g. for yacht skippers. The nature of a waterway depicted by a chart may change, therefore, old or uncorrected charts should never be used for navigation. Every producer of nautical charts also provides a system to inform mariners of changes that affect the chart. In the U. S. NOAA also has a partner who prints the POD NOAA charts. To give notice to mariners, radio broadcasts provide advance notice of urgent corrections, a good way to keep track of corrections is with a Chart and Publication Correction Record Card system. When the time comes to use the chart, he pulls the chart and charts card and this system ensures that every chart is properly corrected prior to use. A prudent mariner should obtain a new chart if he has not kept track of corrections, various Digital Notices to Mariners systems are available on the market such as Digitrace, Voyager, or ChartCo, to correct British Admiralty charts as well as NOAA charts. These systems provide only vessel relevant corrections via e-mail or web downloads, tracings to assist corrections are provided at the same time. The Canadian Coast Guard produces the Notice to Mariners publication which informs mariners of important navigational safety matters affecting Canadian Waters and this electronic publication is published on a monthly basis and can be downloaded from the Notices to Mariners Web site. The information in the Notice to Mariners is formatted to simplify the correction of paper charts, various and diverse methods exist for the correction of electronic navigational charts
20.
Map
–
A map is a symbolic depiction emphasizing relationships between elements of some space, such as objects, regions, or themes. Many maps are static, fixed to paper or some other durable medium, although the earliest maps known are of the heavens, geographic maps of territory have a very long tradition and exist from ancient times. The word map comes from the medieval Latin Mappa mundi, wherein mappa meant napkin or cloth, thus, map became the shortened term referring to a two-dimensional representation of the surface of the world. Cartography or map-making is the study and practice of crafting representations of the Earth upon a flat surface, in addition to location information maps may also be used to portray contour lines indicating constant values of elevation, temperature, rainfall, etc. The orientation of a map is the relationship between the directions on the map and the compass directions in reality. The word orient is derived from Latin oriens, meaning East, in the Middle Ages many maps, including the T and O maps, were drawn with East at the top. Today, the most common – but far from universal – cartographic convention is that North is at the top of a map, several kinds of maps are often traditionally not oriented with North at the top, Maps from non-Western traditions are oriented a variety of ways. Old maps of Edo show the Japanese imperial palace as the top, labels on the map are oriented in such a way that you cannot read them properly unless you put the imperial palace above your head. Medieval European T and O maps such as the Hereford Mappa Mundi were centred on Jerusalem with East at the top, indeed, prior to the reintroduction of Ptolemys Geography to Europe around 1400, there was no single convention in the West. Portolan charts, for example, are oriented to the shores they describe, Maps of cities bordering a sea are often conventionally oriented with the sea at the top. Route and channel maps have traditionally been oriented to the road or waterway they describe, polar maps of the Arctic or Antarctic regions are conventionally centred on the pole, the direction North would be towards or away from the centre of the map, respectively. Typical maps of the Arctic have 0° meridian towards the bottom of the page, reversed maps, also known as Upside-Down maps or South-Up maps, reverse the North is up convention and have south at the top. Buckminster Fullers Dymaxion maps are based on a projection of the Earths sphere onto an icosahedron, the resulting triangular pieces may be arranged in any order or orientation. Modern digital GIS maps such as ArcMap typically project north at the top of the map, compass decimal degrees can be converted to math degrees by subtracting them from 450, if the answer is greater than 360, subtract 360. The scale statement may be taken as exact when the region mapped is small enough for the curvature of the Earth to be neglected, over larger regions where the curvature cannot be ignored we must use map projections from the curved surface of the Earth to the plane. Thus for map projections we must introduce the concept of point scale, which is a function of position, although the scale statement is nominal it is usually accurate enough for all but the most precise of measurements. Large scale maps, say 1,10,000, cover relatively small regions in detail and small scale maps, say 1,10,000,000, cover large regions such as nations, continents. The large/small terminology arose from the practice of writing scales as numerical fractions, there is no exact dividing line between large and small but 1/100,000 might well be considered as a medium scale
21.
