Theseasonal year is the time between successive recurrences of aseasonal event such as theflooding of ariver, themigration of a species of bird, or the flowering of a species of plant.
The need for farmers to predict seasonal events led to the development ofcalendars. However, the variability from year to year of seasonal events (due toclimate change or just random variation) makes the seasonal year very hard to measure. This means that calendars are based onastronomical years (which are regular enough to be easily measured) as surrogates for the seasonal year. For example, theancient Egyptians used theheliacal rising ofSirius to predict the flooding of theNile.
A study of temperature records over the past 300 years[1] suggests that the seasonal year is governed by theanomalistic year rather than thetropical year.This suggestion is surprising because theseasons have been thought to be governed by thetilt of the Earth's axis (seeEffect of sun angle on climate). The two types of years differ by a mere 4 days over 300 years, so Thompson's result may not be significant. However, the result is not unreasonable. The seasons can be considered to be anoscillating system driven by two inputs with slightly different frequencies: the total input of energy from the sun varies with the anomalistic year, while the distribution of this energy between the hemispheres varies with the tropical year. In other physical situations, oscillating systems driven by two similar frequencies can latch onto either one. One point that must be considered is that the oscillation arising from the tilt of the axis is much greater than that arising from the distance of the sun.
