* #REDIRECT[[Secular variation]] {{R from merge}}▼{{More references|date=April 2011}}
In [[astronomy]], '''secular phenomena''' are contrasted with phenomena observed to repeat periodically. In particular, astronomical [[ephemeris|ephemerides]] use ''secular'' to label the longest-lasting or non-oscillatory perturbations in the motion of planets, as opposed to ''periodic'' perturbations which exhibit repetition over the course of a time frame of interest. [[Solar system]] ephemerides are essential for the navigation of [[spacecraft]] and for all kinds of space observations of the [[planets]], their [[natural satellite]]s, [[star]]s and [[galaxies]].
Most of the known perturbations to motion in stable, regular, and well-determined dynamical systems tend to be periodic at some level, but in many-body systems, [[chaos|chaotic dynamics]] result in some effects which are one-way (for example, [[planetary migration]]).
== In the solar system ==
Secular phenomena create variations in the orbits of the Moon and the planets. The [[Sun|solar]] [[emission spectrum]] and the [[solar wind]] are undergoing [[Sun#Possible long-term cycle|secular trends]] due to [[Habitable zone#Galactic habitable zone|migration]] through the [[galactic plane]], leading to [[Sun#Motion and ___location within the galaxy|effects]], that may impact on [[climate]] and cause [[extinction event]]s. [[Solar system]] efemerides are essencial for [[spacecraft]] [[navigation]] and [[astronomical observation]]s.
The secular effects of solar [[accretion]] of [[Dark Matter]] at some rate, during billions of years, is also attracting interest.<ref>L.Iorio; [http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.1697v7.pdf ''Effect of Sun and Planet-Bound Dark Matter on Planet and Satellite Dynamics in the Solar System''], arXiv (May 2010)</ref>
=== Moon ===
The [[secular acceleration of the Moon]] depends on [[tidal force]]s. It was discovered early and has received a number of explanations.<ref>Jyri B. Kolesnik; [http://adsabs.harvard.edu/abs/2001jsrs.meet..231K ''Revision of the tidal acceleration of the Moon and the tidal deceleration of the Earth's rotation from historical optical observations of planets''], in ISBN 2-901057-45-4 (2001) pp. 231 - 234.</ref>
=== Earth ===
Depending on what time frames are considered, perturbations can appear secular even if they are actually periodic. An example of this is the [[Axial precession (astronomy)|precession of the Earth's axis]] considered over the time frame of a few hundred or thousand years. When viewed in this time frame the so-called "precession of the equinoxes" can appear to mimic a secular phenomenon since the axial precession takes 25,771.5 years and monitoring it over a much smaller timeframe appears to simply result in a "drift" of the position of the [[equinox]] in the [[plane of the ecliptic]] of approximately one degree every 71.6 years,<ref>{{cite book
|last = Lowrie
|first = William
|title = Fundamentals of Geophysics
|publisher = [[Cambridge University Press]]
|date = 2004
|isbn=0521461642
}}</ref> influencing the [[Milankovitch cycles]].<ref>Jurij B. Kolesnik; [http://adsabs.harvard.edu/abs/2001jsrs.meet..119K ''A new appraoch to interpretation of the non-precessional equinox motion''], in Journées 2000 - systèmes de référence spatio-temporels. J2000, a fundamental epoch for origins of reference systems and astronomical models, Paris, Septembre 2000, edited by N. Capitaine, Observatoire de Paris (2001), pp. 119 – 120. ISBN 2-901057-45-4 </ref>
[[Magnetic declination]] varies both from place to place, and with the passage of time. The spatial variation reflects irregularities of the flows deep in the earth; in some areas, deposits of [[iron]] [[ore]] or [[magnetite]] in the Earth's crust may contribute substantially and secular changes to these flows result in slow changes to the [[field strength]] and direction at the same point on the Earth. The declination in a given area will most likely change slowly over time, the order of 2–2.5 degrees every hundred years or so, depending on distance from the magnetic poles.
