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Typical home loudspeakers have sensitivities of about 85 to 95 dB for 1 W @ 1 m—an efficiency of 0.5–4%. Sound reinforcement and public address loudspeakers have sensitivities of perhaps 95 to 102 dB for 1 W @ 1 m—an efficiency of 4–10%. Rock concert, stadium PA, marine hailing, etc. speakers generally have higher sens... | Loudspeaker | Wikipedia | 454 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
The interaction of a loudspeaker system with its environment is complex and is largely out of the loudspeaker designer's control. Most listening rooms present a more or less reflective environment, depending on size, shape, volume, and furnishings. This means the sound reaching a listener's ears consists not only of so... | Loudspeaker | Wikipedia | 460 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
Directivity
Acousticians, in studying the radiation of sound sources have developed some concepts important to understanding how loudspeakers are perceived. The simplest possible radiating source is a point source, sometimes called a simple source. An ideal point source is an infinitesimally small point radiating sound... | Loudspeaker | Wikipedia | 449 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
In real life, individual drivers are complex 3D shapes such as cones and domes, and they are placed on a baffle for various reasons. A mathematical expression for the directivity of a complex shape, based on modeling combinations of point sources, is usually not possible, but in the far field, the directivity of a loud... | Loudspeaker | Wikipedia | 446 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
Directivity is an important issue because it affects the frequency balance of sound a listener hears, and also the interaction of the speaker system with the room and its contents. A very directive (sometimes termed 'beamy') speaker (i.e. on an axis perpendicular to the speaker face) may result in a reverberant field l... | Loudspeaker | Wikipedia | 401 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
Piezoelectric speakers are frequently used as beepers in watches and other electronic devices, and are sometimes used as tweeters in less-expensive speaker systems, such as computer speakers and portable radios. Piezoelectric speakers have several advantages over conventional loudspeakers: they are resistant to overloa... | Loudspeaker | Wikipedia | 442 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
Magnetostrictive speakers
Magnetostrictive transducers, based on magnetostriction, have been predominantly used as sonar ultrasonic sound wave radiators, but their use has spread also to audio speaker systems. Magnetostrictive speaker drivers have some special advantages: they can provide greater force (with smaller ex... | Loudspeaker | Wikipedia | 447 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
Electrostatics are usually driven through a step-up transformer that multiplies the voltage swings produced by the power amplifier. This transformer also multiplies the capacitive load that is inherent in electrostatic transducers, which means the effective impedance presented to the power amplifiers varies widely by f... | Loudspeaker | Wikipedia | 499 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
Bending wave loudspeakers
Bending wave transducers use a diaphragm that is intentionally flexible. The rigidity of the material increases from the center to the outside. Short wavelengths radiate primarily from the inner area, while longer waves reach the edge of the speaker. To prevent reflections from the outside bac... | Loudspeaker | Wikipedia | 360 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
Flat panel loudspeakers
There have been many attempts to reduce the size of speaker systems, or alternatively to make them less obvious. One such attempt was the development of exciter transducer coils mounted to flat panels to act as sound sources, most accurately called exciter/panel drivers. These can then be made i... | Loudspeaker | Wikipedia | 492 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
Digital speakers have been the subject of experiments performed by Bell Labs as far back as the 1920s. The design is simple; each bit controls a driver, which is either fully 'on' or 'off'. Problems with this design have led manufacturers to abandon it as impractical for the present. First, for a reasonable number of b... | Loudspeaker | Wikipedia | 368 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
Thermoacoustic speakers
In 2008, researchers of Tsinghua University demonstrated a thermoacoustic loudspeaker (or thermophone) of carbon nanotube thin film, whose working mechanism is a thermoacoustic effect. Sound frequency electric currents are used to periodically heat the CNT and thus result in sound generation in... | Loudspeaker | Wikipedia | 226 | 45871 | https://en.wikipedia.org/wiki/Loudspeaker | Technology | Media and communication | null | vital_articles |
In probability theory and statistics, the exponential distribution or negative exponential distribution is the probability distribution of the distance between events in a Poisson point process, i.e., a process in which events occur continuously and independently at a constant average rate; the distance parameter could... | Exponential distribution | Wikipedia | 475 | 45906 | https://en.wikipedia.org/wiki/Exponential%20distribution | Mathematics | Statistics and probability | null | vital_articles |
When T is interpreted as the waiting time for an event to occur relative to some initial time, this relation implies that, if T is conditioned on a failure to observe the event over some initial period of time s, the distribution of the remaining waiting time is the same as the original unconditional distribution. For ... | Exponential distribution | Wikipedia | 494 | 45906 | https://en.wikipedia.org/wiki/Exponential%20distribution | Mathematics | Statistics and probability | null | vital_articles |
is not exponentially distributed, if X1, ..., Xn do not all have parameter 0.
