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Seasonal year
 
 
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The seasonal year is the time between successive recurrences of a season
Season

A season is one of the major divisions of the year, generally based on yearly periodic changes in weather.Seasons result from the yearly revolution of the Earth around the Sun and the Axial tilt....
al event such as the flooding of a river, the migration 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 of calendar
Calendar

A calendar is a system of organize days for a social, religious, commercial or administrative purpose. This organization is done by giving names to periods of time ? typically days, weeks, months and years....
s. However, the variability from year to year of seasonal events (due to climate change
Climate change

Climate change is any long-term significant change in the expected patterns of average weather of a specific region over an appropriately significant period of time....
 or just random variation) makes the seasonal year very hard to measure.






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The seasonal year is the time between successive recurrences of a season
Season

A season is one of the major divisions of the year, generally based on yearly periodic changes in weather.Seasons result from the yearly revolution of the Earth around the Sun and the Axial tilt....
al event such as the flooding of a river, the migration 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 of calendar
Calendar

A calendar is a system of organize days for a social, religious, commercial or administrative purpose. This organization is done by giving names to periods of time ? typically days, weeks, months and years....
s. However, the variability from year to year of seasonal events (due to climate change
Climate change

Climate change is any long-term significant change in the expected patterns of average weather of a specific region over an appropriately significant period of time....
 or just random variation) makes the seasonal year very hard to measure. This means that calendars are based on astronomical years (which are regular enough to be easily measured) as surrogates for the seasonal year. For example, the ancient Egypt
Ancient Egypt

Ancient Egypt was an Ancient history civilization in eastern North Africa, concentrated along the lower reaches of the Nile in what is now the modern nation of Egypt....
ians used the heliacal rising
Heliacal rising

The heliacal rising of a star occurs when it first becomes visible above the eastern horizon at dawn, after a period when it was hidden below the horizon or when it was just above the horizon but hidden by the brightness of the sun....
 of Sirius
Sirius

Sirius is the list of brightest stars in the night sky with a visual apparent magnitude of −1.46, almost twice as bright as Canopus, the next brightest star....
 to predict the flooding of the Nile
Nile

The Nile is a major north-flowing river in Africa, generally regarded as the List of rivers by length in the world.The Nile has two major tributary, the White Nile and Blue Nile, the latter being the source of most of the Nile's water and silt, but the former being the longer of the two....
.

A study of temperature records over the past 300 years suggests that the seasonal year is governed by the anomalistic year rather than the tropical year
Tropical year

A tropical year is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice....
.

This suggestion is surprising because the season
Season

A season is one of the major divisions of the year, generally based on yearly periodic changes in weather.Seasons result from the yearly revolution of the Earth around the Sun and the Axial tilt....
s have been thought to be governed by the tilt of the Earth's axis
Axial tilt

In astronomy, axial tilt is the inclination angle of a planet axis of rotation in relation to its Orbital plane . It is also called axial inclination or obliquity....
 (see Effect of sun angle on climate
Effect of sun angle on climate

The amount of heat energy received at any location on the globe is a direct effect of sun angle of climate, as the Angle of incidence at which sunlight strikes the earth varies by location, time of day, and season due to the earth's orbit around the sun and the earth's revolution around its tilted axis....
). 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 an oscillating system
Oscillation

Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and Alternating current power....
 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.