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Barycentric Coordinate Time
Barycentric Coordinate Time (TCB, from the French Temps-coordonnée barycentrique) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the Solar System. It is equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the barycenter (center of mass) of the Solar System: that is, a clock that performs exactly the same movements as the Solar System but is outside the system's gravity well. It is therefore not influenced by the gravitational time dilation caused by the Sun and the rest of the system. TCB is the time coordinate for the Barycentric Celestial Reference System (BCRS). TCB was defined in 1991 by the International Astronomical Union, in Recommendation III of the XXIst General Assembly. It was intended as one of the replacements for the problematic 1976 definition of Barycentric Dynamical Time ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
French Language
French ( or ) is a Romance language of the Indo-European family. It descended from the Vulgar Latin of the Roman Empire, as did all Romance languages. French evolved from Gallo-Romance, the Latin spoken in Gaul, and more specifically in Northern Gaul. Its closest relatives are the other langues d'oïl—languages historically spoken in northern France and in southern Belgium, which French ( Francien) largely supplanted. French was also influenced by native Celtic languages of Northern Roman Gaul like Gallia Belgica and by the ( Germanic) Frankish language of the post-Roman Frankish invaders. Today, owing to France's past overseas expansion, there are numerous French-based creole languages, most notably Haitian Creole. A French-speaking person or nation may be referred to as Francophone in both English and French. French is an official language in 29 countries across multiple continents, most of which are members of the ''Organisation internationale de la Francophonie'' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Barycentric Dynamical Time
Barycentric Dynamical Time (TDB, from the French ) is a relativistic coordinate time scale, intended for astronomical use as a time standard to take account of time dilation when calculating orbits and astronomical ephemerides of planets, asteroids, comets and interplanetary spacecraft in the Solar System. TDB is now (since 2006) defined as a linear scaling of Barycentric Coordinate Time (TCB). A feature that distinguishes TDB from TCB is that TDB, when observed from the Earth's surface, has a difference from Terrestrial Time (TT) that is about as small as can be practically arranged with consistent definition: the differences are mainly periodic,The periodic differences, due to relativistic effects, between a coordinate time scale applicable to the Solar-System barycenter, and time measured at the Earth's surface, were first estimated and are explained in: G M Clemence & V Szebehely"Annual variation of an atomic clock" Astronomical Journal, Vol.72 (1967), p.1324-6. and overall w ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Time Scales
Time scale may refer to: *Time standard, a specification of either the rate at which time passes, points in time, or both *A duration or quantity of time: **Orders of magnitude (time) as a power of 10 in seconds; **A specific unit of time * Geological time scale, a scale that divides up the history of Earth into scientifically meaningful periods In astronomy and physics: *Dynamical time scale, in stellar physics, the time in which changes in one part of a body can be communicated to the rest of that body, or in celestial mechanics, a realization of a time-like argument based on a dynamical theory *Nuclear timescale, an estimate of the lifetime of a star based solely on its rate of fuel consumption *Thermal time scale, an estimate of the lifetime of a star once the fuel reserves at its center are used up In cosmology and particle physics: * Planck time, the time scale beneath which quantum effects are comparable in significance to gravitational effects In mathematics: *Time-scale ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Terrestrial Time
Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth. For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time (TDT or TD),TT is equivalent to TDT, see IAU conference 1991, Resolution A4, recommendation IV, note 4. which succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth. The unit of TT is the SI second, the definition of which is based currently on the caesium atomic clock,IAU conference 1991, Resolution A4, recommendation IV, part 2 states that the unit for TT is to agree with the SI second 'on the geoid'. but TT is not itself defined by atomic clocks. It is a theoretical ideal, and real clocks can only ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geocentric Coordinates
The Earth-centered, Earth-fixed coordinate system (acronym ECEF), also known as the geocentric coordinate system, is a cartesian spatial reference system that represents locations in the vicinity of the Earth (including its surface, interior, atmosphere, and surrounding outer space) as ''X'', ''Y'', and ''Z'' measurements from its center of mass. Its most common use is in tracking the orbits of satellites and in satellite navigation systems for measuring locations on the surface of the Earth, but it is also used in applications such as tracking crustal motion. The distance from a given point of interest to the center of Earth is called the geocentric distance, , which is a generalization of the '' geocentric radius'', , not restricted to points on the reference ellipsoid surface. The geocentric altitude is a type of altitude defined as the difference between the two aforementioned quantities: ; it is not to be confused for the ''geodetic altitude''. Conversions between ECE ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gregorian Calendar
