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Axial tilt
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{{short description|Angle between the rotational axis and orbital axis of a body}}{{Redirect|Obliquity|the book|Obliquity (book)}}{{Use dmy dates|date=October 2017}}In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane.BOOK
, U.S. Naval Observatory Nautical Almanac Office
, P. Kenneth Seidelmann
, 1992
, Explanatory Supplement to the Astronomical Almanac
, 733
, University Science Books
, 978-0-935702-68-2
, It differs from orbital inclination.At an obliquity of 0 degrees, the two axes point in the same direction; i.e., the rotational axis is perpendicular to the orbital plane. Earth's obliquity oscillates between 22.1 and 24.5 degreesWEB,weblink Earth Is tilted, timeanddate.com, 2017-08-25, on a 41,000-year cycle; Earth's mean obliquity is currently {{circle of latitude|tropical|convert}} and decreasing.
Over the course of an orbital period, the obliquity usually does not change considerably, and the orientation of the axis remains the same relative to the background of stars. This causes one pole to be directed more toward the Sun on one side of the orbit, and the other pole on the other sideâ€”the cause of the seasons on Earth.">

## Standards {| class"infobox" style"width: 280px;"

|-
| style="padding-top: 12px;" | (File:Planet axis comparison.png|280px|The axial tilt of Earth, Uranus, and Venus)
|}
|The positive pole of a planet is defined by the right-hand rule: if the fingers of the right hand are curled in the direction of the rotation then the thumb points to the positive pole. The axial tilt is defined as the angle between the direction of the positive pole and the normal to the orbital plane. The angles for Earth, Uranus and Venus are approximately 23Â°, 97Â°, and 177Â° respectively.
There are two standard methods of specifying tilt. The International Astronomical Union (IAU) defines the north pole of a planet as that which lies on Earth's north side of the invariable plane of the Solar System;Explanatory Supplement 1992, p. 384 under this system, Venus is tilted 3Â° and spins retrograde, opposite that of most of the other planets.CORREIA, ALEXANDRE C. M. >AUTHOR2=LASKAR, JACQUES TITLE=LONG-TERM EVOLUTION OF THE SPIN OF VENUS I. THEORYVOLUME=163PAGES=1â€“23URL=HTTP://WWW.IMCCE.FR/EQUIPES/ASD/PREPRINTS/PREP.2002/VENUS1.2002.PDFBIBCODE=2003ICAR..163....1C, CORREIA, A. C. M. >AUTHOR2=LASKAR, J., 2003journal=Icarus, 163, 1url=http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus2.2002.pdfbibcode=2003Icar..163...24C, The IAU also uses the right-hand rule to define a positive poleJOURNAL, Report of the IAU/IAG Working Group on cartographic coordinates and rotational elements: 2006, Celestial Mechanics and Dynamical Astronomy, 98, 3, 155â€“180, 10.1007/s10569-007-9072-y, 2007, Seidelmann, P. Kenneth, Archinal, B. A., a'Hearn, M. F., Conrad, A., Consolmagno, G. J., Hestroffer, D., Hilton, J. L., Krasinsky, G. A., Neumann, G., Oberst, J., Stooke, P., Tedesco, E. F., Tholen, D. J., Thomas, P. C., Williams, I. P., for the purpose of determining orientation. Using this convention, Venus is tilted 177Â° ("upside down").

## Earth

missing image!
- AxialTiltObliquity.png -
Earth's axial tilt (obliquity) is currently about 23.4Â°.
Earth's orbital plane is known as the ecliptic plane, and Earth's tilt is known to astronomers as the obliquity of the ecliptic, being the angle between the ecliptic and the celestial equator on the celestial sphere.BOOK
, U.S. Naval Observatory Nautical Almanac Office
, U.K. Hydrographic Office
, H.M. Nautical Almanac Office
, 2008
, The Astronomical Almanac for the Year 2010
, M11
, US Government Printing Office
, 978-0-7077-4082-9
, It is denoted by the Greek letter Îµ.Earth currently has an axial tilt of about 23.44Â°."Glossary" in Astronomical Almanac Online. (2018). Washington DC: United States Naval Observatory. s.v. obliquity. This value remains about the same relative to a stationary orbital plane throughout the cycles of axial precession.BOOK
, Chauvenet, William
, 1906
, A Manual of Spherical and Practical Astronomy
, J. B. Lippincott & Co., J. B. Lippincott
, 1, 604â€“605
, But the ecliptic (i.e., Earth's orbit) moves due to planetary perturbations, and the obliquity of the ecliptic is not a fixed quantity. At present, it is decreasing at a rate of about 47â€³ per century (see details in Short term below).