International Astronomical Union
–
The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the recognized authority for assigning designations to celestial bodies. The IAU is a member of the International Council for Science and its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation. The IAU maintains friendly relations with organizations that include amateur astronomers in their membership, the IAU has its head office on the second floor of the Institut dAstrophysique de Paris in the 14th arrondissement of Paris. The IAU is also responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams, the Minor Planet Center also operates under the IAU, and is a clearinghouse for all non-planetary or non-moon bodies in the Solar System. The Working Group for Meteor Shower Nomenclature and the Meteor Data Center coordinate the nomenclature of meteor showers, the IAU was founded on July 28,1919, at the Constitutive Assembly of the International Research Council held in Brussels, Belgium. The 7 initial member states were Belgium, Canada, France, Great Britain, Greece, Japan, the first executive committee consisted of Benjamin Baillaud, Alfred Fowler, and four vice presidents, William Campbell, Frank Dyson, Georges Lecointe, and Annibale Riccò. Thirty-two Commissions were appointed at the Brussels meeting and focused on topics ranging from relativity to minor planets, the reports of these 32 Commissions formed the main substance of the first General Assembly, which took place in Rome, Italy, May 2–10,1922. By the end of the first General Assembly, ten nations had joined the Union. Although the Union was officially formed eight months after the end of World War I, the first 50 years of the Unions history are well documented. Subsequent history is recorded in the form of reminiscences of past IAU Presidents, twelve of the fourteen past General Secretaries in the period 1964-2006 contributed their recollections of the Unions history in IAU Information Bulletin No.100. Six past IAU Presidents in the period 1976–2003 also contributed their recollections in IAU Information Bulletin No.104, the IAU includes a total of 12,664 individual members who are professional astronomers from 96 countries worldwide. 83% of all members are male, while 17% are female, among them the unions current president. Membership also includes 79 national members, professional astronomical communities representing their countrys affiliation with the IAU, the sovereign body of the IAU is its General Assembly, which comprises all members. The Assembly determines IAU policy, approves the Statutes and By-Laws of the Union, the right to vote on matters brought before the Assembly varies according to the type of business under discussion. On budget matters, votes are weighted according to the subscription levels of the national members. A second category vote requires a turnout of at least two-thirds of national members in order to be valid, an absolute majority is sufficient for approval in any vote, except for Statute revision which requires a two-thirds majority. An equality of votes is resolved by the vote of the President of the Union, since 1922, the IAU General Assembly meets every three years, with the exception of the period between 1938 and 1948, due to World War II
22.
Civil time
–
In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. UTC is calculated by reference to atomic clocks, and was adopted in 1972, in traditional astronomical usage, civil time was mean solar time reckoned from midnight. Before 1925, the astronomical time 00,00,00 meant noon, in 1928, the International Astronomical Union introduced the term Universal Time for GMT beginning at midnight, but the two Nautical Almanac Offices did not accept it until 1952. Category, Time by country Category, Time zones Time Scales
23.
Sun
–
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
24.
Very-long-baseline interferometry
–
Very-long-baseline interferometry is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from a radio source, such as a quasar, is collected at multiple radio telescopes on Earth. The distance between the telescopes is then calculated using the time difference between the arrivals of the radio signal at different telescopes. This allows observations of an object that are made simultaneously by many radio telescopes to be combined, Data received at each antenna in the array include arrival times from a local atomic clock, such as a hydrogen maser. At a later time, the data are correlated with data from other antennas that recorded the same radio signal, the resolution achievable using interferometry is proportional to the observing frequency. VLBI is most well known for imaging distant cosmic radio sources, spacecraft tracking, using VLBI in this manner requires large numbers of time difference measurements from distant sources observed with a global network of antennas over a period of time. Some of the results derived from VLBI include, High resolution radio imaging of cosmic radio sources. Variations in the Earths orientation and length of day, the most sensitive VLBI array in the world is the European VLBI Network. This is an array which brings together the largest European radiotelescopes for typically week-long sessions. The combination of the EVN and VLBA is known as Global VLBI, VLBI has traditionally operated by recording the signal at each telescope on magnetic tapes or disks, and shipping those to the correlation center for replay. Recently, it has become possible to connect VLBI radio telescopes in close to real-time, while employing the local time references of the VLBI technique. The image to the shows the first science produced by the European VLBI Network using e-VLBI. The data from 6 telescopes were processed in time at the European Data Processing centre at JIVE. The Netherlands Academic Research Network SURFnet provides 6 x 1 Gbit/s connectivity between JIVE and the GEANT2 network, in the quest for even greater angular resolution, dedicated VLBI satellites have been placed in Earth orbit to provide greatly extended baselines. Experiments incorporating such space-borne array elements are termed Space Very Long Baseline Interferometry, another space VLBI mission, Spektr-R, was launched in July 2011. In VLBI interferometry, the digitized antenna data are recorded at each of the telescopes. The antenna signal is sampled with a precise and stable atomic clock that is additionally locked onto a GPS time standard. Alongside the astronomical data samples, the output of this clock is recorded on the tape/disk media, the recorded media are then transported to a central location
25.
International Telecommunication Union
–
ITU, based in Geneva, Switzerland, is a member of the United Nations Development Group. ITU has been an intergovernmental public-private partnership organization since its inception, ITU was formed in 1865, in Paris, at the International Telegraph Convention, this makes it one of the oldest intergovernmental organizations in the world. ITU became a United Nations specialized agency in 1947, the ITU comprises three sectors, each managing a different aspect of the matters handled by the Union, as well as ITU Telecom. The sectors were created during the restructuring of ITU at its 1992 Plenipotentiary Conference, radiocommunication Established in 1927 as the International Radio Consultative Committee or CCIR, this sector manages the international radio-frequency spectrum and satellite orbit resources. In 1992, the CCIR became the ITU-R, Standardization Standardization was the original purpose of ITU since its inception. Established in 1956 as the International Telephone and Telegraph Consultative Committee or CCITT, in 1993, the CCITT became the ITU-T. Development Established in 1992, this sector helps spread equitable, sustainable and affordable access to information and communication technologies, ITU Telecom ITU Telecom organizes major events for the worlds ICT community. A permanent General Secretariat, headed by the Secretary General, manages the work of the Union. The basic texts of the ITU are adopted by the ITU Plenipotentiary Conference, the ITU is headed by a Secretary-General, who is elected to a four-year term by the member states at the ITU Plenipotentiary Conference. On 23 October 2014 Houlin Zhao was elected 19th Secretary-General of the ITU at the Plenipotentiary Conference in Busan and his four-year mandate started on 1 January 2015, and he was formally inaugurated on 15 January 2015. There are 193 Member States of the ITU, which are all UN member states, the most recent member state to join the ITU is South Sudan, which became a member on 14 July 2011. The Republic of China was blocked from membership by the Peoples Republic of China, palestine was admitted as an observer in 2010. The Summit was held as two conferences in 2003 and 2005 in Geneva and Tunis, respectively, with the aim of bridging the digital divide, in December 2012, the ITU facilitated The World Conference on International Telecommunications 2012 in Dubai. WCIT-12 was a conference to address International Telecommunications Regulations, the international rules for telecommunications. The previous conference to update the Regulations was held in Melbourne in 1988, in August 2012, ITU called for a public consultation on a draft document ahead of the conference. Telecommunications ministers from 193 countries attended the conference in Dubai, the current regulatory structure was based on voice telecommunications, when the Internet was still in its infancy. In 1988, telecommunications operated under regulated monopolies in most countries, as the Internet has grown, organizations such as ICANN have come into existence to manage key resources such as Internet addresses and Domain Names. Some outside the United States believe that the United States exerts too much influence over the governance of the Internet, current proposals look to take into account the prevalence of data communications
26.
Tidal acceleration
–
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite, and the primary planet that it orbits. The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, the process eventually leads to tidal locking, usually of the smaller first, and later the larger body. The Earth–Moon system is the best studied case, the similar process of tidal deceleration occurs for satellites that have an orbital period that is shorter than the primarys rotational period, or that orbit in a retrograde direction. The naming is somewhat confusing, because the speed of the relative to the body it orbits is decreased as a result of tidal acceleration. Laplaces initial computation accounted for the effect, thus seeming to tie up the theory neatly with both modern and ancient observations. It took some time for the community to accept the reality. But eventually it became clear that three effects are involved, when measured in terms of solar time. Beside the effects of changes in Earths orbital eccentricity, as found by Laplace and corrected by Adams. First there is a real retardation of the Moons angular rate of orbital motion and this increases the Moons angular momentum around Earth. Secondly there is an apparent increase in the Moons angular rate of orbital motion and this arises from Earths loss of angular momentum and the consequent increase in length of day. Because the Moons mass is a fraction of that of Earth. The mass of the Moon is sufficiently large, and it is sufficiently close, in particular, the water of the oceans bulges out towards and away from the Moon. The average tidal bulge is synchronized with the Moons orbit, however, Earths rotation drags the position of the tidal bulge ahead of the position directly under the Moon. As a consequence, there exists a substantial amount of mass in the bulge that is offset from the line through the centers of Earth and the Moon. Because of this offset, a portion of the pull between Earths tidal bulges and the Moon is not perpendicular to the Earth–Moon line, i. e. there exists a torque between Earth and the Moon. This boosts the Moon in its orbit, and slows the rotation of Earth, as a result of this process, the mean solar day, which is nominally 86,400 seconds long, is actually getting longer when measured in SI seconds with stable atomic clocks. The small difference accumulates over time, which leads to a difference between our clock time on the one hand, and Atomic Time and Ephemeris Time on the other hand. This led to the introduction of the second in 1972 to compensate for differences in the bases for time standardization
27.