The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun. The rule for leap years is: There were two reasons to establish the Gregorian calendar. First, the Julian calendar assumed incorrectly that the average solar year is exactly 365.25 days long, an overestimate of a little under one day per century, and thus has a leap year every four years without exception. The Gregorian reform shortened the average (calendar) year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes.See Wikisource English translation of the (Latin) 1582 papal bull '' Inter gravissimas''. Second, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Julian Date
The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date). The Julian period is a chronological interval of 7980 years; year 1 of the Julian Period was . The Julian calendar year is year of the current Julian Period. The next Julian Period begins in the year . Historians used the period to identify Julian calendar years within which an event occurred when no such year was given in the historical record, or when the year given by previous historians was incorrect. The Julian day number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Universal Time, with Julian day number 0 assigned to the day starting at noon on Monday, January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar), a date at whi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gravitational Potential
In classical mechanics, the gravitational potential at a location is equal to the work (energy transferred) per unit mass that would be needed to move an object to that location from a fixed reference location. It is analogous to the electric potential with mass playing the role of charge. The reference location, where the potential is zero, is by convention infinitely far away from any mass, resulting in a negative potential at any finite distance. In mathematics, the gravitational potential is also known as the Newtonian potential and is fundamental in the study of potential theory. It may also be used for solving the electrostatic and magnetostatic fields generated by uniformly charged or polarized ellipsoidal bodies. Potential energy The gravitational potential (''V'') at a location is the gravitational potential energy (''U'') at that location per unit mass: V = \frac, where ''m'' is the mass of the object. Potential energy is equal (in magnitude, but negative) to the w ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
International Atomic Time
International Atomic Time (abbreviated TAI, from its French name ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. It is a continuous scale of time, without leap seconds, and it is the principal realisation of Terrestrial Time (with a fixed offset of epoch). It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface and which has leap seconds. UTC deviates from TAI by a number of whole seconds. , when another leap second was put into effect, UTC is currently exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972. TAI may be reported using traditional means of specifying days, carried over from non-uniform time standards based on the rotation o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geocentric Coordinate Time
Geocentric Coordinate Time (TCG - Temps-coordonnée géocentrique) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to precession, nutation, the Moon, and artificial satellites of the Earth. It is equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the center of the Earth: that is, a clock that performs exactly the same movements as the Earth but is outside the Earth's gravity well. It is therefore not influenced by the gravitational time dilation caused by the Earth. The TCG is the time coordinate for the Geocentric Celestial Reference System (GCRS). TCG was defined in 1991 by the International Astronomical Union. Unlike former astronomical time scales, TCG is defined in the context of the general theory of relativity. The relationships between TCG and other relativistic time scales are defined with fully general relativistic metrics. Because the reference fr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Metric Tensor (general Relativity)
In general relativity, the metric tensor (in this context often abbreviated to simply the metric) is the fundamental object of study. It may loosely be thought of as a generalization of the gravitational potential of Newtonian gravitation. The metric captures all the geometric and causal structure of spacetime, being used to define notions such as time, distance, volume, curvature, angle, and separation of the future and the past. Notation and conventions Throughout this article we work with a metric signature that is mostly positive (); see sign convention. The gravitation constant G will be kept explicit. This article employs the Einstein summation convention, where repeated indices are automatically summed over. Definition Mathematically, spacetime is represented by a four-dimensional differentiable manifold M and the metric tensor is given as a covariant, second-degree, symmetric tensor on M, conventionally denoted by g. Moreover, the metric is required to be nondegenera ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
