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A mathematical entity consisting of a set of values (the carrier set) and a collection of operations that manipulate them. For example, the Integer abstract data type consists of a carrier set containing the positive and negative whole numbers and…

A diverse group of early jawed fishes from the Paleozoic Era, usually classified as a distinct class of vertebrates. Most acanthodians have tiny body scales with a cross section like that of an onion, the layers reflecting incremental scale growth.…

An order of actinopterygian (ray-finned) fishes represented by the sturgeons and paddlefishes. The order Acipenseriformes, the only extant order of about 10–12 orders of chondrosteans (subclass Chondrostei), probably originated from palaeonisciform…

An order of inarticulated brachiopods. Together with Lingulida and Siphonotretida, Acrotretida is one of the three orders of inarticulated brachiopods currently classified in class Lingulata in subphylum Linguliformea (phylum Brachiopoda).…

A class of teleostom fishes commonly known as the ray-finned fishes, which are distinguished by the structure of the paired fins that are supported by dermal rays. The class Actinopterygii comprises the subclasses Cladistia [Polypteriformes (bichirs…

The science of optical systems in which a controllable optical element, usually a deformable mirror, is used to optimize the performance of the system, for example, to maintain a sharply focused image in the presence of wavefront aberrations. A…

Signal processing is a discipline that deals with the extraction of information from signals. The devices that perform this task can be physical hardware devices, specialized software codes, or combinations of both. In recent years the complexity of…

Radar equipment carried by commercial and military aircraft. These aircraft use airborne radar systems to assist in weather assessment and navigation. Military systems also provide other specialized capabilities such as targeting of hostile aircraft…

An order of extremely elongate, limbless fossil amphibians in the subclass Lepospondyli from Permo-Carboniferous rocks of North America and Europe. The order Aistopoda (also spelled Aïstopoda) consists of extremely elongate, limbless fossil…

The efficiency of a surface in reflecting light. White surfaces have albedos close to 1; black surfaces have albedos close to 0. Because this efficiency can vary with the direction to the light source, the direction to the observer, and the color…

A well-defined procedure to solve a problem. As such, an algorithm describes how to solve a problem (Fig. 1). The study of algorithms is a fundamental area of computer science. In writing a computer program to solve a problem, a programmer expresses…

A book that contains astronomical or meteorological data arranged according to days, weeks, and months of a given year and may also include diverse information of a nonastronomical character. This article is restricted to astronomical and…

Two-way radio communications by individuals as a leisure-time activity. Amateur, or “ham,” radio is defined by international treaty as a “service of self-training, intercommunications, and technical investigation carried on by amateurs; that is, by…

Most commonly, a generic name for all fossil resins, although it has been restricted by some to refer only to succinite, the mineralogical species of fossil resin that includes most of the Baltic Coast deposits. Resins generally are complex mixtures…

An extinct subclass of Cephalopoda (phylum Mollusca). Besides the Ammonoidea (chambered ammonoids), the other subclasses of Cephalopoda are Nautiloidea (chambered nautiloids) and Coleoidea (squid, cuttlefish, octopus, and extinct belemnites), both…

One of four classes (the others are Reptilia, Aves, and Mammalia) composing the superclass Tetrapoda of the subphylum Vertebrata. The living amphibians number over 4700 species and are classified in three orders: Salientia or Anura (frogs and toads,…

The process or result of the process whereby the amplitude of a carrier wave is changed in accordance with a modulating wave. This broad definition includes applications using sinusoidal carriers, pulse carriers, or any other form of carrier, the…

A device for moving the frequency of an information signal, which is generally at baseband (such as an audio or instrumentation signal), to a higher frequency, by varying the amplitude of a mediating (carrier) signal. The motivation to modulate may…

A device for recovering information from an amplitude-modulated (AM) electrical signal. Such a signal is received, usually at radio frequency, with information impressed in one of several forms. The carrier signal may be modulated by the information…

Radio communication employing amplitude modulation (AM) of a radio-frequency (RF) carrier wave as the means of conveying the desired intelligence. In AM, the amplitude of the carrier wave is made to vary in response to the fluctuations of a sound…