Joint moments of i.i.d. exponential order statistics
Let be independent and identically distributed exponential random variables with rate parameter λ.
Let denote the corresponding order statistics.
For , the joint moment of the order ... | Exponential distribution | Wikipedia | 276 | 45906 | https://en.wikipedia.org/wiki/Exponential%20distribution | Mathematics | Statistics and probability | null | vital_articles |
Related distributions
If X ~ Laplace(μ, β−1), then |X − μ| ~ Exp(β).
If X ~ U(0, 1) then −log(X) ~ Exp(1).
If X ~ Pareto(1, λ), then log(X) ~ Exp(λ).
If X ~ SkewLogistic(θ), then .
If Xi ~ U(0, 1) then
The exponential distribution is a limit of a scaled beta distribution:
The exponential distribution is a spec... | Exponential distribution | Wikipedia | 507 | 45906 | https://en.wikipedia.org/wiki/Exponential%20distribution | Mathematics | Statistics and probability | null | vital_articles |
If also λ = 1/2 then ; i.e., X has a chi-squared distribution with 2 degrees of freedom. Hence:
If and ~ Poisson(X) then (geometric distribution)
The Hoyt distribution can be obtained from exponential distribution and arcsine distribution
The exponential distribution is a limit of the κ-exponential distribut... | Exponential distribution | Wikipedia | 89 | 45906 | https://en.wikipedia.org/wiki/Exponential%20distribution | Mathematics | Statistics and probability | null | vital_articles |
Other related distributions:
Hyper-exponential distribution – the distribution whose density is a weighted sum of exponential densities.
Hypoexponential distribution – the distribution of a general sum of exponential random variables.
exGaussian distribution – the sum of an exponential distribution and a normal dist... | Exponential distribution | Wikipedia | 488 | 45906 | https://en.wikipedia.org/wiki/Exponential%20distribution | Mathematics | Statistics and probability | null | vital_articles |
Now the posterior density p has been specified up to a missing normalizing constant. Since it has the form of a gamma pdf, this can easily be filled in, and one obtains:
Here the hyperparameter α can be interpreted as the number of prior observations, and β as the sum of the prior observations.
The posterior mean her... | Exponential distribution | Wikipedia | 345 | 45906 | https://en.wikipedia.org/wiki/Exponential%20distribution | Mathematics | Statistics and probability | null | vital_articles |
In queuing theory, the service times of agents in a system (e.g. how long it takes for a bank teller etc. to serve a customer) are often modeled as exponentially distributed variables. (The arrival of customers for instance is also modeled by the Poisson distribution if the arrivals are independent and distributed ide... | Exponential distribution | Wikipedia | 505 | 45906 | https://en.wikipedia.org/wiki/Exponential%20distribution | Mathematics | Statistics and probability | null | vital_articles |
The Bayesian approach provides a predictive distribution which takes into account the uncertainty of the estimated parameter, although this may depend crucially on the choice of prior.
A predictive distribution free of the issues of choosing priors that arise under the subjective Bayesian approach is
which can be con... | Exponential distribution | Wikipedia | 438 | 45906 | https://en.wikipedia.org/wiki/Exponential%20distribution | Mathematics | Statistics and probability | null | vital_articles |
In probability theory and statistics, the geometric distribution is either one of two discrete probability distributions:
The probability distribution of the number of Bernoulli trials needed to get one success, supported on ;
The probability distribution of the number of failures before the first success, support... | Geometric distribution | Wikipedia | 474 | 45922 | https://en.wikipedia.org/wiki/Geometric%20distribution | Mathematics | Probability | null | vital_articles |
where and are natural numbers, is a geometrically distributed random variable defined over , and is a geometrically distributed random variable defined over . Note that these definitions are not equivalent for discrete random variables; does not satisfy the first equation and does not satisfy the second.
