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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 timesidereal time
(ST), time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian.
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). A frequently observed cycle is the dayday,
period of time for the earth to rotate once on its axis. The ordinary day, or solar day, is measured relative to the sun, being the time between successive passages of the sun over a stationary observer's celestial meridian.
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, 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 timesolar time,
time defined by the position of the sun. The solar day is the time it takes for the sun to return to the same meridian in the sky. Local solar time is measured by a sundial.
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. Another important cycle is the yearyear,
time required for the earth to complete one orbit about the sun. The solar or tropical year is measured relative to the sun and is equal to 365 days, 5 hr, 48 min, 46 sec of mean solar time (see solar time).
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, during which the earth completes an orbit of the sun. The solar year is measured from one vernal equinoxequinox
, either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The vernal equinox, also known as "the first point of Aries," is the point at which the sun appears to cross the celestial equator from south to north.
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 to the next and is equal to 365 days, 5 hr, 48 min, 46 sec of mean solar time (see calendarcalendar
[Lat., from Kalends], system of reckoning time for the practical purpose of recording past events and calculating dates for future plans. The calendar is based on noting ordinary and easily observable natural events, the cycle of the sun through the seasons with equinox
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). The sidereal year, measured relative to the stars, differs in length from the solar year due to the precession of the equinoxesprecession of the equinoxes,
westward motion of the equinoxes along the ecliptic. This motion was first noted by Hipparchus c.120 B.C. The precession is due to the gravitational attraction of the moon and sun on the equatorial bulge of the earth, which causes the earth's axis to
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. 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 29 1-2 days, or one lunar month (see synodic periodsynodic period
, in astronomy, length of time during which a body in the solar system makes one orbit of the sun relative to the earth, i.e., returns to the same elongation.
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). 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, 11 1-3 days the earth, moon, and sun are in very nearly the same relative positions; for this reason, solar and lunar eclipseseclipse
[Gr.,=failing], in astronomy, partial or total obscuring of one celestial body by the shadow of another. Best known are the lunar eclipses, which occur when the earth blocks the sun's light from the moon, and solar eclipses, occurring when the moon blocks the sun's light
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 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 cometHalley's comet
or Comet Halley
, periodic comet named for Edmond Halley, who observed it in 1682 and identified it as the one observed in 1531 and 1607. Halley did not live to see its return in 1758, close to the time he predicted.
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 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.



cyclical phenomena

any repetitive or recurring social processes in which a sequence of events is followed by a similar sequence on completion. Numerous social processes are accepted as manifesting a cyclical pattern (e.g. the life cycle); other suggested cyclical patterns (e.g. historical cycles, the CIRCULATION OF ÉLITES) are more controversial.

Bourdon and Bourricaud (1989) identify an important general category of cyclical phenomena, i.e. those that result when ‘a process, in developing, causes a negative feedback to arise, which ends in a reversal of the process’. In ECONOMICS, the well-known cobweb theorem has this basis: producers tend to estimate future prices on the basis of current process, thus, they tend to produce an excess of products they think will be most profitable, and insufficient quantities of goods which they estimate will be less profitable, producing, when graphically expressed, a cyclical spider's web-like pattern of movements from one equilibrium position to another. A more straightforward example is provided by patterns of take-up of vaccination: high levels of vaccination lead to fewer illnesses due to a particular disease, leading to fewer vaccinations and a return of the disease, leading in turn to a renewal of take-up of vaccination. One attraction of conceptualizations of social reality as involving cyclical processes is that these can often be formulated mathematically although such models rarely manifest themselves in a pure form in social life.



in thermodynamics, a process in which a physical system (such as steam) returns to its initial state after undergoing a series of changes.

At the end of a cycle the thermodynamic parameters and characteristic functions of the system’s state (such as the temperature T, pressure p, volume V, internal energy U, and entropy S) once again assume their original values, and consequently the changes in them during the cycle are equal to zero (Δ £7 = 0, and so on). All changes arising as a result of a cycle occur only in the medium surrounding the system. In some sections a system or working body accomplishes positive work through its internal energy and the quantities of heat Qn gained from external sources, whereas in other sections of the cycle extrinsic forces perform work on the system, part of which goes to restore the system’s internal energy.

According to the first law of thermodynamics (the law of conservation of energy), the work accomplished in a cycle by or on a system (A) is equal to the algebraic sum of the quantities of heat (Q) received or given up in each section of the cycle (ΔU = Q - A = 0, A = Q). The ratio A/Qn (the ratio of the work accomplished by the system to the amount of heat received by it) is called the efficiency of the cycle.