A computer or computational device in which the problem variables are represented as continuous, varying physical quantities. An analog computer implements a model of the system being studied. The physical form of the analog may be functionally…

A device for converting the information contained in the value or magnitude of some characteristics of an input signal, compared to a standard or reference, to information in the form of discrete states of a signal, usually with numerical values…

Formerly, a group of reptiles that was recognized in rank-based classifications. Amniotes were once divided into subclasses, based on the patterns of temporal fenestrae (openings in the skull roof posterior to the openings for the eyes). Subclass…

A group of fossil jawless fishes, roughly similar in appearance to extant lampreys, known from Early Silurian–Early Devonian deposits (about 410–430 million years ago) that represent marine, coastal environments. Anaspid fossils are known…

A prominently located constellation in the northern sky (see illustration), named for the daughter of Cassiopeia in Greek mythology: When Cassiopeia bragged that her daughter Andromeda was more beautiful than Poseidon's daughters, the Nereids,…

The giant spiral galaxy closest to Earth. The Andromeda Galaxy is seen in the constellation of Andromeda, which is visible from Earth in the autumn skies. The galaxy is also known as M31 and NGC 224, based on its entries in two widely used catalogs.…

Modulation in which the instantaneous angle varies in proportion to the modulating waveform. When used in an analog manner, it is referred to as either frequency modulation (FM) or phase modulation (PM), depending upon whether the instantaneous…

The phylum comprising the multisegmented, invertebrate wormlike animals, of which the most numerous are the marine bristle worms and the most familiar the terrestrial earthworms. The Annelida (meaning little annuli or rings) include the Polychaeta…

α Scorpii, a cool supergiant star of spectral type M1Ib, whose red color stands out in the midsummer sky. With an effective temperature of approximately 6000°F (3600 K), Antares resembles Betelgeuse, the brightest of the red supergiants, and would…

The device that couples the transmitter or receiver network of a radio system to space. Radio waves are used to transmit signals from a source through space. The information is received at a destination which in some cases, such as radar, can be…

An order of Paleozoic tetrapods that arose Carboniferous (Mississippian) (345 million years ago) and lasted until the Early Triassic (about 245 MYA). The term anthracosaurs (meaning “coal reptiles”) encompasses a range of genera that may not all be…

The smallest order of the class Amphibia, consisting of the wormlike caecilians. The order Apoda is sometimes called Gymnophiona. Its members are known commonly as caecilians, which are wormlike, legless amphibians with indistinct or even hidden…

In astronomy, the two points in an elliptical orbit that are closest to, and farthest from, the primary body about which the secondary revolves. In the orbit of a planet or comet about the Sun, the apsides are, respectively, perihelion and aphelion.…

The Water Bearer, in Greek mythology, is a zodiacal constellation, meaning that the path of the Sun and planets passes through it. (The Sun actually passes through 13 of the modern constellations each year, not the traditional 12.) It is almost…

An order of Paleozoic diapsid reptiles including the families Petrolacosauridae and Araeoscelidae. Members of these families resemble primitive modern lizards, such as the green iguana, in size and most body proportions, but are distinguished by…

An extinct group of mainly Lower Cambrian marine sponges which, although lacking spicules, possessed an intricate, highly porous skeleton of calcite. It was probably a monophyletic group; that is, all representatives were derived from a single…

An order of extinct, free-sporing plants (cryptogams) that lived during the Late Devonian through Early Mississippian. The order includes the genera Archaeopteris (which has been extensively investigated), Actinoxylon, Actinopodium, Svalbardia,…

A genus of fossil birds in the extinct order Archaeopterygiformes, characterized by flight feathers like those of modern birds. Archaeopteryx is generally accepted as the oldest known fossil bird. It represents a unique snapshot of evolution; most…

The most diverse group of early Tertiary mammals are the archaic ungulates (condylarths). Although closely related, they are not a monophyletic group but, along with various extinct and extant mammals, form the taxon Ungulata (Fig. 1). They are…

The establishment of the temporal sequence of prehistoric events. An event can be defined as a duration of time during which certain features came together to form a set, for example, when certain materials were formed into an object (manufacturing…

