Astronomical Cycles by Length

• 23h 56m 4s
  Sidereal dayRotation & tides

  The time for Earth to rotate once relative to the stars, about 4 minutes shorter than a solar day. It is very nearly Earth's true rotation period; the stellar day, measured against the truly fixed stars, is the exact inertial rotation period and is about 0.0084 s longer, the tiny difference arising from precession of the equinoxes.

  Defines sidereal time, the timescale astronomers use to point telescopes and track satellites independent of the Sun.
• 24 hours (86,400 s)
  Mean solar dayRotation & tides

  The average interval between successive passages of the Sun across the same meridian, averaged over the year. It is slightly longer than the sidereal day because Earth must rotate a little farther to face the Sun again after advancing along its orbit.

  The basis of civil timekeeping and the 24-hour clock.
• 24h 50m (about 24.84 hours)
  Tidal (lunar) dayRotation & tides

  The interval between two successive passages of the Moon over the same meridian, roughly 50 minutes longer than a solar day because the Moon advances in its orbit while Earth rotates. In some regions (e.g. parts of the Gulf of Mexico) this produces a single high and low tide per lunar day, the diurnal tide pattern.

  Sets the daily rhythm of the tides, explaining why high tides arrive about 50 minutes later each day. Learn: Tides →
• about 14.77 days (half a synodic month)
  Spring-neap tide cycleRotation & tides

  The roughly fortnightly alternation between large spring tides (at new and full moon, when Sun and Moon align) and smaller neap tides (at the quarter moons), driven by the changing relative positions of the Sun and Moon.

  Governs the fortnightly variation in tidal range, critical for coastal flooding, shipping, and intertidal ecology. Learn: Tides →
• 27.21 days
  Draconic (nodal) monthLunar months

  The time for the Moon to return to the same orbital node, where its orbit crosses the ecliptic. It is shorter than the sidereal month because the lunar nodes slowly regress westward.

  Eclipses occur only when a new or full Moon falls near a node, so the draconic month underlies eclipse seasons and the Saros cycle. Learn: Eclipses → Full profile →
• about 27.3 days (synodic, as seen from Earth); about 25.4 days sidereal at the equator
  Solar (Carrington) rotationRotation & tides

  The Sun's mean rotation period in the standard Carrington frame as seen from the moving Earth. The Sun rotates differentially, so the equator (about 25.4 days sidereal) turns faster than the poles (about 34 days); the Carrington value is the conventional reference.

  Defines the Carrington rotation number used throughout solar physics, with active regions and high-speed solar-wind streams recurring on this about 27-day cadence.
• 27.32 days
  Tropical monthLunar months

  The time for the Moon to return to the same ecliptic longitude, the same point among the zodiac, a shade shorter than the sidereal month because the equinox it is measured from slowly precesses westward.

  The fifth of the standard lunar months; it tracks the Moon against the seasons rather than the fixed stars. The same 27.32 days as the sidereal month below, to a different reference: the drifting equinox, not the stars.
• 27.32 days
  Sidereal monthLunar months

  The time for the Moon to complete one orbit of Earth relative to the fixed stars, returning to the same position against the stellar background.

  The Moon's true orbital period and the baseline from which the other lunar month lengths are derived. Not a repeat of the tropical month above: this one is measured against the fixed stars. The Moon page → Full profile →
• 27.55 days
  Anomalistic monthLunar months

  The time for the Moon to travel from perigee back to perigee, longer than the sidereal month because the line of apsides slowly rotates forward. This period also governs the dominant libration in longitude as orbital speed varies.

  Sets the rhythm of the Moon's varying distance and apparent size, governing perigee 'supermoons' and modulating tidal range. Full profile →
• 29.53 days
  Synodic month (lunation)Lunar months

  The time between successive identical lunar phases, such as new moon to new moon, longer than the sidereal month because Earth moves along its orbit so the Moon must travel farther to restore the same Sun-Earth-Moon geometry. Over this same period Earth traces a small monthly loop around the Earth-Moon barycenter.

