||It has been suggested that Secular phenomena be merged into this article. (Discuss) Proposed since August 2013.|
The secular variation of a time series is its long-term non-periodic variation (see Decomposition of time series). Whether something is perceived as a secular variation or not depends on the available timescale: a secular variation over a time scale of centuries may be part of a periodic variation over a time scale of millions of years. Natural quantities often have both periodic and secular variations. Secular variation is sometimes called secular trend or secular drift when the emphasis is on a linear long-term trend.
The term secular variation is used wherever time series are applicable in economics, operations research, biological anthropology, astronomy (particularly celestial mechanics) such as VSOP (planets) etc.
In astronomy, secular phenomena refers to long-term perturbations in the motion of planets. The precession of the Earth's axis has a period of 25,771.5 years, so on a much shorter time frame it appears to be a "drift" of the position of the equinox in the plane of the ecliptic of approximately one degree every 71.6 years.
Geomagnetic secular variation
Geomagnetic secular variation refers to changes in the Earth's magnetic field. The field has variations on time scales from milliseconds to millions of years, but rapid changes mostly come from currents in the ionosphere and magnetosphere. The secular variation is the changes over periods of a year or more, reflecting changes in the Earth's core. Phenomena associated with this secular variation include geomagnetic jerk, westward drift and geomagnetic reversals.
A secular trend has been observed in the age of onset of puberty (menarche/first menstruation and beginning of breast development) of girls around the world beginning roughly 4 months earlier each decade. This is largely believed to be caused by nutritional changes in children over time.
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