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Time In Physics
In physics, time is defined by its operational definition, measurement: time is what a clock reads. In classical, non-relativistic physics, it is a scalar (physics), scalar quantity (often denoted by the symbol t) and, like length, mass, and electric charge, charge, is usually described as a fundamental quantity. Time can be combined mathematically with other physical quantities to Formal proof, derive other concepts such as motion (physics), motion, kinetic energy and time-dependent Field (physics), fields. '':category:Timekeeping, Timekeeping'' is a complex of technological and scientific issues, and part of the foundation of ''recordkeeping''. Markers of time Before there were clocks, time was measured by those physical processes which were understandable to each epoch of civilization: * the first appearance (see: heliacal rising) of Sirius to mark the flooding of the Nile each yearOtto Neugebauer ''The Exact Sciences in Antiquity''. Princeton: Princeton University Pre ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Second
The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of Units (SI) is more precise: The second ..is defined by taking the fixed numerical value of the caesium frequency, Δ''ν''Cs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be when expressed in the unit Hz, which is equal to s−1. This current definition was adopted in 1967 when it became feasible to define the second based on fundamental properties of nature with caesium clocks. As the speed of Earth's rotation varies and is slowing ever so slightly, a leap second is added at irregular intervals to civil time to keep clocks in sync with Earth's rotation. The definition that is based on of a rotation of the earth is still used by the Universal Time 1 (UT1) system. Etymology "Minute" ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Field (physics)
In science, a field is a physical quantity, represented by a scalar (mathematics), scalar, vector (mathematics and physics), vector, or tensor, that has a value for each Point (geometry), point in Spacetime, space and time. An example of a scalar field is a weather map, with the surface temperature described by assigning a real number, number to each point on the map. A surface wind map, assigning an arrow to each point on a map that describes the wind velocity, speed and direction at that point, is an example of a vector field, i.e. a 1-dimensional (rank-1) tensor field. Field theories, mathematical descriptions of how field values change in space and time, are ubiquitous in physics. For instance, the electric field is another rank-1 tensor field, while electrodynamics can be formulated in terms of Mathematical descriptions of the electromagnetic field, two interacting vector fields at each point in spacetime, or as a Covariant formulation of classical electromagnetism, single-ra ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Eratosthenes
Eratosthenes of Cyrene (; ; – ) was an Ancient Greek polymath: a Greek mathematics, mathematician, geographer, poet, astronomer, and music theory, music theorist. He was a man of learning, becoming the chief librarian at the Library of Alexandria. His work is comparable to the study of geography, and he introduced some of the terminology, even coining the terms geography and geographer. He is best known for being the first person known to calculate the Earth's circumference, which he did by using the extensive survey results he could access in his role at the Library. His calculation was remarkably accurate (his error margin turned out to be less than 1%). He was the first to calculate Earth's axial tilt, which similarly proved to have remarkable accuracy. He created the Eratosthenes' Map of the World, first global projection of the world, incorporating Circle of latitude, parallels and Longitude, meridians based on the available geographic knowledge of his era. Eratosth ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Noon
Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 ( military time). Solar noon is the time when the Sun appears to contact the local celestial meridian. This is when the Sun reaches its apparent highest point in the sky, at 12 noon apparent solar time and can be observed using a sundial. The local or clock time of solar noon depends on the date, longitude, and time zone A time zone is an area which observes a uniform standard time for legal, Commerce, commercial and social purposes. Time zones tend to follow the boundaries between Country, countries and their Administrative division, subdivisions instead of ..., with Daylight Saving Time tending to place solar noon closer to 1:00pm. Etymology The word ''noon'' is derived from Latin ''nona hora'', the ninth c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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William Shakespeare
William Shakespeare ( 23 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of River Avon, Warwickshire, Avon" or simply "the Bard". His extant works, including William Shakespeare's collaborations, collaborations, consist of some Shakespeare's plays, 39 plays, Shakespeare's sonnets, 154 sonnets, three long narrative poems and a few other verses, some of uncertain authorship. His plays List of translations of works by William Shakespeare, have been translated into every major modern language, living language and are performed more often than those of any other playwright. Shakespeare remains arguably the most influential writer in the English language, and his works continue to be studied and reinterpreted. Shakespeare was born and raised in Stratford-upon-Avon, Warwickshire. At the age of 18 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Night
Night, or nighttime, is the period of darkness when the Sun is below the horizon. Sunlight illuminates one side of the Earth, leaving the other in darkness. The opposite of nighttime is daytime. Earth's rotation causes the appearance of sunrise and sunset. Moonlight, airglow, starlight, and light pollution dimly illuminate night. The duration of day, night, and twilight varies depending on the time of year and the latitude. Night on other celestial bodies is affected by their rotation and orbital periods. The planets Mercury and Venus have much longer nights than Earth. On Venus, night lasts about 58 Earth days. The Moon's rotation is tidally locked, rotating so that one of the sides of the Moon always faces Earth. Nightfall across portions of the near side of the Moon results in lunar phases visible from Earth. Organisms respond to the changes brought by nightfall: darkness, increased humidity, and lower temperatures. Their responses include direct reactions a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Otto Neugebauer