  The familiar 'month' of phases underlying lunar and lunisolar calendars. Learn: Synodic vs sidereal → The Moon page → Full profile →
• 87.97 days
  Mercury sidereal orbital periodOrbital periods

  The time Mercury takes to complete one orbit of the Sun relative to the fixed stars, the shortest planetary year in the Solar System.

  Defines Mercury's year and underlies its 3:2 spin-orbit resonance.
• 100.9 days
  Mercury-Mars synodic periodConjunctions

  The average time for Mercury and Mars to return to the same alignment as seen from Earth, set by their very different orbital speeds.

  The fastest of the planet-to-planet conjunction cycles, since quick Mercury laps every other planet in short order.
• 115.88 days
  Mercury synodic periodSynodic periods

  The average time for Mercury to return to the same alignment with the Sun as seen from Earth (for example inferior conjunction to inferior conjunction), set by Mercury's and Earth's differing orbital speeds.

  Determines how often Mercury swings between morning-star and evening-star visibility and the timing of its greatest elongations and transits. Synodic calculator → Mercury page →
• 144.6 days
  Mercury-Venus synodic periodConjunctions

  The average interval between successive conjunctions of Mercury and Venus, the two inner planets, as seen from Earth.

  Both worlds stay near the Sun, so their meetings appear low in bright twilight.
• 177.2 days (about 5.87 months)
  Eclipse season interval (semester)Eclipse & nodes

  The interval of roughly 177 days between successive eclipse seasons, equal to about six synodic months, with eclipses recurring at alternating nodes after this span.

  Explains why solar and lunar eclipses cluster into two periods each year, roughly every six months. Learn: Eclipses →
• 224.70 days
  Venus sidereal orbital periodOrbital periods

  The time Venus takes to complete one orbit of the Sun relative to the fixed stars.

  Shorter than Venus's about 243-day retrograde rotation, making its day longer than its year.
• 333.9 days (about 11 months)
  Venus-Mars synodic periodConjunctions

  The average time between successive conjunctions of Venus and Mars as seen from Earth.

  Brings the brilliant morning or evening star close to the red planet a little less than once a year.
• 346.62 days
  Eclipse (draconic) yearEclipse & nodes

  The time for the Sun to return to the same lunar node along the ecliptic, about 19 days shorter than a tropical year because the nodes regress westward.

  Governs the spacing of eclipse seasons and closely matches an integer count of draconic years in the Saros cycle. Learn: Eclipses → Full profile →
• 354.37 days (12 lunar months)
  Lunar yearLunar months

  Twelve synodic months, the length of a purely lunar calendar year such as the Islamic year. It runs about 11 days shorter than the solar year, so its months drift steadily backward through the seasons.

  The basis of the Islamic (Hijri) calendar, which is why its holy months shift earlier each year.
• 365.2422 days
  Tropical yearYear & seasons

  The time from one vernal equinox to the next, tracking the cycle of the seasons as the Sun returns to the same equinox point.

  Governs the seasons and is the basis for the design of solar calendars such as the Gregorian calendar. Learn: The seasons → Full profile →
• 365.2564 days
  Sidereal yearYear & seasons

  The time Earth takes to complete one orbit around the Sun relative to the fixed stars, returning to the same position against the distant stellar background.

  Earth's true orbital period; slightly longer than the calendar (tropical) year because of the precession of the equinoxes. Learn: Precession → Full profile →
• 365.2596 days
  Anomalistic yearYear & seasons

  The time for Earth to travel from one perihelion to the next, slightly longer than the sidereal year because perihelion slowly advances.

  Tracks Earth's distance-from-Sun cycle and underlies long-term changes in the timing and intensity of the seasons. Learn: Apsidal precession → Full profile →
• 367.49 days
  Neptune synodic periodSynodic periods

  The average time between successive oppositions of Neptune as seen from Earth. Because Neptune crawls along its 165-year orbit, Earth laps it only about two days later each year, so its opposition drifts slowly through the calendar.

  Sets the yearly recurrence of Neptune at opposition, when the distant planet is closest and brightest in a telescope. Synodic calculator → Neptune page →
• 369.66 days
  Uranus synodic periodSynodic periods

  The average time between successive oppositions of Uranus as seen from Earth. Uranus moves slowly along its 84-year orbit, so Earth catches up to the same alignment only about four days later each year.