### History

Earth's obliquity may have been reasonably accurately measured as early as 1100 BC in India and China.JOURNAL
, Wittmann, A.
, 1979
, The Obliquity of the Ecliptic
, Astronomy and Astrophysics
, 73, 1â€“2, 129â€“131
, 1979A&A....73..129W
, The ancient Greeks had good measurements of the obliquity since about 350 BC, when Pytheas of Marseilles measured the shadow of a gnomon at the summer solstice.BOOK
, Gore, J. E.
, 1907
, Astronomical Essays Historical and Descriptive
, 61
, About 830 AD, the Caliph Al-Mamun of Baghdad directed his astronomers to measure the obliquity, and the result was used in the Arab world for many years.BOOK
, Marmery, J. V.
, 1895
, Progress of Science
, 33
, In 1437, Ulugh Beg determined the Earth's axial tilt as 23Â°30â€²17â€³ (23.5047Â°).BOOK, L.P.E.A., SÃ©dillot, ProlÃ©gomÃ¨nes des tables astronomiques d'OlougBeg: Traduction et commentaire, Paris, Firmin Didot FrÃ¨res, 1853, 87 & 253, It was widely believed, during the Middle Ages, that both precession and Earth's obliquity oscillated around a mean value, with a period of 672 years, an idea known as trepidation of the equinoxes. Perhaps the first to realize this was incorrect (during historic time) was Ibn al-Shatir in the fourteenth centuryBOOK
, Saliba, George
, 1994
, A History of Arabic Astronomy: Planetary Theories During the Golden Age of Islam
, 235,
and the first to realize that the obliquity is decreasing at a relatively constant rate was Fracastoro in 1538.BOOK
, Dreyer, J. L. E.
, 1890
, Tycho Brahe
, 355
, The first accurate, modern, western observations of the obliquity were probably those of Tycho Brahe from Denmark, about 1584,Dreyer (1890), p. 123 although observations by several others, including al-Ma'mun, al-Tusi,BOOK
, Sayili, Aydin
, 1981
, The Observatory in Islam
, 78, Purbach, Regiomontanus, and Walther, could have provided similar information.

### Seasons

Earth's axis remains tilted in the same direction with reference to the background stars throughout a year (regardless of where it is in its orbit). This means that one pole (and the associated hemisphere of Earth) will be directed away from the Sun at one side of the orbit, and half an orbit later (half a year later) this pole will be directed towards the Sun. This is the cause of Earth's seasons. Summer occurs in the Northern hemisphere when the north pole is directed toward the Sun. Variations in Earth's axial tilt can influence the seasons and is likely a factor in long-term climate change (also see Milankovitch cycles).(File:axial_tilt_vs_tropical_and_polar_circles.svg|thumb|center|420px|Relationship between Earth's axial tilt (Îµ) to the tropical and polar circles)