A diversified group of diapsid reptiles, comprising dinosaurs (and their descendants, birds), crocodylians, pterosaurs (flying reptiles), and various forms closely related to these groups. In the past, the primitive archosaurs from the Triassic…

The brightest star in the northern sky, apparent magnitude −0.05, also known as α Boötis. It is a yellow giant star of spectral type K1.5, one of the nearest giants to the Earth at a distance of 11.25 parsecs (2.16 × 1014 mi or 3.47 × 1014 km).…

The Ram, a northern zodiacal constellation, meaning that the path of the Sun and planets passes through it (see illustration). In Greek mythology, the golden fleece of this ram was pursued by Jason and the Argonauts, whose ship is memorialized in…

A heavily armored mammal in the order Cingulata. The order Cingulata (formerly Edentata or Xenarthra) comprises the armadillos, which are divided into two extant families: Chlamyphoridae and Dasypodidae. These creatures are characterized by bony…

The largest phylum in the animal kingdom, including the well-known insects, spiders, ticks, and crustaceans, as well as many smaller, less well-known groups. Members of the phylum Arthropoda, that is, arthropods, are the dominant metazoans…

The only surviving subclass of the echinoderm class Crinoidea. The Articulata include all living members of the echinoderm class Crinoidea and are generally regarded as the post-Paleozoic crinoids. Crinoids are exclusively marine organisms, as are…

An order comprising the even-toed ungulates (hoofed mammals). There are two main radiations: the predominantly omnivorous Bunodontia, including suoids (such as pigs, peccaries, and hippos); and the more herbivorous Selenodontia, including camels and…

A small order of specialized holostean fishes which are first recorded from Middle Jurassic deposits of Europe. Later records indicate that they were a very successful group in terms of geographic range, and probably had a worldwide distribution in…

One of the many thousands of small planets (minor planets) revolving around the Sun, mainly between the orbits of Mars and Jupiter. The presence of a gap in J. A. Bode's empirical law of planetary spacings motivated a search for the missing planet.…

A relatively small order of extinct South American ungulates in the infraclass Eutheria, known from the late Paleocene to the late Miocene. Members of the order Astrapotheria are customarily divided into two suborders: the late Paleocene–Eocene…

The scientific discipline that studies the origin, evolution, distribution, and future of life in the universe. Astrobiology is an interdisciplinary science integrating contributions from biology, geology, astronomy, paleontology, and planetary…

That part of astronomy dealing with the position, distance, and motion of celestial objects, including solar system objects, stars, radio sources, and galaxies. New astrometry Astrometry has changed since the 1980s. Very long baseline interferometry…

Books, CD-ROMs, or computer-accessible sets showing stars, constellations, or other astronomical phenomena and their locations in the sky. Astronomical atlases provide two-dimensional views of the distributions of objects.The first sets of…

Lists or enumerations of astronomical data relevant to astronomy, navigation, geodesy, and space science applications. Astronomical catalogs vary a great deal in form and content depending upon the type of the data and the objects to which they are…

Schemes for locating astronomical objects in space. To an observer on the Earth's surface, the stars of the night sky appear to be placed upon a spherical shell of infinite radius with the observer at the center. Celestial objects appear to move…

The production of a permanent two-dimensional record of a scene of interest to astronomy. In direct imaging, the scene is a section of the sky optically imaged with a telescope. In other types of imaging, the scene is the output of an instrument,…

A telescope or telescopes, their protective enclosures (if any), support and headquarters buildings, and the staff of astronomers, engineers, technicians, and other support personnel. The telescopes can be optical or infrared (reflecting or…

The use of spectroscopy (the analysis of light as a function of wavelength) as a tool for obtaining observational data on the chemical compositions, physical conditions, and radial velocities of astronomical objects. Astronomical applications of…

A telescope adapted to the observation of the passage, or transit, of an astronomical object across the meridian of the observer. The astronomical transit instrument is the classic instrument of positional astronomy, which is the study of the…

The basic unit of length in the solar system. The astronomical unit (AU) is also used to a limited extent for interstellar distances through the definition of the parsec (1 pc = 206,265 AU). It is nearly equal to the mean distance a between the…

