Secular variation

The secular variation of a time series is its long-term non-periodic variation, as opposed to its periodic variation. Of course, whether something is perceived as a secular variation or not depends on the available timescale: what appears to be a secular variation over a time scale of centuries can turn out to be a periodic variation over a time scale of millions of years. Natural quantities often have both periodic and secular variations.

The term secular variation is used wherever time series are applicable such as in economics, operations research, astronomy ( particularly celestial mechanics), etc.

Jack McCormac states in Surveying, 5th ed.: "The angle of declination [the angle by which astronomic north varies with magnetic north] at a particular location is not constant but varies with time. For periods of approximately 150 years, there is a gradual unexplainable shift in the earth's magnetic fields in one direction after which a gradual drift occurs in the other direction to complete the cycle in the next 150-year period. This variation, called secular variation, can be large. There is no known method of accurately predicting the secular change and all that can be done is to make observations of its magnitude at various places around the world. Records kept in London for seceral centuries show a range of magnetic declination from 11oE in 1580 to 24oW in 1820. The time period between extreme eastern and western declinations varies with locality. It can be as short as 50 years or even less and as long as 180 years or more."