Moments... | Geometric distribution | Wikipedia | 509 | 45922 | https://en.wikipedia.org/wiki/Geometric%20distribution | Mathematics | Probability | null | vital_articles |
Entropy and Fisher's Information
Entropy (Geometric Distribution, Failures Before Success)
Entropy is a measure of uncertainty in a probability distribution. For the geometric distribution that models the number of failures before the first success, the probability mass function is:
The entropy for this distribution... | Geometric distribution | Wikipedia | 379 | 45922 | https://en.wikipedia.org/wiki/Geometric%20distribution | Mathematics | Probability | null | vital_articles |
The characteristic function is equal to so the geometric distribution's characteristic function, when defined over and respectively, is
The entropy of a geometric distribution with parameter is
Given a mean, the geometric distribution is the maximum entropy probability distribution of all discrete probability di... | Geometric distribution | Wikipedia | 418 | 45922 | https://en.wikipedia.org/wiki/Geometric%20distribution | Mathematics | Probability | null | vital_articles |
If p = 1/n and X is geometrically distributed with parameter p, then the distribution of X/n approaches an exponential distribution with expected value 1 as n → ∞, sinceMore generally, if p = λ/n, where λ is a parameter, then as n→ ∞ the distribution of X/n approaches an exponential distribution with rate λ: therefore ... | Geometric distribution | Wikipedia | 467 | 45922 | https://en.wikipedia.org/wiki/Geometric%20distribution | Mathematics | Probability | null | vital_articles |
The geometric distribution can be generated experimentally from i.i.d. standard uniform random variables by finding the first such random variable to be less than or equal to . However, the number of random variables needed is also geometrically distributed and the algorithm slows as decreases.
Random generation can ... | Geometric distribution | Wikipedia | 180 | 45922 | https://en.wikipedia.org/wiki/Geometric%20distribution | Mathematics | Probability | null | vital_articles |
Index fossils (also known as guide fossils or indicator fossils) are fossils used to define and identify geologic periods (or faunal stages). Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends. Another term, "zone fossil", is used when the fossil has all the chara... | List of index fossils | Wikipedia | 89 | 45930 | https://en.wikipedia.org/wiki/List%20of%20index%20fossils | Physical sciences | Stratigraphy | Earth science | vital_articles |
A set of primary colors (see spelling differences) consists of colorants or colored lights that can be mixed in varying amounts to produce a gamut of colors. This is the essential method used to create the perception of a broad range of colors in, e.g., electronic displays, color printing, and paintings. Perceptions as... | Primary color | Wikipedia | 466 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
Art education materials commonly use red, yellow, and blue as primary colors, sometimes suggesting that they can mix all colors. No set of real colorants or lights can mix all possible colors, however. In other domains, the three primary colors are typically red, green and blue, which are more closely aligned to the se... | Primary color | Wikipedia | 333 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
The perception elicited by multiple light sources co-stimulating the same area of the retina is additive, i.e., predicted via summing the spectral power distributions (the intensity of each wavelength) of the individual light sources assuming a color matching context. For example, a purple spotlight on a dark backgroun... | Primary color | Wikipedia | 462 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
The subtractive color mixing model predicts the resultant spectral power distribution of light filtered through overlaid partially absorbing materials, usually in the context of an underlying reflective surface such as white paper. Each layer partially absorbs some wavelengths of light from the illumination while letti... | Primary color | Wikipedia | 501 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
Traditional color theory is based on experience with pigments, more than on the science of light. In 1920, Snow and Froehlich explained:
The widespread adoption of teaching of RYB as primary colors in post-secondary art schools in the twentieth century has been attributed to the influence of the Bauhaus, where Johanne... | Primary color | Wikipedia | 474 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
Mixing pigments for the purpose of creating realistic paintings with diverse color gamuts is known to have been practiced at least since Ancient Greece (see history section). The identity of a/the set of minimal pigments to mix diverse gamuts has long been the subject of speculation by theorists whose claims have chang... | Primary color | Wikipedia | 419 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
A color space is a subset of a color model, where the primaries have been defined, either directly as photometric spectra, or indirectly as a function of other color spaces. For example, sRGB and Adobe RGB are both color spaces based on the RGB color model. However, the green primary of Adobe RGB is more saturated than... | Primary color | Wikipedia | 452 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
Each integral term in the above equation is known as a tristimulus value and measures amounts in the adopted units. No set of real primary lights can match another monochromatic light under additive mixing so at least one of the color matching functions is negative for each wavelength. A negative tristimulus value corr... | Primary color | Wikipedia | 460 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
Color spaces used in color reproduction must use real primaries that can be reproduced by practical sources, either lights in additive models, or pigments in subtractive models. Most RGB color spaces have real primaries, though some maintain imaginary primaries. For example, all the sRGB primaries fall within the gamut... | Primary color | Wikipedia | 511 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