A device that uses an internal resonance frequency of atoms (or molecules) to measure the passage of time. The terms atomic clock and atomic frequency standard are often used interchangeably. A frequency standard generates pulses at regular…

Time that is based on physics, specifically on a unit of duration that corresponds to a defined number of periods of radiation of a specific atomic transition of a chosen isotope. The fundamental unit of atomic time is the International System…

Any member of an adaptively diverse group of extinct hominin species that inhabited Africa between approximately 4.2 million years ago (MYA) and 1.2 MYA. The term australopith is used in preference to the term australopithecine, which is sometimes…

A mathematical model of computing. The versatility of modern computers and their applications to the study of complex and dynamic systems, such as financial markets, cellular growth, and communication networks, raise questions about the ultimate…

The use of computers to automate decision tasks. A decision task is any task requiring the generation or selection of options. There is an increasing demand for autonomous systems that can perform various complex tasks without human control or…

The automatic control of the intermediate frequency in radio, television, or radar receivers, to correct for variations of the frequency of the transmitted carrier or the local oscillator in the receiver. For many reasons the frequency of the…

The automatic maintenance of a nearly constant output signal level of an amplifier circuit in the presence of a variable input signal level by adjusting its gain in inverse proportion to the input signal level. Automatic gain control (AGC) is also…

The class of animals consisting of the birds. Birds (Fig. 1) are vertebrate animals comprising the class Aves. (In some classification schemes, the term Neornithes may also be used when referring to modern birds.) Primarily adapted for flying, the…

The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every 100 species that have ever lived on Earth, 99 are no longer in existence.…

An elasmobranch subdivision of cartilaginous fishes known collectively as the rays, skates, or batoids. Batoidea is one of the two subdivisions of the subclass Elasmobranchii (class Chondrichthyes) and consists of 4 extant orders, 17–20 families,…

An order of extinct 10-armed cephalopod mollusks with an internal shell. The order Belemnitida belongs to the subclass Coleoidea in the class Cephalopoda (phylum Mollusca). Constituent members, which are all extinct, are known as belemnites (Fig. 1)…

A cool and highly luminous star, prominently located in the right shoulder of the constellation Orion and noticeably red in color. Betelgeuse, or α Orionis, is a supergiant star about 130 parsecs (430 light-years) from the Sun. Its spectral type of…

The collection, storage, and management of huge amounts of digital information. Databases falling into the realm of big data often begin in the hundreds of terabytes but can range up to petabytes (1015) or greater in size.The main characteristics…

Systems composed of two almost equal-size asteroids orbiting around their center of mass, or one or more satellites revolving around a larger primary body. The discovery of binary and multiple asteroids will aid the work of observers and theorists…

Two stars held together by their mutual gravitational attraction in a permanent (or long-term) association. The stellar universe is hierarchical. Stars exist singly and in binary pairs, multiple systems, and clusters. On large scales, roughly 105…

Technologies that employ a distinctive, measurable biological or behavioral characteristic that can reliably distinguish one individual from another (biometric) for authentication, identification, and verification of individuals. Biometric…

An extinct group of epifaunal bivalves that constructed reefs during the late Mesozoic. The valves of rudists are coiled to cylindrical in external form, and the shapes and sizes of rudist shells are extreme relative to other members of the class…

One of the seven existing classes in the phylum Mollusca; sometimes known as Pelecypoda. All members of the Bivalvia (bivalves) are aquatic, living at all depths of the sea and in brackish waters and freshwaters. With about 25,000 living species…

A region of spacetime exerting a gravitational field so strong that neither matter nor radiation can escape. Black holes are extreme cosmic objects predicted by German-born U.S. physicist Albert Einstein's theory of general relativity. Within a…

A class of extinct Pelmatozoa in the subphylum Blastozoa which arose in the Ordovician and flourished in the Carboniferous and Permian. Blastoids did not survive the Permo-Triassic mass extinction event. Historically, two orders of the Blastoidea…

A phylum of solitary, exclusively marine, coelomate, bivalved animals, with both valves symmetrical about a median longitudinal plane. Brachiopods are typically attached to the substrate by a posteriorly located cuticle-covered stalk called a…