  Marks the yearly return of Uranus to opposition, when at magnitude 5.7 it is briefly within reach of sharp eyes under a dark sky. Synodic calculator → Uranus page →
• 378.09 days
  Saturn synodic periodSynodic periods

  The average time between successive oppositions of Saturn as seen from Earth; Earth needs about two weeks beyond a year to lap Saturn's slow orbital motion.

  Governs the yearly recurrence of Saturn at opposition, when its rings are best seen, drifting about two weeks later each year. Synodic calculator → Saturn page →
• 398.88 days (about 13 months)
  Jupiter synodic periodSynodic periods

  The average time between successive oppositions of Jupiter as seen from Earth, since Earth laps slower-moving Jupiter a little more than once per year.

  Sets the roughly 13-month cycle of Jupiter oppositions, when the planet is closest, brightest, and best placed for observation. Synodic calculator → Jupiter page →
• 583.92 days (about 1.6 years)
  Venus synodic periodSynodic periods

  The average time for Venus to return to the same configuration relative to the Sun as seen from Earth (e.g. inferior conjunction to inferior conjunction). Five Venus synodic periods nearly equal eight Earth years.

  Controls Venus's alternation between morning-star and evening-star apparitions and the timing of its rare paired transits. Learn: Venus's 8-year cycle → Venus page →
• 1.88 years
  Mars sidereal orbital periodOrbital periods

  The time Mars takes to complete one orbit around the Sun relative to the fixed stars, nearly twice Earth's year.

  Governs the roughly 26-month spacing of favorable Earth-Mars launch windows.
• 733.8 days (about 2 years)
  Mars-Saturn synodic periodConjunctions

  The average interval between conjunctions of Mars and Saturn, the time for faster Mars to catch up to slow-moving Saturn again.

  Mars sweeps past Saturn roughly every two years, a frequent and easy pairing to spot.
• 779.94 days (about 2.14 years)
  Mars synodic periodSynodic periods

  The average time between successive oppositions of Mars as seen from Earth; because Mars moves at a speed closer to Earth's, it takes well over two years for Earth to lap it.

  Defines the roughly 26-month cadence of Mars oppositions and close approaches that drive mission launch windows. Synodic calculator → Mars page →
• 816.4 days (about 2.2 years)
  Mars-Jupiter synodic periodConjunctions

  The average time between conjunctions of Mars and Jupiter as Mars laps the larger, slower giant.

  A frequent bright-planet pairing, with Mars passing Jupiter about every 27 months.
• about 8 years (2,919.6 days)
  Venus pentagram cycle (8-year near-commensurability)Conjunctions

  Five Venus synodic periods total very nearly eight Earth years, so Venus's inferior conjunctions trace a near-perfect five-pointed star against the sky before slowly drifting.

  A celebrated near-commensurability central to Mayan and Babylonian astronomy and a classic demonstration of orbital commensurability. Learn: Venus's 8-year cycle → Venus page → Full profile →
• about 8 years (2,923.5 days)
  OctaeterisEclipse & nodes

  An eight-year luni-solar cycle in which 99 synodic months almost exactly equal eight solar years, after which the Moon phases fall again on nearly the same dates.

  An early Greek calendar cycle, the forerunner of the more accurate 19-year Metonic cycle.
• about 10.9 years (3,987 days)
  TritosEclipse & nodes

  An eclipse cycle of 135 synodic months, after which a similar eclipse recurs near the opposite lunar node. Paired with the Saros and the Inex, it helps sort eclipses into their long family lineages.

  One of the lesser eclipse cycles astronomers use to trace related eclipses across the centuries. Full profile →
• about 11 years (range about 9-14 years)
  Schwabe (sunspot) cycleSolar activity

  The roughly 11-year rise and fall in sunspot number and overall solar activity, measured minimum to minimum or maximum to maximum. Individual cycles range from about 9 to 14 years, so 11 years is only a long-term average.

