"Even though the metre was initially defined to equal one ten-millionth of the length of the meridian through Paris from pole to the equator, the first prototype metre bar was subsequently found to be short by 0.2 millimetres (because researchers miscalculated the flattening of the Earth). Nevertheless, this original reference metre was retained, leaving the exact distance from equator to pole slightly more than ten million metres. The need for a more practical and reproducible definition of the metre and advances in metrology have led to increased precision in the definition, so that it is now defined as the length"
"2009, the IAU had updated its standard measures to reflect improvements, and calculated the speed of light at (TDB). In 1983, the International Committee for Weights and Measures (CIPM) modified the International System of Units (SI, or ""modern"" metric system) to make the metre defined as the distance travelled in a vacuum by light in 1/299792458 second. This replaced the previous definition, valid between 1960 and 1983, which was that the metre equalled a certain number of wavelengths of a certain emission line of krypton-86. (The reason for the change was an improved method of measuring the speed of light.)"
"is now the most common system of units used in the measuring of space, and is almost universally used. Currently, the standard space interval, called a standard meter or simply meter, is defined as the distance traveled by light in a vacuum during a time interval of exactly 1/299,792,458 of a second. This definition coupled with present definition of the second is based on the special theory of relativity in which the speed of light plays the role of a fundamental constant of nature. Geography is the branch of science concerned with identifying and describing places on Earth, utilizing spatial"
"Light-second The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly metres by the definition of metre (just over miles). Just as the second forms the basis for other units of time, the light-second can form the basis for other units of length, ranging from the light-nanosecond (299.8 mm or just under one international foot) to the light-minute, light-hour and light-day, which are sometimes used in popular science publications. The more commonly used light-year is also"
"Light-second The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly metres by the definition of metre (just over miles). Just as the second forms the basis for other units of time, the light-second can form the basis for other units of length, ranging from the light-nanosecond (299.8 mm or just under one international foot) to the light-minute, light-hour and light-day, which are sometimes used in popular science publications. The more commonly used light-year is also"
"the mesures usuelles system, such that 1m was exactly toise. The Russian verst was 1.0668km. The Swedish mil was 10.688km, but was changed to 10km when Sweden converted to metric units. Metre The metre (British spelling and BIPM spelling) or meter (American spelling) (from the French unit ""mètre"", from the Greek noun μέτρον, ""measure"") is the base unit of length in some metric systems, including the International System of Units (SI). The SI unit symbol is m. The metre is defined as the length of the path travelled by light in a vacuum in second. The metre was originally defined"
"travelled by light in a vacuum during the time interval of of a second. In addition, a reference standard (a rod of platinum-iridium alloy) is maintained by the inter-governmental organisation the International Bureau of Weights and Measures, and calibration of a standard metre is usually achieved (to one part in a billion, or slightly better in some recent installations) by counting 1,579,800.298728 wavelengths of the ultra-fine (3s to 2p) emission line of helium–neon laser light (this wavelength being approximately 632.99139822 nm in a vacuum). These refinements have improved the precision and consistency with which the metre is defined. The adoption"
"correction for the effects of relativity.) The value of the astronomical unit (roughly the distance between Earth and the Sun) in light-seconds is a fundamental measurement for the calculation of modern ephemerides (tables of planetary positions). It is usually quoted as ""light-time for unit distance"" in tables of astronomical constants, and its currently accepted value is s. Multiples of the light-second can be defined, although apart from the light-year, they are more used in popular science publications than in research works. For example, a light-minute is 60 light-seconds, and the average distance between Earth and the Sun is 8.317 light-minutes."
