Sexual reproduction involves the production of new cells by the fusion of sex cells (sperm and ova) to produce a genetically different cell. Asexual reproduction, on the other hand, is the production of new cells by simple division of the parent cell into two daughter cells (called binary fission). Since there is no fusion of two different cells, the daughter cells produced by asexual reproduction…
Asia is the world's largest continent, encompassing an area of 17,177,000 sq mi (44,500,000 sq km), 29.8% of the world's land area. The Himalayan Mountains, which are the highest and youngest mountain range in the world, stretch across the continent from Afghanistan to Burma. The highest of the Himalayan peaks, called Mount Everest, reaches an altitude of 29,028 ft (8,848 m). There a…
Asses include three of the seven genera that make up the family Equidae, which also includes horses and zebras. Wild asses are completely wary and apt to run swiftly away, so they have been difficult to study. Asses can survive in poor habitat such as scrub and near desert regions. Asses have loud voices, most notable in the raucous bray of the domestic burro and a keen sense of hearing. Male asse…
In 1964 Milton M. Gordon (b. 1918) produced a groundbreaking book called Assimilation in American Life. He informed readers that three different theories of assimilation existed in the United States: "Anglo-Conformity," the "Melting Pot," and "Cultural Pluralism." Gordon acknowledged, though, that Anglo-Conformity was "the most prevalent ideology of…
In algebra, a binary operation is a rule for combining the elements of a set two at a time. In most important examples that combination is also another member of the same set. Addition, subtraction, multiplication, and division are familiar binary operations. A familiar example of a binary operation that is associative (obeys the associative principle) is addition (+) of real numbers. For example,…
Late in 2002, astronomers and the International Astronomical Union (IAU) confirmed the discovery the discovery of an asteroid, designated Asteroid 2002 AA29, in a companion orbit to Earth. It is the first object ever identified to be in a companion orbit around the Sun (i.e., it shares at least some of the same orbital path and space). In another 600 years, the asteroid will technically and tempor…
Asthma is a lung disease that affects approximately four million people in the United States. In people with asthma, the airways of the lungs are hypersensitive to irritants such as cigarette smoke or allergens. When these irritants are inhaled, the airways react by constricting, or narrowing. Some people with asthma have only mild, intermittent symptoms that can be controlled without drugs. In ot…
Astrobiology is an area of life science that investigates the origin of life, how the biological components interact to create environment, what makes planets habitable, and searches for life on other planets. Astrobiologists are scientists from many areas of science, including biologists (molecular biologists, microbiologists, ecologists, geneticists), chemists, oceanographers, climatologists, ar…
Astroblemes are the scars left on Earth's surface by the high velocity impact of large objects from outer space. Such colliding bodies are usually meteorites, but some may have been comet heads or asteroids. Few of these impacts are obvious today because our active earth tends to erode meteorite craters over short periods of geologic time. The term astrobleme was coined in 1961 by Robert S.…
At least two forms of the astrolabe have existed. The older form, known as the planispheric astrolabe, consists of two circular metal disks, one representing Earth and the other, the celestial sphere at some particular location (latitude) on the Earth's surface. The first of these disks, called the plate or tympan, is fixed in position on a supporting disk known as the mater. It shows the g…
As originally conceived, the twenty-eight lunar mansions did not technically constitute a zodiac, since, with the exception of comets, novae, and the like, the sun, moon, and planets did not actually appear among their constituent stars: many of the latter in ancient times actually lay closer to the equator than to the ecliptic. Rather, astronomical phenomena occurring within a given astral field …
Greek interest in astrology began in the Hellenistic period, as Alexander the Great's (r. 336–323) conquests exposed Greek thinkers to Mesopotamian culture. The Greeks put astronomy in a cosmological framework, emphasizing the motions of the planets rather than static correlations. Hellenistic Alexandria was a center of astrology, and the Hermetic texts that originated there incorpor…