  The fundamental rhythm of solar activity, driving space weather that affects satellites, radio, and power grids. Learn: The sunspot cycle → The Sun page → Full profile →
• 11.86 years
  Jupiter sidereal orbital periodOrbital periods

  The time Jupiter takes to complete one orbit around the Sun relative to the fixed stars.

  As the most massive planet, Jupiter's near-12-year orbit dominates many solar-system resonances, including the asteroid-belt Kirkwood gaps.
• about 12.78 years (4,669 days)
  Jupiter-Neptune synodic periodConjunctions

  The interval between successive conjunctions of Jupiter and Neptune, when the largest planet laps the most distant of the giants.

  A slow giant-planet alignment tracked in long-term studies of planetary cycles.
• about 13.81 years (5,045 days)
  Jupiter-Uranus synodic periodConjunctions

  The interval between successive conjunctions of Jupiter and Uranus as Jupiter overtakes the slower ice giant.

  One of the recurring outer-planet alignments that pattern the giants over decades.
• about 18 years 11 days (18.03 years)
  SarosEclipse & nodes

  After 223 synodic months the Sun, Moon, and a lunar node return to nearly the same geometry, producing a near-identical eclipse. Because it runs about a third of a day past a whole number, the eclipse shifts roughly 120 degrees westward each cycle.

  The foundational cycle of eclipse prediction since Babylonian times; eclipses are grouped into numbered saros series. Walk a Saros series → Learn: Eclipses → The Moon page → Full profile →
• about 18.6 years
  Principal nutation (18.6-year term)Eclipse & nodes

  The largest periodic nodding of Earth's rotation axis superimposed on the steady axial precession, driven by the regression of the Moon's nodes, with an amplitude of about 9 arcseconds in obliquity.

  Discovered by James Bradley in 1728, it is a distinct axis-motion cycle that must be modeled for precise astrometry, timekeeping, and geodesy. Learn: Precession & nutation → One face of the same 18.6-year motion as the nodal regression and the lunar standstills below.
• 18.61 years
  Lunar nodal precession (regression of the nodes)Eclipse & nodes

  The period for the Moon's orbital nodes to make one complete westward circuit around the ecliptic. Over this cycle the Moon's monthly declination range swings between major and minor standstills.

  The master clock behind eclipse-season drift, the lunar standstill cycle of archaeoastronomical interest, and the largest periodic ocean-tide variation. Learn: The lunar nodes → The Moon page → Full profile → The motion itself; the nutation above and the standstills below are its two visible faces.
• about 18.6 years (major to major)
  Lunar standstillEclipse & nodes

  The widest and narrowest reach of the Moon's monthly swing in declination, driven by the 18.6-year regression of the lunar nodes. A major standstill reaches about 28.6 degrees, a minor about 18.3 degrees.

  The extreme moonrise and moonset that ancient monuments from Stonehenge to Chimney Rock were built to mark. Full profile → Learn: Lunar nodes → The observable face of the 18.6-year nodal regression above, at the horizon.
• about 19 years (6,939.7 days)
  Metonic cycleEclipse & nodes

  A period of 235 synodic months that almost exactly equals 19 tropical years, after which the Moon's phases recur on nearly the same calendar dates. Discovered by the Greek astronomer Meton.

  The basis of luni-solar calendars (Hebrew calendar, ecclesiastical Easter) and of the rough 19-year recurrence of eclipses near the same date. Learn how it works. Full profile →
• about 19.86 years
  Jupiter-Saturn great conjunctionConjunctions

  The interval between successive conjunctions of Jupiter and Saturn, the two slowest-moving bright planets, when they appear closest in the sky. The most recent was in December 2020.

  The longest-known and most historically significant planet-planet alignment cycle, stepping around the ecliptic in a pattern central to early calendrical reckoning. See conjunction dates → Full profile →
• about 22 years
  Hale (magnetic polarity) cycleSolar activity

  The full solar magnetic cycle, spanning two consecutive roughly 11-year Schwabe cycles, after which the Sun's magnetic polarity returns to its original orientation. Discovered by George Ellery Hale in the 1920s.