Philosophical writing from ancient Greece has described notions of primary colors, but they can be difficult to interpret in terms of modern color science. Theophrastus (c. 371–287 BCE) described Democritus' position that the primary colors were white, black, red, and green. In Classical Greece, Empedocles identified w... | Primary color | Wikipedia | 356 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
John Gage's The Fortunes Of Apelles provides a summary of the history of primary colors as pigments in painting and describes the evolution of the idea as complex. Gage begins by describing Pliny the Elder's account of notable Greek painters who used four primaries. Pliny distinguished the pigments (i.e., substances) f... | Primary color | Wikipedia | 453 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
Historical perspectives on color order systems ("catalogs" of color) that were proposed in the 18th and 19th centuries describe them as using red, yellow, and blue pigments as chromatic primaries. Tobias Mayer (a German mathematician, physicist, and astronomer) described a triangular bipyramid with red, yellow and blue... | Primary color | Wikipedia | 478 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
Itten's ideas about RYB primaries have been criticized as ignoring modern color science with demonstrations that some of Itten's claims about mixing RYB primaries are impossible. | Primary color | Wikipedia | 35 | 45939 | https://en.wikipedia.org/wiki/Primary%20color | Physical sciences | Basics_7 | null | vital_articles |
A building or edifice is an enclosed structure with a roof and walls, usually standing permanently in one place, such as a house or factory. Buildings come in a variety of sizes, shapes, and functions, and have been adapted throughout history for numerous factors, from building materials available, to weather condition... | Building | Wikipedia | 455 | 45995 | https://en.wikipedia.org/wiki/Building | Technology | Structures | null | vital_articles |
Single-family residential buildings are most often called houses or homes. Multi-family residential buildings containing more than one dwelling unit are called duplexes or apartment buildings. Condominiums are apartments that occupants own rather than rent. Houses may be built in pairs (semi-detached) or in terraces, w... | Building | Wikipedia | 500 | 45995 | https://en.wikipedia.org/wiki/Building | Technology | Structures | null | vital_articles |
Sometimes a group of inter-related (and possibly inter-connected) builds are referred to as a complex – for example a housing complex, educational complex, hospital complex, etc.
Creation
The practice of designing, constructing, and operating buildings is most usually a collective effort of different groups of profes... | Building | Wikipedia | 506 | 45995 | https://en.wikipedia.org/wiki/Building | Technology | Structures | null | vital_articles |
Buildings may be damaged during construction or during maintenance. They may be damaged by accidents involving storms, explosions, subsidence caused by mining, water withdrawal or poor foundations and landslides. Buildings may suffer fire damage and flooding. They may become dilapidated through lack of proper maintenan... | Building | Wikipedia | 63 | 45995 | https://en.wikipedia.org/wiki/Building | Technology | Structures | null | vital_articles |
Porpoises () are small dolphin-like cetaceans classified under the family Phocoenidae. Although similar in appearance to dolphins, they are more closely related to narwhals and belugas than to the true dolphins. There are eight extant species of porpoise, all among the smallest of the toothed whales. Porpoises are dis... | Porpoise | Wikipedia | 451 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Porpoises were, and still are, hunted by some countries by means of drive hunting. Larger threats to porpoises include extensive bycatch in gill nets, competition for food from fisheries, and marine pollution, in particular heavy metals and organochlorides. The vaquita is nearly extinct due to bycatch in gill nets, wit... | Porpoise | Wikipedia | 257 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
ORDER Artiodactyla
Infraorder Cetacea
Parvorder Odontoceti toothed whales
Superfamily Delphinoidea
Family Phocoenidae – porpoises
Genus †Haborophocoena
H. toyoshimai
Genus Neophocaena
N. phocaeniodes – Indo-Pacific finless porpoise
N. sunameri – East Asian finless porpoise
N. asiaeorientalis – Yangtze finless porpoise
... | Porpoise | Wikipedia | 404 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Odontocetes possess teeth with cementum cells overlying dentine cells. Unlike human teeth, which are composed mostly of enamel on the portion of the tooth outside of the gum, whale teeth have cementum outside the gum. Porpoises have a three-chambered stomach, including a fore-stomach and fundic and pyloric chambers. Po... | Porpoise | Wikipedia | 423 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
The porpoise ear has specific adaptations to the marine environment. In humans, the middle ear works as an impedance equaliser between the outside air's low impedance and the cochlear fluid's high impedance. In whales, and other marine mammals, there is no great difference between the outer and inner environments. Inst... | Porpoise | Wikipedia | 471 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Unlike most animals, porpoises are conscious breathers. All mammals sleep, but porpoises cannot afford to become unconscious for long because they may drown. While knowledge of sleep in wild cetaceans is limited, porpoises in captivity have been recorded to sleep with one side of their brain at a time, so that they may... | Porpoise | Wikipedia | 480 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Little is known about the diets of other species of porpoises. A dissection of three Burmeister's porpoises shows that they consume shrimp and euphausiids (krill). A dissection of a beached Vaquita showed remains of squid and grunts. Nothing is known about the diet of the spectacled porpoise.