=== The planets ===
''[[Secular variations of the planetary orbits]]'' is a concept describing [[secular variation|long-term trends]] in the orbits of the [[planet]]s [[Mercury (planet)|Mercury]] to [[Neptune]]. Several attempts have from time to time been undertaken to analyze and predict such [[gravitation]]al deviations from ordinary satellite orbits. Others are often referred to as post [[Kepler's laws of planetary motion|keplerian]] effects.
''Variations Séculaires des Orbites Planétaires'' (VSOP) is a modern [[numerical]] [[model]]<ref>P. Bretagnon; [http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1982A%26A...114..278B&data_type=PDF_HIGH&type=PRINTER&filetype=.pdf "Théorie du mouvement de l'ensemble des planètes. Solution VSOP82"], (PDF 1.23MB), ''[[Astronomy & Astrophysics]]'' '''114''' (1982) 278–288.</ref>, that tries to address the problem.
[[Jet Propulsion Laboratory]] has an important role in providing scientific efemeris data. In spite of methods to deal with perturbations of numerous [[asteroids]], most of whose masses and orbits are poorly known, remains a tiny secular trend, causing the [[JPL]] to revise its published ephemerides at intervals of 20 years. Russian astronomers have taken notice of this problem. <ref>Kharin, A. S. and [[Yuri B. Kolesnik|Kolesnik, Y. B.]]; ''On the Errors of the Ephemerides Derived from Optical Observations of Planets.'' (1990), [[IAU]] SYMP.141 P.189, 1989.</ref><ref>[[Georgij A. Krasinsky]] and [[Victor A. Brumberg]], ''Secular Increase of Astronomical Unit from Analysis of the Major Planet Motions, and its Interpretation'' [http://iau-comm4.jpl.nasa.gov/GAKVAB.pdf Celestial Mechanics and Dynamical Astronomy 90: 267–288, (2004)].</ref>
Kolesnik has followed this up with an ambitious undertaking, which can be condensed in an [[empiric]] formula, that may help solving JPL's problem.<ref>[[Yuri B. Kolesnik]]; [http://adsabs.harvard.edu/abs/2002HiA....12..330K ''Analysis of the secular variations of longitudes of the Sun, Mercury and Venus from optical observations''], in Highlights of Astronomy, Vol. 12, as presented at the XXIVth General Assembly of the IAU - 2000 [Manchester, UK, 7 - 18 August 2000]. Edited by H. Rickman. San Francisco, CA: Astronomical Society of the Pacific, ISBN 1-58381-086-2, 2002, p. 330 – 333.</ref>
The net result is that the planet accelerates in its orbit, while slowly falling toward the sun. The relation for the change in angular velocity is given by
:<math>\frac{d\omega}{dt}= 3\omega{H_0}</math>
where 1/<math>{H_0}</math> is the [[Hubble's law#Units derived from the Hubble constant|Hubble time]] and '''ω''' is the increasing [[angular velocity]]. This tiny change will cause the Earth to fall roughly 22 meters closer to the Sun per year. <ref>Y. B Kolesnik;[http://adsabs.harvard.edu/full/2001ASPC..245...83K ''On the Relationship Between Dynamical Time and Atomic Time''], Astrophysical Ages and Times Scales, ASP Conference Series Vol. 245. Edited by Ted von Hippel, Chris Simpson, and Nadine Manset. San Francisco: Astronomical Society of the Pacific, ISBN: 1-58381-083-8, 2001., p.83</ref> One theoretical explanation claims that this secular acceleration is of a [[Physical cosmology|cosmological]] origin. <ref>Masreliez C. J.; [http://redshift.vif.com/JournalFiles/V11NO4PDF/V11N4MA2.pdf ''Scale Expanding Cosmos Theory II–Cosmic Drag''], [[Apeiron (tidskrift)|Apeiron]] Okt (2004)</ref>
== See also ==
* [[Nemesis (hypothetical star)]]
* [[Parameterized post-Newtonian formalism]]
== Notes and References ==
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