  The true period of the solar magnetic dynamo, reflected in galactic cosmic-ray intensity at Earth and in odd/even cycle differences. Learn: The sunspot cycle → Full profile →
• about 29 years (10,572 days)
  InexEclipse & nodes

  A span of 358 synodic months, after which an eclipse recurs at nearly the opposite lunar node and roughly the opposite side of the sky. Paired with the Saros it sorts every eclipse into an orderly family tree.

  Astronomers use the Saros and the Inex together to label and trace the entire catalog of eclipse series. Learn: Eclipses → Full profile →
• 29.45 years
  Saturn sidereal orbital periodOrbital periods

  The time Saturn takes to complete one orbit around the Sun relative to the fixed stars.

  Saturn's roughly 29.5-year orbit carries it about one constellation of the zodiac every two and a half years and pairs with Jupiter in the great-conjunction cycle.
• about 35.87 years (13,102 days)
  Saturn-Neptune synodic periodConjunctions

  The time between conjunctions of Saturn and Neptune, two slow outer worlds, as Saturn gradually laps the more distant planet.

  A multi-decade alignment that helps complete the matrix of giant-planet conjunction cycles.
• about 45.36 years (16,567 days)
  Saturn-Uranus synodic periodConjunctions

  The interval between successive conjunctions of Saturn and Uranus as the two slower giants drift past one another.

  The longest of the Saturn pairings, recurring only about once every human generation and a half.
• about 54 years 33 days (3 Saros)
  Exeligmos (triple Saros)Eclipse & nodes

  Three Saros cycles end on a whole number of days, so after one exeligmos an eclipse returns to nearly the same longitude and the same time of day, visible from roughly the same part of Earth. The name is Greek for 'turn of the wheel.'

  It cancels the Saros's one-third-day drift, which otherwise carries each eclipse about 120 degrees west around the globe. Saros series calculator → Learn: Eclipses → Full profile →
• about 59.6 years (three great conjunctions)
  Jupiter-Saturn trigon (triple conjunction)Conjunctions

  Each Jupiter-Saturn conjunction lands about 117 degrees further around the ecliptic, so after three of them, close to sixty years, the meeting returns within about nine degrees of its starting region. Kepler drew these as a slowly turning triangle, the trigon.

  Over roughly eight centuries the conjunction point migrates all the way around the ecliptic and back to a similar region, a long pattern Kepler and earlier astronomers charted. Alignment calculator →
• about 75.3 years
  1P/Halley sidereal orbital periodOrbital periods

  The time the famous comet 1P/Halley takes to complete one orbit around the Sun on its retrograde, highly eccentric path.

  The only short-period comet reliably visible to the naked eye, recorded for over two millennia; it next returns in 2061.
• about 76 years (4 Metonic cycles minus a day)
  Callippic cycleEclipse & nodes

  Four Metonic cycles run about a day long, so Callippus of Cyzicus dropped a single day every 76 years to keep the Moon's phases locked to the calendar even more tightly. It sharpens the 19-year Metonic fit between the months and the years.

  An early Greek refinement of luni-solar timekeeping and a step toward the precise calendars that followed. Full profile →
• 84.02 years
  Uranus sidereal orbital periodOrbital periods

  The time Uranus takes to complete one orbit around the Sun relative to the fixed stars.

  Combined with its extreme axial tilt, Uranus's 84-year orbit gives each pole roughly 21 years of continuous sunlight at a time.
• about 88 years (range about 80-100 years)
  Gleissberg cycleSolar activity

  A centennial-scale modulation of solar activity, with a characteristic period near 88 years, that amplifies and damps the strength of successive 11-year cycles. Named after Wolfgang Gleissberg.

  Explains why some sequences of solar cycles run stronger and others weaker, framing long-term solar-activity predictions and cosmogenic-isotope records.
• 164.8 years
  Neptune sidereal orbital periodOrbital periods

  The time Neptune takes to complete one orbit of the Sun relative to the fixed stars, the longest year of the eight planets.

  Neptune completed its first full orbit since its 1846 discovery only in 2011, a benchmark for the slow dynamics of the outer solar system.
• about 171 years
  Uranus-Neptune synodic periodConjunctions

  The time between conjunctions of the two outermost planets, the slowest such meeting among the major planets, because Uranus barely outpaces Neptune. The most recent fell in 1993 and the next is due around 2165.