Interactions with humans
... | Porpoise | Wikipedia | 359 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Harbour porpoises have historically been kept in captivity, under the assumption that they would fare better than their dolphin counterparts due to their smaller size and shallow-water habitats. Up until the 1980s, they were consistently short-lived. Harbour porpoises have a very long captive history, with poorly docum... | Porpoise | Wikipedia | 497 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Finless porpoises have commonly been kept in Japan, as well as China and Indonesia. As of 1984, ninety-four in total had been in captivity in Japan, eleven in China, and at least two in Indonesia. As of 1986, three establishments in Japan had bred them, and there had been five recorded births. Three calves died moments... | Porpoise | Wikipedia | 460 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Porpoises and other smaller cetaceans have traditionally been hunted in many areas, at least in Asia, Europe and North America, for their meat and blubber. A dominant hunting technique is drive hunting, where a pod of animals is driven together with boats and usually into a bay or onto a beach. Their escape is prevente... | Porpoise | Wikipedia | 397 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Porpoises are very sensitive to anthropogenic disturbances, and are keystone species, which can indicate the overall health of the marine environment. Populations of harbor porpoises in the North and Baltic Seas are under increasing pressure from anthropogenic causes such as offshore construction, ship traffic, fishing... | Porpoise | Wikipedia | 478 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
As of 2014, only 505 Yangtze finless porpoises remained in the main section of the Yangtze, with an alarming population density in Ezhou and Zhenjiang. While many threatened species decline rate slows after their classification, population decline rates of the porpoise are actually accelerating. While population declin... | Porpoise | Wikipedia | 412 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Because vaquitas are indigenous to the Gulf of California, Mexico is leading conservation efforts with the creation of the International Committee for the Recovery of the Vaquita (CIRVA), which has tried to prevent the accidental deaths of vaquitas by outlawing the use of fishing nets within the vaquita's habitat. CIRV... | Porpoise | Wikipedia | 184 | 46037 | https://en.wikipedia.org/wiki/Porpoise | Biology and health sciences | Cetaceans | null | vital_articles |
Halley's Comet is the only known short-period comet that is consistently visible to the naked eye from Earth, appearing every 72–80 years, though with the majority of recorded apparations (25 of 30) occurring after 75–77 years. It last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-... | Halley's Comet | Wikipedia | 441 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
Computation of orbit
Halley was the first comet to be recognised as periodic. Until the Renaissance, the philosophical consensus on the nature of comets, promoted by Aristotle, was that they were disturbances in Earth's atmosphere. This idea was disproven in 1577 by Tycho Brahe, who used parallax measurements to show t... | Halley's Comet | Wikipedia | 496 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
Another independent recognition that the comet had returned was made by the Jamaican astronomer Francis Williams, but his observations did not reach Europe. A unique portrait commissioned by Williams demonstrates the impact of the comet's return on period astronomers. Williams' hand rests on the page of Newton's Princi... | Halley's Comet | Wikipedia | 451 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
Halley's orbital period has varied between 74 and 80 years since 240 BC. Its orbit around the Sun is highly elliptical, with an orbital eccentricity of 0.967 (with 0 being a circle and 1 being a parabolic trajectory). The perihelion, the point in the comet's orbit when it is nearest the Sun, is . This is between the or... | Halley's Comet | Wikipedia | 406 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the giant planets and directed into the inner Solar System. If Halley was once a long-period comet, it is likely to have originated in the Oort cloud, a sphere of cometary bodies aroun... | Halley's Comet | Wikipedia | 325 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
In 1989, Boris Chirikov and Vitold Vecheslavov performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations, which showed that its dynamics were chaotic and unpredictable on long timescales. Halley's projected dynamical lifetime is estimated to be about 10 million yea... | Halley's Comet | Wikipedia | 474 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
Despite the vast size of its coma, Halley's nucleus is relatively small: barely long, wide and perhaps thick. Based on a reanalysis of images taken by the Giotto and Vega spacecraft, Lamy et al. determined an effective diameter of . Its shape has been variously compared to that of a peanut, a potato, or an avocado. ... | Halley's Comet | Wikipedia | 438 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
Giotto provided the first evidence in support of Fred Whipple's "dirty snowball" hypothesis for comet construction; Whipple postulated that comets are icy objects warmed by the Sun as they approach the inner Solar System, causing ices on their surfaces to sublime (change directly from a solid to a gas), and jets of vol... | Halley's Comet | Wikipedia | 360 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
The apparition of 87 BC was recorded in Babylonian tablets which state that the comet was seen "day beyond day" for a month. This appearance may be recalled in the representation of Tigranes the Great, an Armenian king who is depicted on coins with a crown that features, according to Vahe Gurzadyan and R. Vardanyan, "a... | Halley's Comet | Wikipedia | 508 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
The 374 AD and 607 approaches each came within 0.09 au of Earth. The 451 AD apparition was said to herald the defeat of Attila the Hun at the Battle of Chalons.