Astrometry literally means measuring the stars. This type of measurement determines a specific star's location in the sky with great precision. In order to establish a star's location, it is necessary to first establish a coordinate system in which the location can be specified. Traditionally, very distant stars, which show very little motion as viewed from Earth, have been used to e…
An astronomical unit (AU) is a unit of length that astronomers use for measuring distances within the solar system. One astronomical unit is the mean distance between the Earth and the Sun, called the semimajor axis, or 92,919,000 mi (149,597,870 km). Kepler's third law (in a more accurate form derived by Isaac Newton) can be used to calculate a precise value of the AU, if the exact distanc…
The sun and the moon, the planets and the stars are the same the world over. One might hypothesize, therefore, that diverse cultures of the world would think the same of them. The two American continents, having been cut off from the Old World from the time of the Asian migration across the Bering land bridge more than ten thousand years ago up to European contact in the fifteenth century, provide…
The term atheism usually refers to the belief that there is no God or are no gods. This position has been called positive atheism, since it involves an actual belief and not just the absence of belief. In contrast, negative atheism involves the absence of belief in a God or gods. Atheism is typically contrasted with agnosticism, the view that one cannot know if a deity exists. Negative atheism, ho…
Earth's atmosphere is composed of about 78% nitrogen, 21% oxygen, and 0.93% argon. The remainder, less than 0.1%, contains many small but important trace gases, including water vapor, carbon dioxide, and ozone. All of these trace gases have important effects on the earth's climate. The atmosphere can be divided into vertical layers determined by the way temperature changes with heigh…
Atmospheric circulation is the movement of air at all levels of the atmosphere over all parts of the planet. The driving force behind atmospheric circulation is solar energy, which heats the atmosphere with different intensities at the equator, the middle latitudes, and the poles. Differential heating causes air to rise in the atmosphere at some locations on the planet and then to sink back to the…
The earth's atmosphere consists of gases that surround the surface, and like any gas, the atmosphere exerts a pressure on everything within it. A gas is made up of molecules that are constantly in motion. If the gas is in a container, some gas molecules are always bouncing off the container walls. When they do so, they exert a tiny force on the walls. With a sufficient number of molecules, …
Atomic clocks are the world's most accurate time keepers—more accurate than astronomical time or quartz clocks. Originally, a second was defined as 1/86,400 of a mean solar day. Today it is defined as 9,192,631,770 periods or wavelengths of the radiation absorbed by the cesium-133 atom as it changes between two hyperfine energy levels. The change in definition was the result of the …
The atomic number of an element is equal to the number of protons in the nucleus of its atom. For example, the nucleus of an oxygen atom contains eight protons and eight neutrons. Oxygen's atomic number is, therefore, eight. Since each proton carries a single positive charge, the atomic number is also equal to the total positive charge of the atomic nucleus of an element. The concept of ato…
Atomic spectroscopy is the technique of analyzing the energy emitted by atoms in order to determine the energy levels of the atom's electrons. Electrons can have only certain discrete energies. These energies are characteristic of each element; that is, every atom of an element has the same set of available energies. Normally, electrons in atoms are distributed in the lowest energy levels. …
Atomic weights are actually atomic masses but historically they were called atomic weights because the method used to determine them was called weighing. This terminology has persisted and is more familiar to most people even though the values obtained are actually atomic masses. …
Atoms are the smallest particles of matter that have distinct physical and chemical properties. Each different type of atom makes up an element which is characterized by an atomic weight and an atomic symbol. Since the atomic theory was first proposed in the early nineteenth century, scientists have discovered a number of subatomic particles. This experimental method was suggested by Robert Boyle …
Attention deficit/hyperactivity disorder (ADHD), also known as hyperkinetic disorder (HKD) outside of the United States, is estimated to affect 3–9% of children, and afflicts boys more often than girls. Although difficult to assess in infancy and toddlerhood, signs of ADHD may begin to appear as early as age two or three, but the symptom picture changes as adolescence approaches. Many sympt…
Auks are penguinlike seabirds found in the Northern Hemisphere. These birds spend most of their lives in the coastal waters north of 25°N latitude, coming ashore only to lay their eggs and raise their young. There are 22 species of auks, including the Atlantic puffin, the common murre, the dovekie or lesser auk, and the extinct great auk. Called alcids, the members of the auk family fill an…
Of the seven continents, Australia is the flattest, smallest, and except for Antarctica, the most arid. Including the southeastern island of Tasmania, the island continent is roughly equal in area to the United States, excluding Alaska and Hawaii. Millions of years of geographic isolation from other landmasses accounts for Australia's unique animal species, notably marsupial mammals like th…
Ideologies of authenticity within African thought are self-affirming and counterideological positions adopted by individuals, groups, and communities of resistance who have all had their identities traumatically impacted upon or disrupted by forces of imperialism and conquest—namely slavery, colonialism, and neocolonialism. African authenticity is in this sense fundamentally an ongoing cult…
Grand claims have been made about the superiority and inevitability of liberal democracy. Do they hold true for East Asian countries? According to typologies in political science, most East Asian countries are considered authoritarian. Japan, Philippines, Taiwan, Thailand, and South Korea are considered democracies; Indonesia is considered ambiguous while all other East Asian governments (Brunei, …
Traditional interpretations of authoritarianism in Latin America root this phenomenon in the style of Iberian colonization in the region. The Hispanic world, this argument alleges, was naturally more authoritarian than Anglo-Saxon cultures. Furthermore, the cultures they encountered in the New World (particularly the Aztec and Inca Empires) were themselves very hierarchical, which further facilita…
The term authoritarianism can be applied to a great variety of contexts. It can refer to authoritarian behavior, leadership styles, or personality types in families, industrial enterprises, bureaucracies, and other forms of organizations. Here, it refers to political regimes that fall under this broad label. The major characteristics of authoritarian regimes include a limited political pluralism w…
The conceptual history of authority reveals it to be an essentially contested concept because of the many debates about its sources, purposes, and limits, as well as its proximity to the concept of power. Since Plato's critique of Athenian democracy, physical force and rhetorical persuasion have been viewed as types of power but not authority. Hannah Arendt observes that "[i]f author…
Growing scholarly interest in the relationship between truth and fiction, along with popular interest in personal life-narratives and the "culture of confession," have brought new prominence to the genre of autobiography. Indeed, according to Leigh Gilmore, the number of English-language autobiographies and memoirs roughly tripled from the 1940s to the 1990s (p. 1, n. 1), and scholar…
Autoimmunity is a condition where the immune system mistakenly recognizes host tissue or cells as foreign. (The word "auto" is the Greek word for self.) Because of this false recognition, the immune system reacts against the host components. There are a variety of autoimmune disorders (also called autoimmune diseases). An autoimmune disease can be very specific, involving a single or…
The automatic pilot has it roots in the gyroscope, a weighted, balanced wheel mounted in bearings and spinning at high velocity. As early as 1852 the French scientist Jean-Bernard-Léon Foucault had experimented with the gyroscope and found that it tended to stay aligned with its original position and also tended to orient itself parallel to Earth's axis in a north-south direction. Th…
Automation is the use of scientific and technological principles in the manufacture of machines that take over work normally done by humans. This definition has been disputed by professional scientists and engineers, but in any case, the term is derived from the longer term automatization or from the phrase automatic operation. Delmar S. Harder, a plant manager for General Motors, is credited with…
Few inventions in modern times have had as much impact on human life and on the global environment as the automobile. Automobiles and trucks have had a strong influence on the history, economy, and social life of much of the world. Entire societies, especially those of the industrialized countries, have been restructured around the power of rapid, long-distance movement that the automobile confers…
Autonomy was first used by the ancient Greeks to describe city-states that had the power to legislate their own laws and direct the course of their own affairs. The etymology (auto [self] nomos [law]) suggests self-governance or the imposition of law on oneself. The original implication of autonomy was pejorative when applied to the individual. When, for example, in Sophocles' tragedy, the …
Gustave Courbet (1819–1877) was the first artist to assume the mantle of the avant-garde, savoring its military associations from within his outpost in the Pavillon du Réalisme he had constructed to exhibit his paintings that had been rejected by the official salon of the 1855 World's Fair. While the largest of these works—Studio of the Painter: A Real Allegory (1855)…
Beginning in the nineteenth century, the term avant-garde has been applied to a wide range of social activities, from military to political to artistic. Since the early twentieth century, however, it has most commonly been used to designate those artists who, in making works of art, knowingly transgress aesthetic and social norms, seeking thus to scandalize, to disrupt established canons of taste,…
The concept that a mole of any substance contains the same number of particles arose out of research conducted in the early 1800s by the Italian physicist Amedeo Avogadro (1776-1856). Avogadro based his work on the earlier discovery by Joseph Gay-Lussac that gases combine with each other in simple, whole-number ratios of volumes. For example, one liter of oxygen combines with two liters of hydroge…
The aye-aye (Daubentonia madagascariensis) is a rare tree-dwelling animal that is found only at a few localities along the eastern half of Madagascar, off eastern Africa. It is a member of a group of primitive primates known as Prosimians, most of which are lemurs. The aye-aye is the only surviving member of the family Daubentoniidae; a slightly larger relative (D. robusta) became extinct about 1,…
As a region in mythical geography, Aztlán (the land of the [white] herons) has a long history. According to the Náhuatl myth, the Aztecs (whose name is derived from Aztlán) were the last remaining tribe of seven, and they were advised by their god Huitzilopochtli to leave Aztlán in search of the promised land, which they would know by an eagle sitting on a cactus, devouring a s…
Babblers are small to medium-sized passerine (perching) birds characterized by soft, fluffy plumage, strong, stout legs, and short rounded wings. Their wings make them poor fliers, and most are largely sedentary birds. Many species, particularly those that stay close to the ground, are gray, brown, or black, while the tree-living (arboreal) species are often green, yellow, or olive. Wren-babblers …
Baboons are ground-living monkeys in the primate family Cercopithecidae and are found in Africa and the Arabian Peninsula. Some taxonomists classify baboons in two genera, while others classify them in three or four. All baboons have a strong torso, a snout-like face, the same dentition with long, sharp canine teeth, powerful jaws, a ground-walking habit, coarse body hair, a naked rump, and a simi…
Bacteria are mostly unicellular organisms that lack chlorophyll and are among the smallest living things on earth—only viruses are smaller. Multiplying rapidly under favorable conditions, bacteria can aggregate into colonies of millions or even billions of organisms within a space as small as a drop of water. The Dutch merchant and amateur scientist Anton van Leeuwenhoek was the first to ob…
Bacteriophage (also known as phages) are viruses that target and infect only bacterial cells. The first observation of what since turned out to be bacteriophage was made in 1896. Almost twenty years later, the British bacteriologist Frederick Twort demonstrated that an unknown microorganism that could pass through a filter that excluded bacteria was capable of destroying bacteria. He did not explo…
Ball bearings help reduce friction and improve efficiency by minimizing the frictional contact between machine parts through bearings and lubrication. Ball bearings allow rotary or linear movement between two surfaces. As the name indicates, a ball bearing involves a number of balls, typically steel, sandwiched between a spinning inner race (a small steel ring with a rounded grove on its outer sur…
Any missile that lofts an explosive payload which descends to its target as a ballistic projectile—that is, solely under the influence of gravity and air resistance—is a ballistic missile. Missiles that do not deliver a free-falling payload, such as engine powered-cruise missiles (which fly to their targets as robotic airplanes), are not "ballistic." A ballistic missile…