  The longest planet-to-planet alignment cycle of the giant planets, a slow metronome in the outer solar system. Alignment calculator →
• about 210 years
  Suess / de Vries cycleSolar activity

  A roughly 210-year (205-210 year) cycle in solar activity detected in cosmogenic isotopes such as carbon-14 and beryllium-10, named after Hans Suess and Hessel de Vries.

  One of the clearest long-term solar periodicities, closely associated with the recurrence of grand solar minima such as the Maunder and Dalton minima.
• 243 years
  Venus transit cycleConjunctions

  Transits of Venus across the Sun recur in a 243-year cycle, with pairs eight years apart separated by gaps of about 121.5 and 105.5 years; the most recent pair occurred in 2004 and 2012.

  Among the rarest predictable astronomical alignments; historical transit expeditions provided the first accurate measurement of the Earth-Sun distance.
• 248 years
  Pluto sidereal orbital periodOrbital periods

  The time the dwarf planet Pluto takes to complete one orbit of the Sun relative to the fixed stars.

  Pluto's eccentric, inclined orbit is locked in a stable 3:2 mean-motion resonance with Neptune, and for part of each orbit it comes closer to the Sun than Neptune. Pluto page →
• about 25,920 years
  Axial precession (Great Year)Milankovitch

  The slow, conical wobble of Earth's rotational axis, which traces one full circle against the fixed stars roughly every 25,920 years, causing the equinoxes to drift around Earth's orbit and the identity of the pole star to change. This complete circuit is the classical Great Year or Platonic Year.

  It shifts which star marks the celestial pole, slowly changes which constellations the Sun appears against on a given date over millennia, and combines with orbital precession to produce the climatic precession that paces ice ages. Learn: Precession → Full profile →
• about 41,000 years
  Axial obliquity (tilt) cycleMilankovitch

  Earth's axial tilt oscillates between roughly 22.1 and 24.5 degrees over about 41,000 years; the current tilt is about 23.4 degrees and slowly decreasing.

  Greater tilt means stronger seasons and more intense high-latitude summers; this cycle dominated the rhythm of ice ages during the early Pleistocene (the '41,000-year world'). Learn: Milankovitch cycles → Learn: The seasons →
• about 100,000 years
  Short eccentricity cycleMilankovitch

  A roughly 100,000-year variation in the shape of Earth's orbit (its eccentricity), arising from a cluster of terms near 95,000 and 125,000 years driven mainly by Jupiter and Venus. The orbit shifts between more circular and more elliptical.

  It matches the dominant about 100,000-year rhythm of the major glacial-interglacial cycles over roughly the past million years.
• about 112,000 years
  Apsidal precession of Earth's orbitMilankovitch

  The slow rotation of Earth's elliptical orbit in space, so that the line of apsides (perihelion-to-aphelion axis) sweeps once around relative to the fixed stars in about 112,000 years. It is driven mainly by gravitational tugs from Jupiter and Saturn.

  Combined with axial precession it produces the about 19,000- to 23,000-year climatic precession cycles that govern where in the orbit the seasons fall. Learn: Apsidal precession →
• about 405,000 years
  Long eccentricity cycleMilankovitch

  A very stable about 405,000-year oscillation in Earth's orbital eccentricity driven chiefly by the gravitational interaction between Jupiter and Venus. It is the most regular and long-lasting of the orbital cycles.

  Its stability over hundreds of millions of years makes it an astronomical 'metronome' used to calibrate the geologic time scale (astrochronology). Learn: Milankovitch cycles →
• about 230 million years
  Galactic year (cosmic year)Galactic & deep time

  The time the Sun takes to complete one orbit around the center of the Milky Way at a galactocentric distance of about 26,000-27,000 light-years, traveling at roughly 220-230 km/s. Mainstream estimates span about 225 to 250 million years.

  The fundamental timescale of the Solar System's journey through the Galaxy; the Sun has completed only about 20 galactic years since its formation. Learn: The galactic year →

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