The 684 AD apparition was reported in Chinese records as the "broom star".
The 760 AD apparition was recorded in the Zuqnin Chronicle'''s entry for iyyōr 107... | Halley's Comet | Wikipedia | 502 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
The Irish Annals of the Four Masters recorded the comet as "A star [that] appeared on the seventh of the Calends of May, on Tuesday after Little Easter, than whose light the brilliance or light of The Moon was not greater; and it was visible to all in this manner till the end of four nights afterwards." Chaco Native Am... | Halley's Comet | Wikipedia | 502 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
In 1456, the year of Halley's next apparition, the Ottoman Empire invaded the Kingdom of Hungary, culminating in the siege of Belgrade in July of that year. In a papal bull, Pope Callixtus III ordered special prayers be said for the city's protection. In 1470, the humanist scholar Bartolomeo Platina wrote in his that,... | Halley's Comet | Wikipedia | 456 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
In the Sikh scriptures of the Guru Granth Sahib, the founder of the faith Guru Nanak makes reference to "a long star that has risen" at Ang 1110, and it is believed by some Sikh scholars to be a reference to Halley's appearance in 1531.
Halley's periodic returns have been subject to scientific investigation since the ... | Halley's Comet | Wikipedia | 279 | 46083 | https://en.wikipedia.org/wiki/Halley%27s%20Comet | Physical sciences | Solar System | null | vital_articles |
An eared seal, otariid, or otary is any member of the marine mammal family Otariidae, one of three groupings of pinnipeds. They comprise 15 extant species in seven genera (another species became extinct in the 1950s) and are commonly known either as sea lions or fur seals, distinct from true seals (phocids) and the wal... | Eared seal | Wikipedia | 368 | 46086 | https://en.wikipedia.org/wiki/Eared%20seal | Biology and health sciences | Pinnipeds | null | vital_articles |
Traditionally, otariids had been subdivided into the fur seal (Arctocephalinae) and sea lion (Otariinae) subfamilies, with the major distinction between them being the presence of a thick underfur layer in the former. Under this categorization, the fur seals comprised two genera: Callorhinus in the North Pacific with a... | Eared seal | Wikipedia | 447 | 46086 | https://en.wikipedia.org/wiki/Eared%20seal | Biology and health sciences | Pinnipeds | null | vital_articles |
Otariids are further distinguished by a more dog-like head, sharp, well-developed canines, and the aforementioned visible external pinnae. Their postcanine teeth are generally simple and conical in shape. The dental formula for eared seals is: . Sea lions are covered with coarse guard hairs, while fur seals have a thic... | Eared seal | Wikipedia | 461 | 46086 | https://en.wikipedia.org/wiki/Eared%20seal | Biology and health sciences | Pinnipeds | null | vital_articles |
Species
Family Otariidae
Subfamily Arctocephalinae (fur seals)
Genus Arctocephalus
Brown fur seal, A. pusillus
South African fur seal, A. pusillus pusillus
Australian fur seal, A. pusillus doriferus
Antarctic fur seal, A. gazella
Guadalupe fur seal, A. townsendi
Juan Fernández fur seal, A. philippii
Galápagos fur... | Eared seal | Wikipedia | 359 | 46086 | https://en.wikipedia.org/wiki/Eared%20seal | Biology and health sciences | Pinnipeds | null | vital_articles |
In mathematical logic, Russell's paradox (also known as Russell's antinomy) is a set-theoretic paradox published by the British philosopher and mathematician, Bertrand Russell, in 1901. Russell's paradox shows that every set theory that contains an unrestricted comprehension principle leads to contradictions. Accordin... | Russell's paradox | Wikipedia | 496 | 46095 | https://en.wikipedia.org/wiki/Russell%27s%20paradox | Mathematics | Discrete mathematics | null | vital_articles |
Informal presentation
