Seasonal year

The need for farmers to predict seasonal events led to the development of calendars.

For example, the ancient Egyptians used the heliacal rising of Sirius to predict the flooding of the Nile.

This suggestion is surprising because the seasons have been thought to be governed by the tilt of the Earth's axis (see Effect of sun angle on climate).

The seasons can be considered to be an oscillating 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.