### Oscillation

#### Short term

(File:Obliquity of the ecliptic laskar.PNG|thumb|Obliquity of the ecliptic for 20,000 years, from Laskar (1986). The red point represents the year 2000.)The exact angular value of the obliquity is found by observation of the motions of Earth and planets over many years. Astronomers produce new fundamental ephemerides as the accuracy of observation improves and as the understanding of the dynamics increases, and from these ephemerides various astronomical values, including the obliquity, are derived.Annual almanacs are published listing the derived values and methods of use. Until 1983, the Astronomical Almanac's angular value of the mean obliquity for any date was calculated based on the work of Newcomb, who analyzed positions of the planets until about 1895:
{{math|Îµ {{=}} 23Â° 27â€² 8.26â€³ âˆ’ 46.845â€³ T âˆ’ 0.0059â€³ T2 + {{val|0.00181}}â€³ T3}}
where {{math|Îµ}} is the obliquity and {{math|T}} is tropical centuries from B1900.0 to the date in question.BOOK
, U.S. Naval Observatory Nautical Almanac Office
, H.M. Nautical Almanac Office
, 1961
, Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac
, H.M. Stationery Office
, Section 2B
, From 1984, the Jet Propulsion Laboratory's DE series of computer-generated ephemerides took over as the fundamental ephemeris of the Astronomical Almanac. Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated:
{{math|Îµ {{=}} 23Â° 26â€² 21.448â€³ âˆ’ 46.8150â€³ T âˆ’ 0.00059â€³ T2 + {{val|0.001813}}â€³ T3}}
where hereafter {{math|T}} is Julian centuries from J2000.0.BOOK
, U.S. Naval Observatory
, H.M. Nautical Almanac Office
, 1989
, The Astronomical Almanac for the Year 1990
, B18
, US Government Printing Office
, 978-0-11-886934-8
, JPL's fundamental ephemerides have been continually updated. For instance, the Astronomical Almanac for 2010 specifies:Astronomical Almanac 2010, p. B52
{{math|Îµ {{=}} 23Â° 26â€² 21.406â€³ âˆ’ {{val|46.836769}}â€³ T âˆ’ {{val|0.0001831}}â€³ T2 + {{val|0.00200340}}â€³ T3 âˆ’ 5.76â€³ Ã— 10âˆ’7 T4 âˆ’ 4.34â€³ Ã— 10âˆ’8 T5}}
These expressions for the obliquity are intended for high precision over a relatively short time span, perhaps {{math|Â±}} several centuries.BOOK
, Newcomb, Simon
, 1906
, A Compendium of Spherical Astronomy
, Macmillan Publishers, MacMillan
, 226â€“227
, J. Laskar computed an expression to order {{math|T10}} good to 0.02â€³ over 1000 years and several arcseconds over 10,000 years.
{{math|Îµ {{=}} 23Â° 26â€² 21.448â€³ âˆ’ 4680.93â€³ t âˆ’ 1.55â€³ t2 + 1999.25â€³ t3 âˆ’ 51.38â€³ t4 âˆ’ 249.67â€³ t5 âˆ’ 39.05â€³ t6 + 7.12â€³ t7 + 27.87â€³ t8 + 5.79â€³ t9 + 2.45â€³ t10}}
where here {{math|t}} is multiples of 10,000 Julian years from J2000.0.See table 8 and eq. 35 in JOURNAL
, 1986
, Secular terms of classical planetary theories using the results of general theory
, Astronomy and Astrophysics
, 157, 59â€“70
, 1986A&A...157...59L
, and erratum to articleLASKAR >FIRST = J., Erratum: Secular terms of classical planetary theories using the results of general theory, Astronomy and Astrophysics, 164, 1986, 437, 1986A&A...164..437L, Units in article are arcseconds, which may be more convenient.These expressions are for the so-called mean obliquity, that is, the obliquity free from short-term variations. Periodic motions of the Moon and of Earth in its orbit cause much smaller (9.2 arcseconds) short-period (about 18.6 years) oscillations of the rotation axis of Earth, known as nutation, which add a periodic component to Earth's obliquity.Explanatory Supplement (1961), sec. 2CWEB
, Basics of Space Flight, Chapter 2
, 29 October 2013
, 26 March 2015
, Jet Propulsion Laboratory/NASA
, The true or instantaneous obliquity includes this nutation.BOOK
, Meeus, Jean
, 1991
, Chapter 21
, Astronomical Algorithms
, Willmann-Bell
, 978-0-943396-35-4
,

#### Long term

Using numerical methods to simulate Solar System behavior, long-term changes in Earth's orbit, and hence its obliquity, have been investigated over a period of several million years. For the past 5 million years, Earth's obliquity has varied between {{nowrap|22Â° 2â€² 33â€³}} and {{nowrap|24Â° 30â€² 16â€³}}, with a mean period of 41,040 years. This cycle is a combination of precession and the largest term in the motion of the ecliptic. For the next 1 million years, the cycle will carry the obliquity between {{nowrap|22Â° 13â€² 44â€³}} and {{nowrap|24Â° 20â€² 50â€³}}.JOURNAL
, Berger, A.L.
, 1976
, Obliquity and Precession for the Last 5000000 Years
, Astronomy and Astrophysics
, 51, 1, 127â€“135
, 1976A&A....51..127B
, The Moon has a stabilizing effect on Earth's obliquity. Frequency map analysis conducted in 1993 suggested that, in the absence of the Moon, the obliquity can change rapidly due to orbital resonances and chaotic behavior of the Solar System, reaching as high as 90Â° in as little as a few million years (also see Orbit of the Moon).JOURNAL
, Robutel, P.
, 1993
, The Chaotic Obliquity of the Planets
, Nature (journal), Nature
, 361
, 6413
, 608â€“612
, 1993Natur.361..608L
, 10.1038/361608a0
, yes
, 23 November 2012
,
, JOURNAL
, Joutel, F.
, Robutel, P.
, 1993
, Stabilization of the Earth's Obliquity by the Moon
, Nature
, 361, 6413, 615â€“617
, 1993Natur.361..615L
, 10.1038/361615a0
, However, more recent numerical simulationsJOURNAL
, Lissauer, J.J.
, Barnes, J.W.
, Chambers, J.E.
, 2011
, Obliquity variations of a moonless Earth
, Icarus (journal), Icarus
, 217, 1, 77â€“87
, 10.1016/j.icarus.2011.10.013
, 2012Icar..217...77L
, made in 2011 indicated that even in the absence of the Moon, Earth's obliquity might not be quite so unstable; varying only by about 20â€“25Â°.To resolve this contradiction, diffusion rate of obliquity has been calculated, and it was found that it takes more than billions of years for Earth's obliquity to reach near 90Â°.JOURNAL, Li, Gongjie, Batygin, Konstantin, 20 July 2014, On the Spin-axis Dynamics of a Moonless Earth, Astrophysical Journal, 790, 1, 69â€“76, 1404.7505, 2014ApJ...790...69L, 10.1088/0004-637X/790/1/69, The Moon's stabilizing effect will continue for less than 2 billion years. As the Moon continues to recede from Earth due to tidal acceleration, resonances may occur which will cause large oscillations of the obliquity.JOURNAL
, Ward, W.R.
, 1982
, Comments on the Long-Term Stability of the Earth's Obliquity
, Icarus (journal), Icarus
, 50, 2â€“3, 444â€“448
, 1982Icar...50..444W
, 10.1016/0019-1035(82)90134-8
, {{multiple image
|direction = horizontal
|align= center
|width1= 264
|width2= 272
|image1=Obliquity berger -5000000 to 0.png
|image2=Obliquity berger 0 to 1000000.png
|footer=Long-term obliquity of the ecliptic. Left: for the past 5 million years; note that the obliquity varies only from about 22.0Â° to 24.5Â°. Right: for the next 1 million years; note the approx. 41,000-year period of variation. In both graphs, the red point represents the year 1850. (Source: Berger, 1976).
}}