Moon
–
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
28.
Mean solar day
–
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of time is the day. Two types of time are apparent solar time and mean solar time. Fix a tall pole vertically in the ground, at some instant on any day the shadow will point exactly north or south. That instant is local apparent noon,12,00 local apparent time, about 24 hours later the shadow will again point north/south, the Sun seeming to have covered a 360-degree arc around the Earths axis. When the Sun has covered exactly 15 degrees, local apparent time is 13,00 exactly, after 15 more degrees it will be 14,00 exactly. As explained in the equation of time article, this is due to the eccentricity of the Earths orbit, the effect of this is that a clock running at a constant rate – e. g. This is mean solar time, which is not perfectly constant from one century to the next but is close enough for most purposes. Currently a mean solar day is about 86,400.002 SI seconds, the two kinds of solar time are among the three kinds of time reckoning that were employed by astronomers until the 1950s. By the 1950s it had become clear that the Earths rotation rate was not constant, so astronomers developed ephemeris time, the apparent sun is the true sun as seen by an observer on Earth. Apparent solar time or true solar time is based on the apparent motion of the actual Sun and it is based on the apparent solar day, the interval between two successive returns of the Sun to the local meridian. Solar time can be measured by a sundial. The equivalent on other planets is termed local true solar time, the length of a solar day varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean. The effect has two main causes, first, Earths orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun and slower when it is farthest from the Sun. Second, due to Earths axial tilt, the Suns annual motion is along a circle that is tilted to Earths celestial equator. In June and December when the sun is farthest from the equator a given shift along the ecliptic corresponds to a large shift at the equator. So apparent solar days are shorter in March and September than in June or December and these lengths will change slightly in a few years and significantly in thousands of years. Mean solar time is the angle of the mean Sun plus 12 hours
29.
International System of Units
–
The International System of Units is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, the system also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system was published in 1960 as the result of an initiative began in 1948. It is based on the system of units rather than any variant of the centimetre-gram-second system. The motivation for the development of the SI was the diversity of units that had sprung up within the CGS systems, the International System of Units has been adopted by most developed countries, however, the adoption has not been universal in all English-speaking countries. The metric system was first implemented during the French Revolution with just the metre and kilogram as standards of length, in the 1830s Carl Friedrich Gauss laid the foundations for a coherent system based on length, mass, and time. In the 1860s a group working under the auspices of the British Association for the Advancement of Science formulated the requirement for a coherent system of units with base units and derived units. Meanwhile, in 1875, the Treaty of the Metre passed responsibility for verification of the kilogram, in 1921, the Treaty was extended to include all physical quantities including electrical units originally defined in 1893. The units associated with these quantities were the metre, kilogram, second, ampere, kelvin, in 1971, a seventh base quantity, amount of substance represented by the mole, was added to the definition of SI. On 11 July 1792, the proposed the names metre, are, litre and grave for the units of length, area, capacity. The committee also proposed that multiples and submultiples of these units were to be denoted by decimal-based prefixes such as centi for a hundredth, on 10 December 1799, the law by which the metric system was to be definitively adopted in France was passed. Prior to this, the strength of the magnetic field had only been described in relative terms. The technique used by Gauss was to equate the torque induced on a magnet of known mass by the earth’s magnetic field with the torque induced on an equivalent system under gravity. The resultant calculations enabled him to assign dimensions based on mass, length, a French-inspired initiative for international cooperation in metrology led to the signing in 1875 of the Metre Convention. Initially the convention only covered standards for the metre and the kilogram, one of each was selected at random to become the International prototype metre and International prototype kilogram that replaced the mètre des Archives and kilogramme des Archives respectively. Each member state was entitled to one of each of the prototypes to serve as the national prototype for that country. Initially its prime purpose was a periodic recalibration of national prototype metres. The official language of the Metre Convention is French and the version of all official documents published by or on behalf of the CGPM is the French-language version
30.
Planet
–
The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye and these were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, in 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of mass based on where or what they orbit. The planets were thought by Ptolemy to orbit Earth in deferent, at about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler found the planets orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the planets share characteristics such as volcanism, hurricanes, tectonics. Planets are generally divided into two types, large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System, in order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, several thousands of planets around other stars have been discovered in the Milky Way. e. in the habitable zone. On December 20,2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way, around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include not only in the Solar System. The ambiguities inherent in defining planets have led to much scientific controversy, the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the fixed stars, ancient Greeks called these lights πλάνητες ἀστέρες or simply πλανῆται, from which todays word planet was derived. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the planets circled Earth. The first civilization known to have a theory of the planets were the Babylonians
31.
Radar
–
Radar is an object-detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the objects location and speed. Radar was developed secretly for military use by several nations in the period before, the term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging or RAdio Direction And Ranging. The term radar has since entered English and other languages as a common noun, high tech radar systems are associated with digital signal processing, machine learning and are capable of extracting useful information from very high noise levels. Other systems similar to make use of other parts of the electromagnetic spectrum. One example is lidar, which uses ultraviolet, visible, or near infrared light from lasers rather than radio waves, as early as 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov, an instructor at the Imperial Russian Navy school in Kronstadt. The next year, he added a spark-gap transmitter, in 1897, while testing this equipment for communicating between two ships in the Baltic Sea, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, the German inventor Christian Hülsmeyer was the first to use radio waves to detect the presence of distant metallic objects. In 1904, he demonstrated the feasibility of detecting a ship in dense fog and he obtained a patent for his detection device in April 1904 and later a patent for a related amendment for estimating the distance to the ship. He also got a British patent on September 23,1904 for a radar system. It operated on a 50 cm wavelength and the radar signal was created via a spark-gap. In 1915, Robert Watson-Watt used radio technology to advance warning to airmen. Watson-Watt became an expert on the use of direction finding as part of his lightning experiments. As part of ongoing experiments, he asked the new boy, Arnold Frederic Wilkins, Wilkins made an extensive study of available units before selecting a receiver model from the General Post Office. Its instruction manual noted that there was fading when aircraft flew by, in 1922, A. Hoyt Taylor and Leo C. Taylor submitted a report, suggesting that this might be used to detect the presence of ships in low visibility, eight years later, Lawrence A. Australia, Canada, New Zealand, and South Africa followed prewar Great Britain, and Hungary had similar developments during the war. Hugon, began developing a radio apparatus, a part of which was installed on the liner Normandie in 1935
32.
Newton's laws of motion
–
Newtons laws of motion are three physical laws that, together, laid the foundation for classical mechanics. They describe the relationship between a body and the forces acting upon it, and its motion in response to those forces. More precisely, the first law defines the force qualitatively, the second law offers a measure of the force. These three laws have been expressed in different ways, over nearly three centuries, and can be summarised as follows. The three laws of motion were first compiled by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica, Newton used them to explain and investigate the motion of many physical objects and systems. For example, in the volume of the text, Newton showed that these laws of motion, combined with his law of universal gravitation. Newtons laws are applied to objects which are idealised as single point masses, in the sense that the size and this can be done when the object is small compared to the distances involved in its analysis, or the deformation and rotation of the body are of no importance. In this way, even a planet can be idealised as a particle for analysis of its orbital motion around a star, in their original form, Newtons laws of motion are not adequate to characterise the motion of rigid bodies and deformable bodies. Leonhard Euler in 1750 introduced a generalisation of Newtons laws of motion for rigid bodies called Eulers laws of motion, if a body is represented as an assemblage of discrete particles, each governed by Newtons laws of motion, then Eulers laws can be derived from Newtons laws. Eulers laws can, however, be taken as axioms describing the laws of motion for extended bodies, Newtons laws hold only with respect to a certain set of frames of reference called Newtonian or inertial reference frames. Other authors do treat the first law as a corollary of the second, the explicit concept of an inertial frame of reference was not developed until long after Newtons death. In the given mass, acceleration, momentum, and force are assumed to be externally defined quantities. This is the most common, but not the interpretation of the way one can consider the laws to be a definition of these quantities. Newtonian mechanics has been superseded by special relativity, but it is useful as an approximation when the speeds involved are much slower than the speed of light. The first law states that if the net force is zero, the first law can be stated mathematically when the mass is a non-zero constant, as, ∑ F =0 ⇔ d v d t =0. Consequently, An object that is at rest will stay at rest unless a force acts upon it, an object that is in motion will not change its velocity unless a force acts upon it. This is known as uniform motion, an object continues to do whatever it happens to be doing unless a force is exerted upon it. If it is at rest, it continues in a state of rest, if an object is moving, it continues to move without turning or changing its speed
33.
General relativity
–
General relativity is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. General relativity generalizes special relativity and Newtons law of gravitation, providing a unified description of gravity as a geometric property of space and time. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter, the relation is specified by the Einstein field equations, a system of partial differential equations. Examples of such differences include gravitational time dilation, gravitational lensing, the redshift of light. The predictions of relativity have been confirmed in all observations. Although general relativity is not the only theory of gravity. Einsteins theory has important astrophysical implications, for example, it implies the existence of black holes—regions of space in which space and time are distorted in such a way that nothing, not even light, can escape—as an end-state for massive stars. The bending of light by gravity can lead to the phenomenon of gravitational lensing, General relativity also predicts the existence of gravitational waves, which have since been observed directly by physics collaboration LIGO. In addition, general relativity is the basis of current cosmological models of an expanding universe. Soon after publishing the special theory of relativity in 1905, Einstein started thinking about how to incorporate gravity into his new relativistic framework. In 1907, beginning with a thought experiment involving an observer in free fall. After numerous detours and false starts, his work culminated in the presentation to the Prussian Academy of Science in November 1915 of what are now known as the Einstein field equations. These equations specify how the geometry of space and time is influenced by whatever matter and radiation are present, the Einstein field equations are nonlinear and very difficult to solve. Einstein used approximation methods in working out initial predictions of the theory, but as early as 1916, the astrophysicist Karl Schwarzschild found the first non-trivial exact solution to the Einstein field equations, the Schwarzschild metric. This solution laid the groundwork for the description of the stages of gravitational collapse. In 1917, Einstein applied his theory to the universe as a whole, in line with contemporary thinking, he assumed a static universe, adding a new parameter to his original field equations—the cosmological constant—to match that observational presumption. By 1929, however, the work of Hubble and others had shown that our universe is expanding and this is readily described by the expanding cosmological solutions found by Friedmann in 1922, which do not require a cosmological constant. Lemaître used these solutions to formulate the earliest version of the Big Bang models, in which our universe has evolved from an extremely hot, Einstein later declared the cosmological constant the biggest blunder of his life
34.
Polar motion
–
Polar motion of the Earth is the motion of the Earths rotational axis relative to its crust. This is measured with respect to a frame in which the solid Earth is fixed. This variation is only a few meters, polar motion is defined relative to a conventionally defined reference axis, the CIO, being the poles average location over the year 1900. The mean displacement far exceeds the magnitude of the wobbles, the latter are determined based on the International Terrestrial Reference System, which follows the polar motion. e. The slow rise of land that was formerly burdened with ice sheets or glaciers, the drift is roughly along the 80th meridian west. However, since about year 2000, the pole has found new direction of drift and this dramatic eastward shift in drift direction is attributed to the global scale mass transport between the oceans and the continents. Major earthquakes cause abrupt polar motion by altering the distribution of the Earths solid mass. These shifts, however, are small in magnitude relative to the long-term core/mantle. In the absence of external torques, the vector of the angular momentum M of a system remains constant and is directed toward a fixed point in space. In the case of the Earth, it is almost identical with its axis of rotation, the vector of the figure axis F of the system wobbles around M. This motion is called Eulers free nutation, for a rigid Earth which is an oblate spheroid to a good approximation, the figure axis F is its geometric axis defined by the geographic north and south pole. It is identical with the axis of its moment of inertia. The observed angle between M and F is a few hundred milliarcseconds which gives rise to a displacement of several meters between the figure axis of the Earth and its angular momentum. This is the polar motion. This observed free nutation is called Chandler wobble, there exist, in addition, polar motions with smaller periods of the order of decades. Finally, a secular polar drift of about 0, the annual variation was discovered by Karl Friedrich Küstner in 1885 by exact measurements of the variation of the latitude of stars, while S. C. Chandler found the free nutation in 1891. Both periods superpose, giving rise to a frequency with a period of about 5 to 8 years. They are also called nutations, except for the slowest, which is the precession of the equinoxes, polar motion is observed routinely by very-long-baseline interferometry, lunar laser ranging and satellite laser ranging
35.