"favored the ""seconds pendulum"" (a pendulum with a half-period of one second), but this was rejected when it was discovered that it varied from place to place with local gravity. A new unit of length, the ""metre"" was introduced - defined as one ten-millionth of the distance from the North Pole to the equator. For practical purposes however, the standard metre was made available in the form of a platinum bar held in Paris. This in turn was replaced in 1889 by thirty platinum-iridium bars kept across the globe. However, using such physical objects as the standard had been something"
"light to travel from the Sun to the Earth as 8 minutes and 12 seconds. Depending on the value assumed for the astronomical unit, this yields the speed of light as just a little more than 300,000 kilometres per second. The modern value is 8 minutes and 19 seconds, and a speed of 299,792.458 km/s. A plaque at the Observatory of Paris, where the Danish astronomer happened to be working, commemorates what was, in effect, the first measurement of a universal quantity made on this planet. In addition to inventing the first street lights in Copenhagen, Rømer also invented the"
"a standard metre or simply metre, is defined as the distance traveled by light in a vacuum during a time interval of 1/299792458 of a second (exact). In classical physics, space is a three-dimensional Euclidean space where any position can be described using three coordinates and parameterised by time. Special and general relativity use four-dimensional spacetime rather than three-dimensional space; and currently there are many speculative theories which use more than four spatial dimensions. Quantum mechanics is a large focus of contemporary philosophy of physics, specifically concerning the correct interpretation of quantum mechanics. Very broadly, much of the philosophical work"
"of 10 hours to the day, 100 minutes to the hour and 100 seconds to the minute - introduced in 1793. Instead of the seconds pendulum method, the commission – whose members included Lagrange, Laplace, Monge and Condorcet – decided that the new measure should be equal to one ten-millionth of the distance from the North Pole to the Equator (the quadrant of the Earth's circumference), measured along the meridian passing through Paris. Apart from the obvious consideration of safe access for French surveyors, the Paris meridian was also a sound choice for scientific reasons: a portion of the quadrant"
"denote the standard unit of length derived from a pendulum. As a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. On 7 October 1790 that commission advised the adoption of a decimal system, and on 19 March 1791 advised the adoption of the term ""mètre"" (""measure""), a basic unit of length, which they defined as equal to one ten-millionth of the distance between the North Pole and the Equator. In 1793, the French National Convention adopted the proposal; this use of ""metre"" in English began at least"
"direct relationship to fundamental physical constants. In contrast, the reference standard for the metre is no longer defined by a physical object (as the former international prototype metre (IPM) or originally the ""mètre des Archives""). In 1983, the standard metre was redefined as the distance light travels in a vacuum during 1/299 792 458 of a second. Secondary reference standards are very close approximations of primary reference standards. For example, major national measuring laboratories such as the US's National Institute of Standards and Technology (NIST) will hold several ""national standard"" kilograms, which are periodically calibrated against the IPK and each"
"Nautical mile A nautical mile (also known as an air mile) is a unit of measurement defined as exactly . Historically, it was defined as one minute of latitude, which is one sixtieth of a degree of latitude – i.e. since a minute of a degree is x = , the Earth's polar circumference is approximately 21,600 nautical miles. It is a non-SI unit which today has a continued use in both air and marine navigation, and for the definition of territorial waters. There is no single internationally agreed symbol. While using M itself, the International Bureau of Weights and"
"prototype are fashioned and issued to many nations to represent the national standard of the kilogram and are periodically compared to the Paris prototype. Until 1960, the meter was defined by a platinum-iridium prototype bar with two scratch marks on it (that were, by definition, spaced apart by one meter), the International Prototype Metre, and in 1983 the meter was redefined to be the distance in free space covered by light in 1/299,792,458 of a second (thus ""defining"" the speed of light to be 299,792,458 meters per second). It is widely believed that the kilogram prototype standard will be replaced."
"a unit of distance in cosmology, defined as formula_77 — the speed of light multiplied by the Hubble time. It is equivalent to 4,550 million parsecs or 14.4 billion light years. (The numerical value of the Hubble length in light years is, by definition, equal to that of the Hubble time in years.) The Hubble distance would be the distance between the Earth and the galaxies which are ""currently"" receding from us at the speed of light, as can be seen by substituting formula_78 into the equation for Hubble's law, . The Hubble volume is sometimes defined as a volume"
"sound travels about 343 meters in one second in air; light takes 1.3 seconds to reach Earth from the surface of the Moon, a distance of 384,400 kilometers. A second is part of other units, such as frequency measured in hertz (inverse seconds or second), speed (meters per second) and acceleration (meters per second squared). The metric system unit becquerel, a measure of radioactive decay, is measured in inverse seconds. The meter is defined in terms of the speed of light and the second; definitions of the metric base units ampere and candela also depend on the second. Of the"
"Astronomical unit The astronomical unit (symbol: au, ua, or AU) is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly metres or about . The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length,"
"the combined 1761 and 1769 transit data, the French astronomer Jérôme Lalande calculated the astronomical unit to have a value of 153 million kilometers (±1 million km). The precision was less than had been hoped for because of the black drop effect, but still a considerable improvement on Horrocks's calculations. Maximilian Hell published the results of his expedition in 1770, in Copenhagen. Based on the results of his own expedition, and of Wales and Cook, in 1772 he presented another calculation of the astronomical unit: 151.7 million kilometers. Lalande queried the accuracy and authenticity of the Hell expedition, but later"
"metre as: ""The metre is the length of the path travelled by light in vacuum during a time interval of 1/ of a second."" As a result of this definition, the value of the speed of light in vacuum is exactly and has become a defined constant in the SI system of units. Improved experimental techniques that prior to 1983 would have measured the speed of light, no longer affect the known value of the speed of light in SI units, but instead allow a more precise realization of the metre by more accurately measuring the wavelength of Krypton-86 and"