## Solar System bodies

All four of the innermost, rocky planets of the Solar System may have had large variations of their obliquity in the past. Since obliquity is the angle between the axis of rotation and the direction perpendicular to the orbital plane, it changes as the orbital plane changes due to the influence of other planets. But the axis of rotation can also move (axial precession), due to torque exerted by the sun on a planet's equatorial bulge. Like Earth, all of the rocky planets show axial precession. If the precession rate were very fast the obliquity would actually remain fairly constant even as the orbital plane changes.JOURNAL, William Ward, Large-Scale Variations in the Obliquity of Mars, Science, 181, 4096, 260â€“262, 20 July 1973, 10.1126/science.181.4096.260, 17730940, 1973Sci...181..260W, The rate varies due to tidal dissipation and core-mantle interaction, among other things. When a planet's precession rate approaches certain values, orbital resonances may cause large changes in obliquity. The amplitude of the contribution having one of the resonant rates is divided by the difference between the resonant rate and the precession rate, so it becomes large when the two are similar. Mercury and Venus have most likely been stabilized by the tidal dissipation of the Sun. Earth was stabilized by the Moon, as mentioned above, but before its capture, Earth, too, could have passed through times of instability. Mars's obliquity is quite variable over millions of years and may be in a chaotic state; it varies as much as 0Â° to 60Â° over some millions of years, depending on perturbations of the planets.JOURNAL
, Touma, J.
, Wisdom, J.
, 1993
, The Chaotic Obliquity of Mars
, Science (journal), Science
, 259, 5099, 1294â€“1297
, 1993Sci...259.1294T
, 10.1126/science.259.5099.1294, 17732249
, Some authors dispute that Mars's obliquity is chaotic, and show that tidal dissipation and viscous core-mantle coupling are adequate for it to have reached a fully damped state, similar to Mercury and Venus.JOURNAL
, Correia, Alexandre C.M
, Mercury's capture into the 3/2 spin-orbit resonance including the effect of core-mantle friction
, Icarus, 2009
, 10.1016/j.icarus.2008.12.034
, 0901.1843
, 201
, 1
, 1â€“11, 2009Icar..201....1C,
The occasional shifts in the axial tilt of Mars have been suggested as an explanation for the appearance and disappearance of rivers and lakes over the course of the existence of Mars. A shift could cause a burst of methane into the atmosphere, causing warming, but then the methane would be destroyed and the climate would become arid again.JOURNAL, Rebecca Boyle, Methane burps on young Mars helped it keep its liquid water, New Scientist, 7 October 2017,weblink JOURNAL, Edwin Kite, et al, Methane bursts as a trigger for intermittent lake-forming climates on post-Noachian Mars, Nature Geoscience, 10, 10, 737â€“740, 2 October 2017, 10.1038/ngeo3033, 2017NatGe..10..737K,weblink The obliquities of the outer planets are considered relatively stable.{|class="wikitable" style="margin: 0.5em auto; text-align:center;"|+ Axis and rotation of selected Solar System bodies! rowspan=3 style="background:#ccc; | Body! colspan=4 style="background:#F2FEEC;"| NASA, J2000.0Planetary Fact Sheets, atweblink colspan=4 style="background:#edf3fe;" | IAU, 0 January 2010, 0h TTAstronomical Almanac 2010, pp. B52, C3, D2, E3, E55! style="background: #F2FEEC;" rowspan=2 | Axial tilt(degrees)! style="background: #F2FEEC;" colspan=2 | North Pole! style="background: #F2FEEC;" rowspan=2 |Rotation(hours)! style="background: #edf3fe;" rowspan=2 | Axial tilt(degrees)! style="background: #edf3fe;" colspan=2 | North Pole! style="background: #edf3fe;" rowspan=2 |Rotation(deg/day)
! style="background: #F2FEEC;"| R.A. (degrees)! style="background: #F2FEEC;"| Dec. (degrees)! style="background: #edf3fe;"| R.A. (degrees)! style="background: #edf3fe;"| Dec. (degrees)
Sun >| 14.18
Mercury (planet)>Mercury 0.03 281.01 61.42 1407.6 0.01 281.01 61.45 6.14
Venus >| âˆ’1.48
Earth >| 360.99
Moon >| 13.18
Mars >| 350.89
Jupiter >| 870.54D
Saturn >| 810.79D
Uranus >| âˆ’501.16D
Neptune >| 536.31D
PlutoE >| âˆ’56.36
!colspan=9 style="font-weight: normal; text-align: left; padding: 8px 16px; font-size: 0.85em;"|A with respect to the ecliptic of 1850B at 16Â° latitude; the Sun's rotation varies with latitudeC with respect to the ecliptic; the Moon's orbit is inclined 5.16Â° to the eclipticD from the origin of the radio emissions; the visible clouds generally rotate at different rateE NASA lists the coordinates of Pluto's positive pole; values in (parentheses) have been reinterpreted to correspond to the north/negative pole.