Most sets commonly encountered are not members of themselves. Let us call a set "normal" if it is not a member of itself, and "abnormal" if it is a member of itself. Clearly every set must be either normal or abnormal. For example, consider the set of all squares in a plane. This set is not itsel... | Russell's paradox | Wikipedia | 486 | 46095 | https://en.wikipedia.org/wiki/Russell%27s%20paradox | Mathematics | Discrete mathematics | null | vital_articles |
Set-theoretic responses
From the principle of explosion of classical logic, any proposition can be proved from a contradiction. Therefore, the presence of contradictions like Russell's paradox in an axiomatic set theory is disastrous; since if any formula can be proved true it destroys the conventional meaning of tru... | Russell's paradox | Wikipedia | 393 | 46095 | https://en.wikipedia.org/wiki/Russell%27s%20paradox | Mathematics | Discrete mathematics | null | vital_articles |
ZFC is silent about types, although the cumulative hierarchy has a notion of layers that resemble types. Zermelo himself never accepted Skolem's formulation of ZFC using the language of first-order logic. As José Ferreirós notes, Zermelo insisted instead that "propositional functions (conditions or predicates) used for... | Russell's paradox | Wikipedia | 495 | 46095 | https://en.wikipedia.org/wiki/Russell%27s%20paradox | Mathematics | Discrete mathematics | null | vital_articles |
Through the work of Zermelo and others, especially John von Neumann, the structure of what some see as the "natural" objects described by ZFC eventually became clear: they are the elements of the von Neumann universe, V, built up from the empty set by transfinitely iterating the power set operation. It is thus now poss... | Russell's paradox | Wikipedia | 442 | 46095 | https://en.wikipedia.org/wiki/Russell%27s%20paradox | Mathematics | Discrete mathematics | null | vital_articles |
Ernst Zermelo in his (1908) A new proof of the possibility of a well-ordering (published at the same time he published "the first axiomatic set theory") laid claim to prior discovery of the antinomy in Cantor's naive set theory. He states: "And yet, even the elementary form that Russell9 gave to the set-theoretic antin... | Russell's paradox | Wikipedia | 454 | 46095 | https://en.wikipedia.org/wiki/Russell%27s%20paradox | Mathematics | Discrete mathematics | null | vital_articles |
Russell and Alfred North Whitehead wrote their three-volume Principia Mathematica hoping to achieve what Frege had been unable to do. They sought to banish the paradoxes of naive set theory by employing a theory of types they devised for this purpose. While they succeeded in grounding arithmetic in a fashion, it is not... | Russell's paradox | Wikipedia | 492 | 46095 | https://en.wikipedia.org/wiki/Russell%27s%20paradox | Mathematics | Discrete mathematics | null | vital_articles |
One way that the paradox has been dramatised is as follows: Suppose that every public library has to compile a catalogue of all its books. Since the catalogue is itself one of the library's books, some librarians include it in the catalogue for completeness; while others leave it out as it being one of the library's bo... | Russell's paradox | Wikipedia | 475 | 46095 | https://en.wikipedia.org/wiki/Russell%27s%20paradox | Mathematics | Discrete mathematics | null | vital_articles |
Paradoxes that fall in this scheme include:
The barber with "shave".
The original Russell's paradox with "contain": The container (Set) that contains all (containers) that do not contain themselves.
The Grelling–Nelson paradox with "describer": The describer (word) that describes all words, that do not describe them... | Russell's paradox | Wikipedia | 250 | 46095 | https://en.wikipedia.org/wiki/Russell%27s%20paradox | Mathematics | Discrete mathematics | null | vital_articles |
GLONASS (, ; ) is a Russian satellite navigation system operating as part of a radionavigation-satellite service. It provides an alternative to Global Positioning System (GPS) and is the second navigational system in operation with global coverage and of comparable precision.