"dimension was a decimal fraction of the unit of length. The proposals were accepted by the French Assembly on 30 March 1791. Following acceptance, the ""Académie des sciences"" was instructed to implement the proposals. The ""Académie"" broke the tasks into five operations, allocating each part to a separate working group: The panel decided that the new measure of length should be equal to one ten-millionth of the distance from the North Pole to the Equator (the quadrant of the Earth's circumference), measured along the meridian passing through Paris. Using Jean Picard's survey of 1670 and Jacques Cassini's survey of 1718,"
"Light-year The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres or 5.88 trillion miles. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word ""year"", the term light-year is sometimes misinterpreted as a unit of time. The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in nonspecialist and popular science publications. The unit most commonly used in professional astrometry is the parsec"
"speed of light.) A possible additional reason is that the radical French Academy didn't want to base their new system on the second, a traditional and nondecimal unit from the ""ancien regime"". Although not defined by the pendulum, the final length chosen for the metre, 10 of the pole-to-equator meridian arc, was very close to the length of the seconds pendulum (0.9937 m), within 0.63%. Although no reason for this particular choice was given at the time, it was probably to facilitate the use of the seconds pendulum as a secondary standard, as was proposed in the official document. So"
"value in astronomical units per second (via the exact speed of light in metres per second). Previously, the inverse of expressed in seconds per astronomical unit was measured by comparing the time for radio signals to reach different spacecraft in the Solar System, with their position calculated from the gravitational effects of the Sun and various planets. By combining many such measurements, a best fit value for the light time per unit distance could be obtained. For example, in 2009, the best estimate, as approved by the International Astronomical Union (IAU), was: The relative uncertainty in these measurements is 0.02"
") is known as the geographical mile. Nautical mile A nautical mile (also known as an air mile) is a unit of measurement defined as exactly . Historically, it was defined as one minute of latitude, which is one sixtieth of a degree of latitude – i.e. since a minute of a degree is x = , the Earth's polar circumference is approximately 21,600 nautical miles. It is a non-SI unit which today has a continued use in both air and marine navigation, and for the definition of territorial waters. There is no single internationally agreed symbol. While using M"
"and Snellius's mathematics for doing so, Picard achieved this by measuring one degree of latitude along the Paris Meridian using triangulation along thirteen triangles stretching from Paris to the clocktower of Sourdon, near Amiens. His measurements produced a result of 110.46 km for one degree of latitude, which gives a corresponding terrestrial radius of 6328.9 km. Isaac Newton was to use this value in his theory of universal gravitation. The polar radius has now been measured at just over 6357 km. This was an error only 0.44% less than the modern value. This was another example of advances in astronomy"
"base units representing seven fundamental dimensions of measure: length, mass, time, electromagnetism, thermodynamic temperature, luminous intensity, and quantity of substance. The units are: Together they are sufficient for measuring any known quantity, without reference to further quantities or phenomena. The metre, ampere, candela, and mole are all defined in terms of other base units. For example, the speed of light is defined as 299792458 metres per second, and the metre is derived from that constant and the definition of a second. As a result, in dimensional analysis, they remain wholly separate concepts. There are currently 22 derived units with special"
"general relativity was proposed, and ""vigorous debate"" ensued until August 2012 when the IAU adopted the current definition of 1 astronomical unit = metres. The astronomical unit is typically used for stellar system scale distances, such as the size of a protostellar disk or the heliocentric distance of an asteroid, whereas other units are used for other distances in astronomy. The astronomical unit is too small to be convenient for interstellar distances, where the parsec and light-year are widely used. The parsec (parallax arcsecond) is defined in terms of the astronomical unit, being the distance of an object with a"
"pratique"" [practical realisation] that describes in detail at least one way in which the base unit can be measured. Where possible, definitions of the base units were developed so that any laboratory equipped with proper instruments would be able to realise a standard without reliance on an artefact held by another country. In practice, such realisation is done under the auspices of a mutual acceptance arrangement (MAA). The standard metre is defined as exactly 1/299,792,458 of the distance that light travels in a second. The realisation of the metre depends in turn on precise realisation of the second. There are"
"speed, the old definition of the astronomical unit of length had the accepted value: 1 ua = ""c""τ = ± 3 m, where τ is the transit time of light across the astronomical unit. The astronomical unit of length was determined by the condition that the measured data in the ephemeris match observations, and that in turn decides the transit time τ. The distances to distant galaxies are typically not quoted in distance units at all, but rather in terms of redshift. The reasons for this are that converting redshift to distance requires knowledge of the Hubble constant which was"
"when the eleventh CGPM defined the metre in the new International System of Units (SI) as equal to 1 650 763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. To further reduce uncertainty, the 17th CGPM in 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of the second and the speed of light: This definition fixed the speed of light in vacuum at exactly metres per second (≈). An intended by-product of the 17th CGPM's definition was that it"
"for historical or archaeological purposes; however, the classical second has been retained as the basis of timekeeping. Second : The time taken for light to travel 299,792,458 meters in vacuum. Kilosecond : 16 minutes Diurn ""100 kiloseconds"" : 27 hours, 1 day and 3 hours Megasecond (Cycle) ""10 diurns"" : 11 days and 6 hours M-year ""30 megaseconds"" : 337 Earth days, 11 months Gigasecond : approximately 31 Earth years Terasecond : approximately 31,000 Earth years (half age of human species) Petasecond : approximately 31,000,000 Earth years (half elapsed time since end of Cretaceous era) T-gates : (Transporter gates). These"