A phylum of sessile aquatic invertebrates (also called Polyzoa) that form colonies of zooids. Each zooid, in its basic form, has a lophophore of ciliated tentacles situated distally on an introvert, a looped gut with the mouth inside the lophophore…

Part of a clay and silt sequence that accumulated at the foot of a colossal “reef” during the Cambrian explosion, a dramatic evolutionary radiation of animals beginning about 545 million years ago. Although this explosion is most obvious from the…

A class of the phylum Porifera, including exclusively marine sponges with a skeleton composed of spicules of calcium carbonate to which can be added a solid basal calcitic skeleton. The sponges that constitute the phylum Porifera are assigned to…

Primitive fossil members of the phylum Chordata with a calcite skeleton of echinoderm type. They occur in marine rocks of Cambrian to Pennsylvanian age (530–300 million years old) and, because of their skeletons, have traditionally been placed in…

An extinct subclass of stalked Crinoidea; a class of the phylum Echinodermata. The Camerata include more than 235 genera confined to the Paleozoic Era, ranging from the Early Ordovician to the Late Permian. Camerate crinoids were one of the four…

An order of crepuscular (active during predawn or twilight hours) or mainly nocturnal birds collectively known as the nightjars or goatsuckers. Members of the order Caprimulgiformes are apparently most closely related to the owls (Strigiformes).…

A moderately coherent group of primitive reptiles constituting an order of the subclass Anapsida. Most members are characterized by a closed cheek (temporal) region. The order is divided into four suborders: Captorhinomorpha, Millerosauria,…

The first interplanetary orbiter and lander mission dedicated to the study of Saturn and its moons. A collaborative mission by NASA (National Aeronautics and Space Administration), ESA (European Space Agency), and ISA (Italian Space Agency), the…

The application of dynamical theory to the motion of celestial bodies, including planets, moons, asteroids, comets, and stars, influenced primarily by the force of gravity. Celestial mechanics has enabled humankind to describe with great precision…

Cameras in mobile phones or smartphones that can capture images and videos.The cellphone, the world’s favorite must-have gadget, has been with us for nearly a half-century, while experiencing continuous and often phenomenal improvements. Much of the…

A class in the phylum Mollusca that is exclusively marine and contains the extant members—squids, cuttlefishes, octopuses, and the chambered nautiluses—and the fossil taxa—ammonoids, nautiloids, and belemnoids. The earliest known cephalopod fossils…

The largest object in the asteroid belt between Mars and Jupiter. As an astronomical body, Ceres contains sufficient mass for gravitation to have pulled it into a round shape, in contrast to lumpy, irregularly shaped asteroids. This characteristic…

An aquatic mammalian order comprising approximately 90 living species of whales, dolphins, and porpoises and their fossil relatives. Like all other mammals, members of the order Cetacea (cetaceans) nurse their young with milk produced by the mother,…

The process of converting scanned images of machine-printed or handwritten text (numerals, letters, and symbols) into a computer-processable format; also known as optical character recognition (OCR). Systems A typical OCR system (Fig. 1) contains…

An anapsid order of the class Reptilia, including the turtles, terrapins, and tortoises; also termed Chelonii, Testudinata, and Testudines. The order Chelonia (often referred to as Chelonii, Testudinata, and Testudines) comprises one of the oldest…

The name for several species of small ruminant mammals, also called mouse deer, which constitute the family Tragulidae in the order Artiodactyla. Chevrotains, often known as mouse deer (also spelled mouse-deer), belong to the family Tragulidae in…

The only extant order of the chondrichthyan subclass Holocephali, comprising the ratfishes. Members of the order Chimaeriformes (chimaeriforms or ratfishes; Fig. 1) are the closest living relatives of members of the subclass Elasmobranchii (sharks…

A class of vertebrates comprising the cartilaginous, jawed fishes. Members of the Chondrichthyes (chondrichthyans, or the cartilaginous fishes) are characterized by well-developed jaws and bony teeth; pectoral and pelvic girdles, with each…

An order of sea urchins (phylum Echinodermata, class Echinoidea) in which the 20 columns of plates in the essentially spherical test consist of five pairs of narrow and numerous ambulacral plates and five pairs of broad and few interambulacral…