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Realisation (metrology)
In metrology, the realisation of a unit of measure is the conversion of its definition into reality. An example of the concept of realisation, is the unit of length, the meter. The metre was originally defined in 1791 as one ten-millionth of the distance from the equator to the North Pole along a great circle. To actually measure a length, this definition must be converted into a physical tool, which can be used to complete the measurement. The meter stick is the realisation of the meter. The International Bureau of Weights and Measures maintains the techniques for realisation of the base units in the International System of Units (SI), all seven of which are defined in terms of natural physical constants, rather than human artefacts such as the standard kilogram. Following the 2019 revision of the SI all fundamental units of metrology are now defined in terms of natural physical constants, rather than human artefacts. The realization of these units is also defined by a publi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Metrology
Metrology is the scientific study of measurement. It establishes a common understanding of Unit of measurement, units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the ''International Bureau of Weights and Measures, Bureau International des Poids et Mesures'' (BIPM) was established by the Metre Convention. This has evolved into the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (CGPM) in 1960. Metrology is divided into three basic overlapping activities: * The definition of units of measurement * ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sundial
A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate (the ''dial'') and a gnomon, which casts a shadow onto the dial. As the Sun diurnal motion, appears to move through the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The ''style'' is the time-telling edge of the gnomon, though a single point or ''nodus'' may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be polar alignment, parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude. The term ''sundial'' can r ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Foot (unit)
The foot (standard symbol: ft) is a Units of measurement, unit of length in the imperial units, British imperial and United States customary units, United States customary systems of metrology, measurement. The prime (symbol), prime symbol, , is commonly used to represent the foot. In both customary and imperial units, one foot comprises 12 inches, and one yard comprises three feet. Since international yard and pound, an international agreement in 1959, the foot is defined as equal to exactly 0.3048 meters. Historically, the "foot" was a part of many local systems of units, including the Ancient Greek units of measurement, Greek, Ancient Roman units of measurement, Roman, Chinese units of measurement, Chinese, Units of measurement in France before the French Revolution, French, and English units, English systems. It varied in length from country to country, from city to city, and sometimes from trade to trade. Its length was usually between 250 mm and 335 mm and ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Barleycorn (unit)
The barleycorn is an English unit of length equal to of an inch (i.e. about ). It is still used as the basis of shoe sizes in English-speaking countries. History Under the 1300 Composition of Yards and Perches, one of the statutes of uncertain date that was notionally in force until the 1824 Weights and Measures Act, "3 dry and " were to serve as the basis for the inch and thence the larger units of feet, yards, perches and thus of the acre, an important unit of area. The notion of three barleycorns composing an inch certainly predates this statute, however, appearing in the 10th-century Welsh Laws of Hywel Dda. In practice, various weights and measures acts of the English kings were standardized with reference to some particular yard-length iron, brass, or bronze bar held by the king or the Royal Exchequer. The formal barleycorn was of its length. As modern studies show, the actual length of a kernel of barley varies from as short as to as long as depending on the cu ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Natural Units
In physics, natural unit systems are measurement systems for which selected physical constants have been set to 1 through nondimensionalization of physical units. For example, the speed of light may be set to 1, and it may then be omitted, equating mass and energy directly rather than using as a conversion factor in the typical mass–energy equivalence equation . A purely natural system of units has all of its dimensions collapsed, such that the physical constants completely define the system of units and the relevant physical laws contain no conversion constants. While natural unit systems simplify the form of each equation, it is still necessary to keep track of the non-collapsed dimensions of each quantity or expression in order to reinsert physical constants (such dimensions uniquely determine the full formula). Systems of natural units Summary table where: * is the fine-structure constant ( ≈ 0.007297) * ≈ * ≈ * A dash (—) indicates where the system ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sundial
A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate (the ''dial'') and a gnomon, which casts a shadow onto the dial. As the Sun diurnal motion, appears to move through the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The ''style'' is the time-telling edge of the gnomon, though a single point or ''nodus'' may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be polar alignment, parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude. The term ''sundial'' can r ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chartres
