Earth and Moon

The 4 seasons of the year

The 4 seasons of the year

On planet Earth there are large areas whose climate has four seasons a year. Depending on the latitude and altitude, the meteorological changes may be minimal, as in the low tropical zones, or maximum, as in the mid latitudes.

In these middle regions of the planet the climate is changing. The year is divided into Seasons, with more or less similar characteristics, which affect the landscape, living beings and human activities.

In general, there are four seasons of about three months each, although there are areas of the Earth where there are only two, the wet and the dry; for example, in monsoon areas.

Causes and effects of the seasons

The axis of the Earth is inclined with respect to the plane of the orbit that the planet travels around the Sun. Therefore, some regions receive different amounts of sunlight according to the dates of the year. The climatic variations that the Earth suffers are more pronounced in the cold and temperate zones, and softer or even imperceptible between the tropics.

The following table shows the start dates, the duration and the inclination of the earth's axis of the four astronomical stations. The climatic seasons, on the other hand, are usually counted for whole months, from the first day of the month in which the corresponding astronomical station begins.

StartH. northH. southDays durationInclination
March 20-21SpringFall92,90 º
June 21-22SummerWinter93,723.5 º North
September 22-24FallSpring89,60 º
December 21-22WinterSummer89,023.5 º South

The four seasons are: Spring, summer, autumn, and winter. The first two make up the half year in which the days last longer than the nights, while in the other two the nights are longer than the days.

Since the variations in the climate are due to the inclination of the terrestrial axis, they do not occur at the same time in the Northern (Boreal) hemisphere as in the Southern (Southern) hemisphere, but are inverted in relation to each other. When in one hemisphere it is summer, in the other it is winter, and when in one the autumn begins, in the other the spring.

While the Earth moves with the axis of the North Pole tilted towards the Sun, that of the South Pole is in the opposite direction and the northern regions receive more solar radiation than those of the south. This process is subsequently reversed and the northern hemisphere areas receive less heat, when the days shorten and the sun's rays fall more inclined.

Solstices and Equinoxes

The astronomical stations they are determined by the four main positions in the Earth's orbit, opposite two to two, which are called solstices and equinoxes. Winter solstice, spring equinox, summer solstice and autumn equinox.

In the equinoxes, the axis of rotation of the Earth is perpendicular to the sun's rays, which fall vertically over the equator. In solstices, the earth's axis is inclined about 23.5º, so that the sun's rays fall vertically on the Tropic of Cancer (summer in the northern hemisphere) or of Capricorn (summer in the southern hemisphere).

Not all seasons last the same since, because of the eccentricity of the Earth's orbit, our planet travels around the Sun with variable speed. The Earth goes faster the closer it is to the Sun and slower the further away.

The rigor of each station is not the same for both hemispheres. The Earth is closer to the Sun at the beginning of January (perihelion) than at the beginning of July (aphelion), which causes it to receive 7% more heat in the first month of the year than half of it. For this reason, together, in addition to other factors, the boreal winter is less cold than the austral, and the austral summer is hotter than the boreal.

On the other hand, because of various disturbances that the Earth experiences while revolving around the Sun, it does not go through solstices and equinoxes exactly, which motivates that the different seasons do not always begin at the same precise date and time.

You have to pay attention to the media or consult a astronomical calendar if you want to know exactly when the four seasons start each year.

Discover more:
• Difference between meteorological and astronomical stations
• Earth movements: translation, rotation, precession and nutation
• The seasons of the year according to Wikipedia

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