Global Positioning System
–
The Global Positioning System is a space-based radionavigation system owned by the United States government and operated by the United States Air Force. The GPS system operates independently of any telephonic or internet reception, the GPS system provides critical positioning capabilities to military, civil, and commercial users around the world. The United States government created the system, maintains it, however, the US government can selectively deny access to the system, as happened to the Indian military in 1999 during the Kargil War. The U. S. Department of Defense developed the system and it became fully operational in 1995. Roger L. Easton of the Naval Research Laboratory, Ivan A, getting of The Aerospace Corporation, and Bradford Parkinson of the Applied Physics Laboratory are credited with inventing it. Announcements from Vice President Al Gore and the White House in 1998 initiated these changes, in 2000, the U. S. Congress authorized the modernization effort, GPS III. In addition to GPS, other systems are in use or under development, mainly because of a denial of access. The Russian Global Navigation Satellite System was developed contemporaneously with GPS, GLONASS can be added to GPS devices, making more satellites available and enabling positions to be fixed more quickly and accurately, to within two meters. There are also the European Union Galileo positioning system and Chinas BeiDou Navigation Satellite System, special and general relativity predict that the clocks on the GPS satellites would be seen by the Earths observers to run 38 microseconds faster per day than the clocks on the Earth. The GPS calculated positions would quickly drift into error, accumulating to 10 kilometers per day, the relativistic time effect of the GPS clocks running faster than the clocks on earth was corrected for in the design of GPS. The Soviet Union launched the first man-made satellite, Sputnik 1, two American physicists, William Guier and George Weiffenbach, at Johns Hopkinss Applied Physics Laboratory, decided to monitor Sputniks radio transmissions. Within hours they realized that, because of the Doppler effect, the Director of the APL gave them access to their UNIVAC to do the heavy calculations required. The next spring, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem — pinpointing the users location and this led them and APL to develop the TRANSIT system. In 1959, ARPA also played a role in TRANSIT, the first satellite navigation system, TRANSIT, used by the United States Navy, was first successfully tested in 1960. It used a constellation of five satellites and could provide a navigational fix approximately once per hour, in 1967, the U. S. Navy developed the Timation satellite, which proved the feasibility of placing accurate clocks in space, a technology required by GPS. In the 1970s, the ground-based OMEGA navigation system, based on comparison of signal transmission from pairs of stations. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy, during the Cold War arms race, the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the United States Congress. This deterrent effect is why GPS was funded and it is also the reason for the ultra secrecy at that time
36.
Greenwich Sidereal Time
–
Sidereal time /saɪˈdɪəriəl/ is a time-keeping system that astronomers use to locate celestial objects. Using sidereal time it is possible to point a telescope to the proper coordinates in the night sky. Briefly, sidereal time is a scale that is based on Earths rate of rotation measured relative to the fixed stars rather than the Sun. From a given point, a star found at one location in the sky will be found at the same location on another night at the same sidereal time. This is similar to how the time kept by a sundial can be used to find the location of the Sun, just as the Sun and Moon appear to rise in the east and set in the west due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earths rotation about its polar axis, a mean sidereal day is 23 hours,56 minutes,4.0916 seconds, the time it takes Earth to make one rotation relative to the vernal equinox. The longer true sidereal period is called a day by the International Earth Rotation. It is also referred to as the period of rotation. Maps of the stars in the night sky use declination and right ascension as coordinates and these correspond to latitude and longitude respectively. In the sky, the meridian is the north to south line that goes through the point directly overhead. Solar time is measured by the apparent diurnal motion of the Sun, a mean solar day is the average time between local solar noons. Earth makes one rotation around its axis in a sidereal day, so after a sidereal day has passed, Earth still needs to rotate slightly more before the Sun reaches local noon according to solar time. A mean solar day is, therefore, nearly 4 minutes longer than a sidereal day, the stars are so far away that Earths movement along its orbit makes nearly no difference to their apparent direction, and so they return to their highest point in a sidereal day. Another way to see this difference is to notice that, relative to the stars, therefore, there is one fewer solar day per year than there are sidereal days. This makes a sidereal day approximately 365. 24/366.24 times the length of the 24-hour solar day, Earths rotation is not a simple rotation around an axis that would always remain parallel to itself. Earths rotational axis itself rotates about an axis, orthogonal to Earths orbit. This phenomenon is called the precession of the equinoxes, because of this precession, the stars appear to move around Earth in a manner more complicated than a simple constant rotation. In this reference frame, Earths rotation is close to constant and it is also in this reference frame that the tropical year, the year related to Earths seasons, represents one orbit of Earth around the Sun
37.