Chartres () is the Prefectures in France, prefecture of the Eure-et-Loir Departments of France, department in the Centre-Val de Loire Regions of France, region in France. It is located about southwest of Paris. At the 2019 census, there were 170,763 inhabitants in the Functional area (France), metropolitan area of Chartres (as defined by the Institut national de la statistique et des études économiques, INSEE), 38,534 of whom lived in the city (Communes of France, commune) of Chartres proper. Chartres is famous worldwide for its Chartres Cathedral, cathedral. Mostly constructed between 1193 and 1250, this Gothic architecture, Gothic cathedral is in an exceptional state of preservation. The majority of the original stained glass windows survive intact, while the architecture has seen only minor changes since the early 13th century. Part of the old town, including most of the library associated with the School of Chartres, was destroyed by Allies of World War II, Allied bombs i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Stonehenge
Stonehenge is a prehistoric Megalith, megalithic structure on Salisbury Plain in Wiltshire, England, west of Amesbury. It consists of an outer ring of vertical sarsen standing stones, each around high, wide, and weighing around 25 tons, topped by connecting horizontal lintel stones, held in place with mortise and tenon joints, a feature unique among contemporary monuments. Inside is a ring of smaller bluestones. Inside these are free-standing trilithons, two bulkier vertical sarsens joined by one lintel. The whole monument, now ruinous, is aligned towards the sunrise on the summer solstice and sunset on the winter solstice. The stones are set within Earthwork (archaeology), earthworks in the middle of the densest complex of Neolithic British Isles, Neolithic and Bronze Age Britain, Bronze Age monuments in England, including several hundred ''tumuli'' (burial mounds). Stonehenge was constructed in several phases beginning about 3100 BC and continuing until about 1600 B ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Astronomical Time
The modern 24-hour clock is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours (and minutes) passed since midnight, from to , with as an option to indicate the end of the day. This system, as opposed to the 12-hour clock, is the most commonly used time notation in the world today, and is used by the international standard ISO 8601. A number of countries, particularly English speaking, use the 12-hour clock, or a mixture of the 24- and 12-hour time systems. In countries where the 12-hour clock is dominant, some professions prefer to use the 24-hour clock. For example, in the practice of medicine, the 24-hour clock is generally used in documentation of care as it prevents any ambiguity as to when events occurred in a patient's medical history. Description A time of day is written in the 24-hour notation in the form hh:mm (for example 01:23) or hh:mm:ss (for example, 01:23:45), where h ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Synchronization
Synchronization is the coordination of events to operate a system in unison. For example, the Conductor (music), conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronous or ''in sync''—and those that are not are ''Asynchronous system, asynchronous''. Today, time synchronization can occur between systems around the world through satellite navigation signals and other time and frequency transfer techniques. Navigation and railways Time-keeping and synchronization of clocks is a critical problem in long-distance ocean navigation. Before radio navigation and Radionavigation-satellite service, satellite-based navigation, navigators required accurate time in conjunction with astronomical observations to determine History of longitude, how far east or west their vessel traveled. The invention of an accurate marine chronometer revolutionized marine navigation. By the end of the 19th cent ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Time Of Day
In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations. In traditional astronomical usage, civil time was mean solar time reckoned from midnight. Before 1925, the astronomical time 00:00:00 meant noon, twelve hours after the civil time 00:00:00 which meant midnight. HM Nautical Almanac Office in the United Kingdom used Greenwich Mean Time (GMT) for both conventions, leading to ambiguity, whereas the Nautical Almanac Office at the United States Naval Observatory used GMT for the pre-1925 convention and Greenwich Civil Time (GCT) for the post-1924 convention until 1952. In 1928, the International Astronomical Union introduced the term Univers ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Clock
A clock or chronometer is a device that measures and displays time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, and the year. Devices operating on several physical processes have been used over the Millennium, millennia. Some predecessors to the modern clock may be considered "clocks" that are based on movement in nature: A sundial shows the time by displaying the position of a shadow on a flat surface. There is a range of duration timers, a well-known example being the hourglass. Water clocks, along with sundials, are possibly the oldest time-measuring instruments. A major advance occurred with the invention of the verge escapement, which made possible the first mechanical clocks around 1300 in Europe, which kept time with oscillating timekeepers like balance wheels., pp. 103–104., p. 31. Traditionally, in horology (the study of timekeeping), the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