Satellite navigation devices supporting bo... | GLONASS | Wikipedia | 481 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
The signals use similar DSSS encoding and binary phase-shift keying (BPSK) modulation as in GPS signals. All GLONASS satellites transmit the same code as their standard-precision signal; however each transmits on a different frequency using a 15-channel frequency-division multiple access (FDMA) technique spanning eithe... | GLONASS | Wikipedia | 424 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
The navigational message is modulated at 50 bits per second. The superframe of the open signal is 7500 bits long and consists of 5 frames of 30 seconds, taking 150 seconds (2.5 minutes) to transmit the continuous message. Each frame is 1500 bits long and consists of 15 strings of 100 bits (2 seconds for each string), w... | GLONASS | Wikipedia | 500 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
Accuracy
At peak efficiency, the standard-precision signal offers horizontal positioning accuracy within 5–10 metres, vertical positioning within , a velocity vector measuring within , and timing within 200 nanoseconds, all based on measurements from four first-generation satellites simultaneously; newer satellites su... | GLONASS | Wikipedia | 378 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
Open L1OC and restricted L1SC signals are centered at 1600.995 MHz, and open L2OC and restricted L2SC signals are centered at 1248.06 MHz, overlapping with GLONASS FDMA signals. Open signals L1OC and L2OC use time-division multiplexing to transmit pilot and data signals, with BPSK(1) modulation for data and BOC(1,1) mo... | GLONASS | Wikipedia | 456 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
The navigational message of the L3OC signal is transmitted at 100 bit/s, with each string of symbols taking 3 seconds (300 bits). A pseudo-frame of 6 strings takes 18 seconds (1800 bits) to transmit. A superframe of 8 pseudo-frames is 14,400 bits long and takes 144 seconds (2 minutes 24 seconds) to transmit the full al... | GLONASS | Wikipedia | 499 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
With the introduction of CDMA signals, the constellation will be expanded to 30 active satellites by 2025; this may require eventual deprecation of FDMA signals. The new satellites will be deployed into three additional planes, bringing the total to six planes from the current three—aided by System for Differential Cor... | GLONASS | Wikipedia | 462 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
The true first generation of GLONASS (also called Uragan) satellites were all three-axis stabilized vehicles, generally weighing and were equipped with a modest propulsion system to permit relocation within the constellation. Over time they were upgraded to Block IIa, IIb, and IIv vehicles, with each block containing ... | GLONASS | Wikipedia | 484 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
In July 2015, ISS Reshetnev announced that it had completed the last GLONASS-M (No. 61) spacecraft and it was putting it in storage waiting for launch, along with eight previously built satellites.
As on 22 September 2017, GLONASS-M No.52 satellite went into operation and the orbital grouping has again increased to 24... | GLONASS | Wikipedia | 506 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
, some of the latest receivers in the Garmin eTrex line also support GLONASS (along with GPS). Garmin also produce a standalone Bluetooth receiver, the GLO for Aviation, which combines GPS, WAAS and GLONASS.
Various smartphones from 2011 onwards have integrated GLONASS capability in addition to their pre-existing GPS ... | GLONASS | Wikipedia | 459 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
Some modern receivers are able to use both GLONASS and GPS satellites together, providing greatly improved coverage in urban canyons and giving a very fast time to fix due to over 50 satellites being available. In indoor, urban canyon or mountainous areas, accuracy can be greatly improved over using GPS alone. For usin... | GLONASS | Wikipedia | 272 | 46149 | https://en.wikipedia.org/wiki/GLONASS | Technology | Navigation | null | vital_articles |
Tillage is the agricultural preparation of soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shoveling, picking, mattock work, hoeing, and raking. Examples of draft-animal-powered or mechanized work include ploug... | Tillage | Wikipedia | 330 | 46191 | https://en.wikipedia.org/wiki/Tillage | Technology | Horticultural techniques | null | vital_articles |
Secondary tillage produces finer soil and sometimes shapes the rows, preparing the seed bed. It also provides weed control throughout the growing season during the maturation of the crop plants, unless such weed control is instead achieved with low-till or no-till methods involving herbicides.
The seedbed preparation ... | Tillage | Wikipedia | 403 | 46191 | https://en.wikipedia.org/wiki/Tillage | Technology | Horticultural techniques | null | vital_articles |
The popularity of tillage as an agricultural technique in early modern times had to do with theories about plant biology proposed by European thinkers. In 1731, English writer Jethro Tull published the book "Horse-Hoeing Husbandry: An Essay on the Principles of Vegetation and Tillage," which argued that soil needed to ... | Tillage | Wikipedia | 500 | 46191 | https://en.wikipedia.org/wiki/Tillage | Technology | Horticultural techniques | null | vital_articles |
Reduced tillage leaves between 15 and 30% crop residue cover on the soil or 500 to 1000 pounds per acre (560 to 1100 kg/ha) of small grain residue during the critical erosion period. This may involve the use of a chisel plow, field cultivators, or other implements. See the general comments below to see how they can aff... | Tillage | Wikipedia | 420 | 46191 | https://en.wikipedia.org/wiki/Tillage | Technology | Horticultural techniques | null | vital_articles |
No-till – plows, disks, et cetera are not used. Aims for 100% ground cover.
Strip-till – Narrow strips are tilled where seeds will be planted, leaving the soil in between the rows untilled.
Mulch-till - Soil is covered with mulch to conserve heat and moisture. 100% soil disturbance.
Rotational tillage – Tilling the ... | Tillage | Wikipedia | 407 | 46191 | https://en.wikipedia.org/wiki/Tillage | Technology | Horticultural techniques | null | vital_articles |
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