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1.7.2. Season of the year

The seasons of the year

The translational movement of the Earth around the Sun is the cause of the seasonal changes that occur.

The imaginary plane on which the Earth moves around the Sun is called the ecliptic. The complete translation lasts 365 days, 5 hours, 48 ​​minutes and 45 seconds, which is called a year. As there are a few hours to spare, every four years one more day is added to the calendar and has 366 days, and that year is said to be a leap year.

The distance between the Earth and the Sun is about 150 million kilometers, although it varies according to the time of year, since the translational movement is not circular, but elliptical, so there is a time of year when they are more closer than others.

  • Aphelion: It is the point at which the Earth is furthest from the Sun.
  • Perihelion: It is the point at which the Earth is closest to the Sun.

Although it seems clear that the closer the Earth is to the Sun it has to be hotter, what influences the seasons is the perpendicularity or obliqueness of the incidence of the solar rays on the Earth, not the distance at which the Earth is. of the sun

The solar radiation that reaches the Earth does not heat it in the same way in all its parts. In areas close to the equator, the rays strike almost perpendicularly and heat more than in the polar areas, where the sun's rays arrive very obliquely.

Furthermore, due to the tilt of the Earth's axis and translational motion , the sun's rays do not strike the Earth's surface in the same way throughout the year.

In the northern hemisphere, in summer, the sun's rays strike more perpendicularly than in the southern hemisphere, which is winter.

In winter in the northern hemisphere, on the other hand, the sun's rays strike more obliquely than in the southern hemisphere. If it weren't for the tilt of the earth's axis, there would be no stations.

Although it may be strange, in the northern hemisphere, when the rays strike perpendicularly in summer, we are further from the Sun. At that time, in the southern hemisphere they receive the most oblique solar rays, and they are in winter.

In winter in the northern hemisphere, on the other hand, despite being closer to the Sun, the sun's rays heat less because they hit obliquely. In the southern hemisphere, on the other hand, they will be in summer.

Estaciones del año

By File:Four season blank.svg: Horst Frank, Gothikaderivative work: El duende alegre (File:Four season blank.svg) [CC-BY-SA-3.0 or GFDL], via Wikimedia Commons

Duration of day and night

As you may have observed, the duration of day and night depends on where the Earth is in its orbit, since the orbit is not circular but elliptical. Thus, it passes twice a year along the major axis and twice a year along the minor axis of the ellipse that it travels.

  • Solstices: Points of the Earth's orbit that coincide with the ends of the major axis of the ellipse. There are two solstices:
    • Summer solstice: It coincides with the beginning of summer in the northern hemisphere, and has the shortest night of the year. It happens around June 21.
    • Winter solstice: It coincides with the beginning of winter in the northern hemisphere, and has the longest night of the year. It happens around December 21.
  • Equinoxes: Points of the Earth's orbit that coincide with the ends of the minor axis of the ellipse. At the equinoxes, the length of the day is the same as that of the night. There are two equinoxes:
    • Spring equinox.
    • Autumn equinox.

After the vernal equinox, the duration of the night is shorter and shorter, and each day has more hours of light. From the summer solstice, the hours of light are reduced until they are equal to those of darkness at the autumn equinox. Thereafter, the nights are longer until they reach their maximum at the winter solstice.

Although the dates are the same, the solstices and equinoxes are different in the northern hemisphere and in the southern hemisphere, since when in one it is summer, in another it is winter, and when in one it is spring, in the other it is autumn.

According to this, in the temperate zone of the northern hemisphere, the seasons follow one another as follows:

  • The winter begins on December 21. Temperatures are cold because the sun's rays reach the earth's surface very steeply and the days are shorter (there are fewer hours of sunlight).
  • The spring begins on March 21 at the equinox of spring. The temperature rises and the hours of sunlight continue to increase.
  • The summer begins on June 21 with the solstice summer (day hours more solar year). The sun's rays strike perpendicularly and cause temperatures to be very high. 
  • The autumn begins on 21 September at the equinox autumn. The sun's rays are increasingly inclined and heat the Earth's surface less. Every day there are fewer hours of light.