Otto Eduard Neugebauer (May 26, 1899 – February 19, 1990) was an Austrian-American mathematician and historian of science who became known for his research on the history of astronomy and the other exact sciences as they were practiced in antiquity and the Middle Ages. By studying clay tablets, he discovered that the ancient Babylonians knew much more about mathematics and astronomy than had been previously realized. The National Academy of Sciences has called Neugebauer "the most original and productive scholar of the history of the exact sciences, perhaps of the history of science, of our age." Career Neugebauer was born in Innsbruck, Austria. His father Rudolph Neugebauer was a railroad construction engineer and a collector and scholar of Oriental carpets. His parents died when he was quite young. During World War I, Neugebauer enlisted in the Austrian Army and served as an artillery lieutenant on the Italian front and then in an Italian prisoner-of-war camp alongside fe ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Flooding Of The Nile
The flooding of the Nile (commonly referred to as ''the Inundation'') and its silt Deposition (geology), deposition was a natural cycle first attested in Ancient Egypt. It was of singular importance in the history and culture of Egypt. Governments and administrators of Egypt began constructing infrastructure to Flood management, control the flooding in the 19th century, and these projects continued into the 20th. The annual flooding cycle in Egypt came to an end in 1970 with the completion of the Aswan Dam, Aswan High Dam. The river's predictability and annual deposits in the Valley, Nile Valley and Nile Delta, Delta made for extraordinarily rich soil—classified today as alluvium on a bed of entisol— enabling the Egyptians to build an empire on the basis of its enormous agricultural wealth and Surplus product, surpluses of cereals which could be stored or traded. Egyptians were one of the History of agriculture, first groups of people to practice agriculture on a large scale. T ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Sirius
Sirius is the brightest star in the night sky. Its name is derived from the Greek word (Latin script: ), meaning 'glowing' or 'scorching'. The star is designated Canis Majoris, Latinized to Alpha Canis Majoris, and abbreviated CMa or Alpha CMa. With a visual apparent magnitude of −1.46, Sirius is almost twice as bright as Canopus, the next brightest star. Sirius is a binary star consisting of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years. Sirius appears bright because of its intrinsic luminosity and its proximity to the Solar System. At a distance of , the Sirius system is one of Earth's nearest neighbours. Sirius is gradually moving closer to the Solar System and it is expected to increase in brightness slightly over t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Heliacal Rising
The heliacal rising ( ) of a star or a planet occurs annually when it becomes visible above the eastern horizon at dawn just before sunrise (thus becoming "the Morning Star (other)#Astronomy, morning star"). A heliacal rising marks the time when a star or planet becomes visible for the first time again in the night sky after having set with the Sun at the western horizon in a previous sunset (its heliacal setting), having since been in the sky only during daytime, obscured by sunlight. Historically, the most important such rising is that of Sirius, which was an important feature of the Egyptian calendar and Egyptian astronomy, astronomical development. The rising of the Pleiades heralded the start of the Ancient Greek sailing season, using celestial navigation, as well as the farming season (attested by Hesiod in his Works and Days). Heliacal rising is one of several types of risings and settings, mostly they are grouped into morning and evening risings and settings of obj ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Pulse
In medicine, the pulse refers to the rhythmic pulsations (expansion and contraction) of an artery in response to the cardiac cycle (heartbeat). The pulse may be felt ( palpated) in any place that allows an artery to be compressed near the surface of the body close to the skin, such as at the neck ( carotid artery), wrist (radial artery or ulnar artery), at the groin (femoral artery), behind the knee ( popliteal artery), near the ankle joint ( posterior tibial artery), and on foot (dorsalis pedis artery). The pulse is most commonly measured at the wrist or neck for adults and at the brachial artery (inner upper arm between the shoulder and elbow) for infants and very young children. A sphygmograph is an instrument for measuring the pulse. Physiology Claudius Galen was perhaps the first physiologist to describe the pulse. The pulse is an expedient tactile method of determination of systolic blood pressure to a trained observer. Diastolic blood pressure is non-palpable and ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Simple Harmonic Oscillator
In mechanics and physics, simple harmonic motion (sometimes abbreviated as ) is a special type of periodic function, periodic motion an object experiences by means of a restoring force whose magnitude is directly proportionality (mathematics), proportional to the distance of the object from an equilibrium position and acts towards the equilibrium position. It results in an oscillation that is described by a sinusoid which continues indefinitely (if uninhibited by friction or any other dissipation of energy). Simple harmonic motion can serve as a mathematical model for a variety of motions, but is typified by the oscillation of a mass on a spring (device), spring when it is subject to the linear elasticity (physics), elastic restoring force given by Hooke's law. The motion is sinusoidal in time and demonstrates a single resonance, resonant frequency. Other phenomena can be modeled by simple harmonic motion, including the motion of a pendulum, simple pendulum, although for it to b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |