cycle, in astronomy, period of time required for the recurrence of some celestial event. The length of a cycle may be measured relative to the sun or to the fixed stars (see sidereal time). A frequently observed cycle is the day, during which the sun seems to circle around the earth due to the earth's rotation on its axis; although the length of the day varies, the average day is defined as exactly 24 hr of mean solar time. Another important cycle is the year, during which the earth completes an orbit of the sun. The solar year is measured from one vernal equinox to the next and is equal to 365 days, 5 hr, 48 min, 46 sec of mean solar time (see calendar). The sidereal year, measured relative to the stars, differs in length from the solar year due to the precession of the equinoxes. The moon goes through a cycle of phases as it orbits the earth, completing a cycle from one full moon to the next in about 291/2 days, or one lunar month (see synodic period). The moon completes an orbit of the earth relative to the stars in one sidereal month, which is about 2 days shorter than the lunar month. Every 18 years, 111/3 days the earth, moon, and sun are in very nearly the same relative positions; for this reason, solar and lunar eclipses recur in a cycle with this period. This cycle was known to the Chaldaeans (fl. 1000-540 B.C.) and was called the saros by them. Halley's comet reappears in a cycle whose period is about 75 years. Astronomers also make use of various other cycles, e.g., those of sunspots and variable stars.

Cycle that involves the continuous circulation of water in the Earth-atmosphere system. Water is transferred from the oceans through the atmosphere to the continents and back to the oceans by means of evaporation, transpiration, precipitation, interception, infiltration, subterranean percolation, overland flow, runoff, and other complex processes. Although the total amount of water within the cycle remains essentially constant, its distribution among the various processes is continually changing.

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or Krebs cycle or citric-acid cycle

Last stage of the chemical processes by which living cells obtain energy from foodstuffs. Described by Hans Adolf Krebs in 1937, the reactions of the cycle have been shown in animals, plants, microorganisms, and fungi, and it is thus a feature of cell chemistry shared by all types of life. It is a complex series of reactions beginning and ending with the compound oxaloacetate. In addition to re-forming oxaloacetate, the cycle produces carbon dioxide and the energy-rich compound ATP. The enzymes that catalyze each step are located in mitochondria in animals, in chloroplasts in plants, and in the cell membrane in microorganisms. The hydrogen atoms and electrons that are removed from intermediate compounds formed during the cycle are channeled ultimately to oxygen in animal cells or to carbon dioxide in plant cells.

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Period in which several important kinds of solar activity repeat, discovered in 1843 by Samuel Heinrich Schwabe (1789–1875). Lasting about 22 years on average, it includes two 11-year cycles of sunspots, whose magnetic polarities alternate between the Sun's northern and southern hemispheres, and two peaks and two declines in the phenomena (e.g., solar prominences, auroras) that vary in the same period. Attempts have been made to connect the solar cycle to various other phenomena, including possible slight variations in the diameter of the Sun, sequences of annual growth rings in trees, and even the stock market's rise and fall.

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Circulation of nitrogen in various forms throughout nature. Nitrogen is essential to life, but in the atmosphere it is in a form (the diatomic molecule N2) unavailable to most organisms. Nitrogen fixation by microbes turns this nitrogen into nitrates and other compounds, which plants or algae assimilate into their tissues. Animals that eat plants in turn incorporate the compounds into their own tissues. Microbes decompose the remains and waste of all living things into ammonia (ammonification); the ammonia may leave the soil through vaporization into the air or leaching into water. Ammonia remaining in soil may be transformed by bacteria into nitrates (nitrification), which then can be reassimilated into living organisms, or into free nitrogen (denitrification), which reenters the atmosphere. Hence, once fixed from air, some nitrogen goes through the cycle repeatedly without returning to the gaseous state.

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Periodic fluctuation in the rate of economic activity, as measured by levels of employment, prices, and production. Economists have long debated why periods of prosperity are eventually followed by economic crises (stock-market crashes, bankruptcies, unemployment, etc.). Some have identified recurring 8-to-10-year cycles in market economies; longer cycles have also been proposed, notably by Nikolay Kondratev. Apart from random shocks to the economy, such as wars and technological changes, the main influences on the level of economic activity are investment and consumption. An increase in investment, as when a factory is built, leads to consumption because the workers employed to build the factory have wages to spend. Conversely, increases in consumer demand cause new factories to be built to satisfy the demand. Eventually the economy reaches its full capacity, and, with little free capital and no new demand, the process reverses itself and contraction ensues. Natural fluctuations in agricultural markets, psychological factors such as a bandwagon mentality, and changes in the money supply have all been proposed as explanations for initial changes in investment and consumption. After World War II many governments used monetary policy to moderate the business cycle, aiming to prevent the extremes of inflation and depression by stimulating the national economy in slack times and restraining it during expansions. Seealso productivity.

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Circulation through nature of carbon in the form of the simple element and its compounds. The source of carbon in living things is carbon dioxide (CO2) from air or dissolved in water. Algae and green plants (producers) use CO2 in photosynthesis to make carbohydrates, which in turn are used in the processes of metabolism to make all other compounds in their tissues and those of animals that consume them. The carbon may pass through several levels of herbivores and carnivores (consumers). Animals and, at night, plants return the CO2 to the atmosphere as a by-product of respiration. The carbon in animal wastes and in the bodies of organisms is released as CO2 in a series of steps by decay organisms (decomposers), chiefly bacteria and fungi (see fungus). Some organic carbon (the remains of organisms) has accumulated in Earth's crust in fossil fuels, limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric times, is being returned in vast quantities as CO2 via industrial and agricultural processes, some accumulating in the oceans as dissolved carbonates and some staying in the atmosphere (see greenhouse effect).

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Periodic biological fluctuation in an organism corresponding to and in response to periodic environmental change, such as day and night or high and low tide. The internal mechanism that maintains this rhythm even without the apparent environmental stimulus is a “biological clock.” When the rhythm is interrupted, the clock's adjustment is delayed, accounting for such phenomena as jet lag when traveling across time zones. Rhythms may have 24-hour (circadian rhythm), monthly, or annual cycles. Seealso photoperiodism.

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or Ulaid cycle

In early Irish literature, a group of legends and tales dealing with the heroic age of the Ulaid, a people of northeast Ireland from whom the modern name Ulster derives. The stories, set in the 1st century BC, were recorded from oral tradition between the 8th and 11th century and are preserved in the 12th-century manuscripts The Book of the Dun Cow and The Book of Leinster and later compilations. Reflecting the customs of a free pre-Christian aristocracy, they combine mythological and legendary elements. Among the stories are “Bricriu's Feast,” containing a beheading game that appeared in medieval narratives, and “The Tragic Death of the Sons of Usnech,” dramatized in the 20th century by William Butler Yeats and John Millington Synge.

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a cycle is something that repeats inself in nature or life over and over again. -Megan Neaton

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