## Extrasolar planets

{{further|Exoplanet#Rotation and axial tilt}}The stellar obliquity {{math|Ïˆs}}, i.e. the axial tilt of a star with respect to the orbital plane of one of its planets, has been determined for only a few systems. But for 49 stars as of today, the sky-projected spin-orbit misalignment {{math|Î»}} has been observed,WEB
, Heller, R.
, Holt-Rossiter-McLaughlin Encyclopaedia
, 24 February 2012
, which serves as a lower limit to {{math|Ïˆs}}. Most of these measurements rely on the Rossiterâ€“McLaughlin effect. So far, it has not been possible to constrain the obliquity of an extrasolar planet. But the rotational flattening of the planet and the entourage of moons and/or rings, which are traceable with high-precision photometry, e.g. by the space-based Kepler space telescope, could provide access to {{math|Ïˆp}} in the near future.Astrophysicists have applied tidal theories to predict the obliquity of extrasolar planets. It has been shown that the obliquities of exoplanets in the habitable zone around low-mass stars tend to be eroded in less than 109 years,JOURNAL
, Heller, R.
, Leconte, J.
, Barnes, R.
, Tidal obliquity evolution of potentially habitable planets
, Astronomy and Astrophysics
, 2011
, 528, A27
, 2011A&A...528A..27H
, 10.1051/0004-6361/201015809, 1101.2156,
JOURNAL
, Heller, R.
, Leconte, J.
, Barnes, R.
, 2011
, Habitability of Extrasolar Planets and Tidal Spin Evolution
, Origins of Life and Evolution of Biospheres
, 41, 6, 539â€“43
, 2011OLEB...41..539H
, 10.1007/s11084-011-9252-3, 22139513
, 1108.4347, which means that they would not have seasons as Earth has.

## References

{{Reflist|30em}}

• National Space Science Data Center
• JOURNAL, 10.1007/s10569-007-9072-y, Seidelmann, P. Kenneth, Archinal, Brent A., A'Hearn, Michael F., 3, Conrad, Albert R., Consolmagno, Guy J., Hestroffer, Daniel, Hilton, James L., Krasinsky, Georgij A., Neumann, Gregory A., Oberst, JÃ¼rgen, Stooke, Philip J., Tedesco, Edward F., Tholen, David J., Thomas, Peter C., Williams, Iwan P., 2007, Report of the IAU/IAG Working Group on cartographic coordinates and rotational elements: 2006, Celestial Mechanics and Dynamical Astronomy, 98, 3, 155â€“180, 2007CeMDA..98..155S, {{sfnRef, Seidelmann Archinal A'hearn et al., 2007, }}
• Obliquity of the Ecliptic Calculator

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