Year
–
A year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earths axial tilt, the course of a year sees the passing of the seasons, marked by changes in weather, the hours of daylight, and, consequently, vegetation and soil fertility. In temperate and subpolar regions around the globe, four seasons are recognized, spring, summer, autumn. In tropical and subtropical regions several geographical sectors do not present defined seasons, but in the seasonal tropics, a calendar year is an approximation of the number of days of the Earths orbital period as counted in a given calendar. The Gregorian, or modern, calendar, presents its calendar year to be either a common year of 365 days or a year of 366 days, as do the Julian calendars. For the Gregorian calendar the average length of the year across the complete leap cycle of 400 years is 365.2425 days. The ISO standard ISO 80000-3, Annex C, supports the symbol a to represent a year of either 365 or 366 days, in English, the abbreviations y and yr are commonly used. In astronomy, the Julian year is a unit of time, it is defined as 365.25 days of exactly 86400 seconds, totalling exactly 31557600 seconds in the Julian astronomical year. The word year is used for periods loosely associated with, but not identical to, the calendar or astronomical year, such as the seasonal year, the fiscal year. Similarly, year can mean the period of any planet, for example. The term can also be used in reference to any long period or cycle, west Saxon ġēar, Anglian ġēr continues Proto-Germanic *jǣran. Cognates are German Jahr, Old High German jār, Old Norse ár and Gothic jer, all the descendants of the Proto-Indo-European noun *yeh₁rom year, season. Cognates also descended from the same Proto-Indo-European noun are Avestan yārǝ year, Greek ὥρα year, season, period of time, Old Church Slavonic jarŭ, Latin annus is from a PIE noun *h₂et-no-, which also yielded Gothic aþn year. Both *yeh₁-ro- and *h₂et-no- are based on verbal roots expressing movement, *h₁ey- and *h₂et- respectively, the Greek word for year, ἔτος, is cognate with Latin vetus old, from the PIE word *wetos- year, also preserved in this meaning in Sanskrit vat-sa- yearling and vat-sa-ras year. Derived from Latin annus are a number of English words, such as annual, annuity, anniversary, etc. per annum means each year, anno Domini means in the year of the Lord. No astronomical year has an number of days or lunar months. Financial and scientific calculations often use a 365-day calendar to simplify daily rates, in the Julian calendar, the average length of a year is 365.25 days. In a non-leap year, there are 365 days, in a year there are 366 days
38.
Accuracy and precision
–
Precision is a description of random errors, a measure of statistical variability. The two concepts are independent of other, so a particular set of data can be said to be either accurate, or precise. In the fields of science, engineering and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantitys true value. The precision of a measurement system, related to reproducibility and repeatability, is the degree to which repeated measurements under unchanged conditions show the same results. Although the two words precision and accuracy can be synonymous in colloquial use, they are contrasted in the context of the scientific method. A measurement system can be accurate but not precise, precise but not accurate, neither, for example, if an experiment contains a systematic error, then increasing the sample size generally increases precision but does not improve accuracy. The result would be a consistent yet inaccurate string of results from the flawed experiment, eliminating the systematic error improves accuracy but does not change precision. A measurement system is considered if it is both accurate and precise. Related terms include bias and error, the terminology is also applied to indirect measurements—that is, values obtained by a computational procedure from observed data. Statistical literature prefers to use the terms bias and variability instead of accuracy and precision, bias is the amount of inaccuracy and variability is the amount of imprecision. In military terms, accuracy refers primarily to the accuracy of fire, ideally a measurement device is both accurate and precise, with measurements all close to and tightly clustered around the true value. The accuracy and precision of a measurement process is established by repeatedly measuring some traceable reference standard. Such standards are defined in the International System of Units and maintained by national organizations such as the National Institute of Standards. This also applies when measurements are repeated and averaged, further, the central limit theorem shows that the probability distribution of the averaged measurements will be closer to a normal distribution than that of individual measurements. With regard to accuracy we can distinguish, the difference between the mean of the measurements and the value, the bias. Establishing and correcting for bias is necessary for calibration, the combined effect of that and precision. A common convention in science and engineering is to express accuracy and/or precision implicitly by means of significant figures, here, when not explicitly stated, the margin of error is understood to be one-half the value of the last significant place. For instance, a recording of 843.6 m, or 843.0 m, or 800.0 m would imply a margin of 0.05 m, while a recording of 8,436 m would imply a margin of error of 0.5 m
39.
DUT1
–
The time correction DUT1 is the difference between Universal Time, which is defined by Earths rotation, and Coordinated Universal Time, which is defined by a network of precision atomic clocks. DUT1 = UT1 − UTC UTC is maintained via leap seconds, forecast values of DUT1 are published by IERS Bulletin A. Weekly updated values of DUT1 with 0.1 s precision are broadcast by several time signal services, including WWV and these services transmit one pulse per second of some sort. To represent positive DUT1 values from +0.1 to +0.8 seconds, the number of emphasized pulses gives the value of DUT1. Negative DUT1 values, from −0.1 to −0.8 seconds, are represented by emphasizing pulses 9 through 16. For example, a DUT1 value of −0.4 would be transmitted by emphasizing pulses 9 through 12, the Russian time signal RWM transmits an additional correction dUT1 in 0.02 s increments. Positive values of dUT1 from +0.02 to +0.08 s are encoded by emphasizing pulses 21 through 24, the actual value of DUT1 is approximated by the sum of the transmitted DUT1 + dUT1. The longwave RBU time signal also transmits dUT1, iTU-R Recommendation TF. 460-4, Standard-frequency and time-signal emissions. IERS Rapid Service/Prediction Center http, //scienceworld. wolfram. com/astronomy/Time. html http, //maia. usno. navy. mil/ser7/deltat. data
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
