An Etymological Dictionary of Astronomy and Astrophysics
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1) A complex series of processes through which all the carbon atoms on Earth is cycled through the air, ground, plants, animals, and fossil fuels. During the cycle, plants absorb → carbon dioxide (CO₂) from the atmosphere and through → photosynthesis incorporate the associated carbon atoms into sugars and other molecules necessary for growth. Plants return carbon atoms back to the atmosphere in the form of CO₂. However, much of the carbon absorbed remains "locked up" in the living organisms until decomposition or fire releases it back to the atmosphere.
2) For nuclear fusion in stars → CNO cycle.

→ carbon; → cycle.

A → cyclic process comprising a sequence of → isothermal and → adiabatic expansions and compressions that bring a system back to its initial state.

Nicolas Léonard Sadi Carnot (1796-1832), a French physicist and military engineer who, in his 1824 Reflections on the Motive Power of Fire, gave the first successful theoretical account of heat engines; → cycle.

A series of → nuclear reactions taking place in stars in which → carbon, → nitrogen, and → oxygen are used to transform → hydrogen into → helium. In → massive stars the carbon cycle is the dominant process of energy generation, whereas in → low-mass stars such as the Sun, the → proton-proton chain of reactions converts hydrogen into helium. The carbon cycle starts and ends with carbon-12, which acts as a catalyst in the sequential production of helium from hydrogen; neutrinos and gamma rays are also produced.

→ CNO; → cycle.

1) A sequence of changes that are repeated regularly, such as revolution, rotation, vibration, oscillation, wave motion.
2) One complete set of changes in the value of a → periodic function during one period.
3) A period of time at the end of which an event or sequence of events are repeated.
→ carbon cycle, → Carnot cycle, → CNO cycle, → cycle shape, → hydrologic cycle, → Metonic cycle, → Milankovich cycle, → solar activity cycle, → solar cycle, → solar magnetic cycle, → sunspot cycle, → epicycle, → recycle.
4) In → graph theory, a closed → path with at least one → edge.

From L.L. cyclus, from Gk. kyklos "circle, wheel," from PIE base *kw(e)-kwl-o- "wheel, circle," from *kwel- "to turn, move around, sojourn, dwell," (cf. Av. caxra- "wheel," caraiti "he moves, approaches;" Mod.Pers. carx "wheel;" Skt. cakra- "wheel, circle; cycle," carati "he moves, wanders;" Gk. polos "axis of a sphere," polein "move around;" L. colere "to dwell in, to cultivate, move around," colonus "farmer, settler;" O.E. hweol "wheel;" Rus. koleso "awheel").

Carx "every thing performing a circulatory motion; a wheel; a cart," Mid.Pers. chr "wheel," Parthian cxr "wheel," Ossetic, Khotanese calx "wheel," Av. caxra- "wheel," cognate with Gk. kyklos "circle, wheel," as above. Carxé from carx + nuance suffix -é.

A graph displaying the average → sunspot number over each → sunspot cycle. It presents a shape of skewed → Gaussian: the rise to maximum is faster than the decline to minimum.

→ cycle; → shape.

1) In → Ptolemaic system, a circular → orbit of a body around a point that itself orbits circularly another point. Such a system was formulated to explain some → planetary orbits in terms of → circular motions in a → geocentric cosmology.
2a) Math.: A circle that rolls, externally or internally on another circle, generating an → epicycloid or → hypocycloid.
2b) In → galactic dynamics models describing the → spiral arms, a → perturbation of simple circular orbits. → epicyclic theory.

→ epi-; → cycle.

1) Falak-e tadvir, from Ar. falak al-tadwir, from falak "sphere" + tadwir "causing to turn in a circle."
2) → epi-; → cycle.

The vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas; often called the water cycle.

Hydrologic, pertaining to → hydrology; → cycle.

A time interval lasting 235 → lunations, or about 19 → tropical years (235 = 19 x 12 + 7), after which → lunar phases recur on the same days of the year.

Named after Meton of Athens, a Gk. mathematician, astronomer, geometer, and engineer who used it in 432 B.C., but it was known to the Babylonians by around 500 B.C. and to the Chinese around 600 B.C.; → cycle.

The theory according to which variations in the elements of Earth-Sun geometry are responsible for the sequence of ice ages during the Pleistocene era. The main elements are the varying tilt of the Earth's rotational axis, and the varying eccentricity of the Earth's orbit.

Named after the Serbian mathematician Milutin Milankovitch (1879-1958), who introduced the concept during the first half of the twentieth century.

1) To treat or process used material so that it can be used again.
2) To repeat or pass through a process again. → recycled pulsar.

→ re-; → cycle.

A → pulsar of abnormally low magnetic field and short period. The short period suggests that the pulasr is young, while the low field suggests a very old pulsar. According to theoretical models, a pulsar at some point in its evolution stops functioning as a pulsar. If it resides in a binary system (→ binary pulsar), its magnetic field decays in the interval when the companion evolves and fills its → Roche lobe. The dead pulsar is eventually spun up to life when → mass transfer from the companion begins. The pulsar gains → angular momentum from the infalling gas and increases its spin rate as more gas falls onto it. → Millisecond pulsars that spin hundreds of times per second are thought to be the result of such a transfer (see, e.g., G. Srinivasan, 2010, New Astronomy Reviews 54, 93, and references therein).

→ recycle; → pulsar.

Same as the → solar cycle.

→ solar activity; → cycle.

The periodic variation in frequency or number of solar active events (→ sunspots, → prominences, → flares, and other solar activity) occurring with an interval of about 11 years. The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe (1789-1875), a German apothecary and amateur astronomer, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk. Solar cycle numbering goes back to the 18-th century, when the Cycle 1 peak occurred in 1760. Cycle 23 peaked in 2000, and the following Cycle 24 will reach its maximum in 2013.

→ solar; → cycle.

The period of time, about 22 years, after which the magnetic → polarity of the Sun returns to its earlier state. It consists of two consecutive → solar cycles.

→ solar; → magnetic; → cycle.

→ solar cycle.

→ sunspot; → cycle.