A distinction is made between equilibrium cycles (or, more accurately, quasiequilibrium cycles), in which the states through which the system passes are close to equilibrium states, and nonequilibrium cycles, in which at least one of the sections is a nonequilibrium process. The efficiency is at a maximum in equilibrium cycles. A graphic representation of an equilibrium (reversible) Carnot cycle with maximum efficiency is shown in Figure 1.

A cycle is called direct if it results in the accomplishment of work on external bodies and in the transfer of a certain amount of heat from a hotter body (the heater) to a colder body (the cooler). A cycle that results in the transfer of a certain amount of heat from the cooler to the heater through extrinsic forces is called a reverse or cooling cycle.

Cycles have played a prominent role in physics, chemistry, and technology. The design of various equilibrium cycles was historically the first method of thermodynamic research. This method made possible, on the basis of analysis of the working cycle of an ideal heat engine (the Carnot cycle), the derivation of a mathematical expression for the second law of thermodynamics and the construction of the thermodynamic temperature scale. Many important thermodynamic relations (such as the Clausius-Clapeyron equation) were found by examining the corresponding

Figure 1. Diagram of a direct Carnot cycle in various coordinate systems: (a) volume V and pressure p, (b) volume V and temperature T, (c) entropy S and temperature T. The area ABCD is proportional to the work of the cycle. The reverse cycle takes place in the opposite order— ADCBA (counterclockwise on the graph).

cycles. Cycles are used in technology as the working cycles of internal-combustion engines and of various thermal-power and refrigeration units.


Krichevskii, I. R. Poniatiia i osnovy termodinamiki. Moscow, 1962.
Kurs fizicheskoi khimii, 2nd ed., vol. 1. Edited by la. I. Gerasimov. Moscow, 1969.


To run a machine through a single complete operation.
(fluid mechanics)
A system of phases through which the working substance passes in an engine, compressor, pump, turbine, power plant, or refrigeration system.
A member of the kernel of a boundary homomorphism.
A closed path in a graph that does not pass through any vertex more than once and passes through at least three vertices. Also known as circuit.
(science and technology)
One complete sequence of values of an alternating quantity.
A set of operations that is repeated as a unit.
A periodic movement in a time series.

alternating current

An electric current that varies periodically in value and direction, first flowing in one direction in the circuit and then flowing in the opposite direction; each complete repetition is called a cycle, and the number of repetitions per second is called the frequency; usually expressed in Hertz (Hz).


i. One complete sequence of events making up a portion of life. For example, a cycle can be startup, taxi, takeoff, climb, cruise, descend, land, taxi, and switch off. It could be an application of full power on the engine, or in an internal combustion engine, four strokes of the Otto cycle.
ii. A complete process of starting at zero, passing through two maximums of opposite direction, and returning to zero again.


1. Lit a group of poems or prose narratives forming a continuous story about a central figure or event
2. a series of miracle plays
3. Music a group or sequence of songs (see song cycle)
4. Astronomy the orbit of a celestial body
5. Biology a recurrent series of events or processes in plants and animals
6. Physics a continuous change or a sequence of changes in the state of a system that leads to the restoration of the system to its original state after a finite period of time
7. Physics one of a series of repeated changes in the magnitude of a periodically varying quantity, such as current or voltage
8. Computing
a. a set of operations that can be both treated and repeated as a unit
b. the time required to complete a set of operations
c. one oscillation of the regular voltage waveform used to synchronize processes in a digital computer


A basic unit of computation, one period of a computer clock.

Each instruction takes a number of clock cycles. Often the computer can access its memory once on every clock cycle, and so one speaks also of "memory cycles".

Every hacker wants more cycles (noted hacker Bill Gosper describes himself as a "cycle junkie"). There are only so many cycles per second, and when you are sharing a computer the cycles get divided up among the users. The more cycles the computer spends working on your program rather than someone else's, the faster your program will run. That's why every hacker wants more cycles: so he can spend less time waiting for the computer to respond.

The use of the term "cycle" for a computer clock period can probably be traced back to the rotation of a generator generating alternating current though computers generally use a clock signal which is more like a square wave. Interestingly, the earliest mechanical calculators, e.g. Babbage's Difference Engine, really did have parts which rotated in true cycles.


(1) A single event that is repeated. For example, in a carrier frequency, one cycle is one complete wave.

(2) A set of events that is repeated. For example, in a polling system, all of the attached terminals are tested in one cycle. See machine cycle and memory cycle.