Change of seasons (intermediate)

Change of seasons (intermediate)

Due to the Earth´s tilted axis, the angle of the Sun's rays at given latitudes continuously changes during the year.

Geography

Keywords

change of seasons, season, seasons, spring, summer, autumn, winter, equinox, winter solstice, summer solstice, solstice, axial tilt, angle of inclination, axis of rotation, circulatory system, culmination, Earth, Sun, sunlight, year, calendar, Tropic of Cancer, Tropic of Capricorn, month, months, rotation, Equator, polar circle, nature, astronomy, geography

Related items

Scenes

Orbit of the Earth

  • spring
  • 21 March: vernal equinox
  • summer
  • 22 June: summer solstice
  • autumn
  • 23 September: autumnal equinox
  • winter
  • 22 December: winter solstice

The change of seasons is the result of the Earth orbiting with its tilted axis around the Sun. The angle of sunlight continuously changes during the year at different locations of the Earth, resulting in them heating up at different rates.
Solar radiation can only hit the surface perpendicularly at noon between the tropics; beyond the tropics this is not possible.

Due to the axial tilt of the Earth, the maximum angle of the Sun's rays (90°) is not always at the Equator; but rather changes its position between 23.5° North and 23.5° South. It reaches each of these latitudes once a year (during the Winter Solstice and the Summer Solstice) but travels across the band between them twice. These latitudes are called the Tropic of Cancer and the Tropic of Capricorn. The region between the two Tropics is the warmest on Earth, as it is only here that the Sun's rays can hit the Earth's surface perpendicularly.

It is also due to the axial tilt of the Earth that at latitudes over 66.5° North and 66.5° South, there is at least one day each year when the Sun does not rise or set. These two latitudes are called the Arctic Circle and the Antarctic Circle. The polar regions of the Earth (the areas bordered by the polar circles) are the coldest, as the angle of the Sun's rays is the smallest here, and because once a year the regions experience total darkness for 24 continuous hours. On the other hand, at the poles, the Sun does not rise for half a year. The angle of the Sun's rays is less than 90° between the circles of the Tropics and the polar circles, but the Sun rises and sets daily here. These areas receive less heat than the Tropics but more than the polar regions.

At the equinoxes, the Sun rises exactly in the East and sets in the West everywhere on the Earth, and the Sun spends the same amount of time above the horizon as below; thus, days and nights have the same length. On these days, the Sun is directly overhead at noon at the Equator.

Above each tropic there is only one day every year when the Sun is directly overhead at noon: this is called the solstice. Then the tropics 'return' the Sun, causing it to pass lower in the sky, and 'turning it back' towards the Equator. One year is necessary for the Earth to revolve once around the Sun. Therefore, seasons repeat in the same sequence every year.

Definitions of terms:

Equinox: the two days in the year when the lengths of days and nights are the same on both hemispheres of the Earth. On these days, the Sun is directly overhead at noon along the Equator.
The spring equinox is on 21 March – astronomical spring starts on this day in the northern hemisphere. The autumn equinox is on 23 September – astronomical autumn starts on this day in the northern hemisphere.

Solstice: the date when the Sun reaches the highest and lowest point on its path across the sky over the tropics. On 22 June, the Sun is directly overhead at noon along the Tropic of Cancer – astronomical summer starts on this day in the northern hemisphere (in the southern hemisphere winter starts).
On 22 December, the Sun is directly overhead at noon along the Tropic of Capricorn – astronomical summer starts on this day in the southern hemisphere (in the northern hemisphere winter starts).

Noontime altitude of the Sun: the highest point of the Sun's apparent daily path in the sky at a given longitude. It changes together with the seasons; it is the lowest in winter and the highest in summer.

Spring

  • 21 March
  • 22 June
  • 23 September
  • 22 December
  • 23.5°
  • 66.5°
  • 90°
  • 66.5°
  • 23.5°
  • spring
  • summer
  • autumn
  • winter
  • Arctic Circle
  • Tropic of Cancer
  • Equator
  • Tropic of Capricorn
  • Antarctic Circle

Astronomical spring in the Northern Hemisphere starts on 21 March and ends on 22 June. On 21 March the Sun's rays hit the Earth at a right angle at the Equator. As the days pass, the area where the rays hit the surface at a right angle move towards the north and it is at the Tropic of Cancer on the last day of Spring. The Northern Hemisphere becomes increasingly warmer.

Meanwhile, on the Southern Hemisphere the angle of the Sun's rays decreases and thus it grows colder and colder.

Summer

  • 21 March
  • 22 June
  • 23 September
  • 22 December
  • 23.5°
  • 66.5°
  • 90°
  • 66.5°
  • 23.5°
  • spring
  • summer
  • autumn
  • winter
  • Arctic Circle
  • Tropic of Cancer
  • Equator
  • Tropic of Capricorn
  • Antarctic Circle

Astronomical summer in the Northern Hemisphere starts on 22 June, on the same day astronomical winter starts in the Southern Hemisphere. On this day the Sun is directly overhead at noon at the Tropic of Cancer. However, the angle of the Sun's rays is very low at the Tropic of Capricorn. As the days pass, the line where the Sun is directly overhead at noon gets nearer to the Equator. When it is summer in the Northern Hemisphere, it warms up far more there than in the Southern Hemisphere.

Summer in the Northern Hemisphere (and winter in the Southern Hemisphere) ends on 23 September.

Autumn

  • 21 March
  • 22 June
  • 23 September
  • 22 December
  • 23.5°
  • 66.5°
  • 90°
  • 66.5°
  • 23.5°
  • spring
  • summer
  • autumn
  • winter
  • Arctic Circle
  • Tropic of Cancer
  • Equator
  • Tropic of Capricorn
  • Antarctic Circle

By 23 September the Sun 'returns' to the Equator, this is where the Sun is directly overhead at noon. The angle of the Sun's rays gradually decreases, and therefore their warming effect also gradually decreases until 22 December.

In the Southern Hemisphere, however, the angle of the Sun's rays gradually increases and therefore it gets increasingly warm.

Winter

  • 21 March
  • 22 June
  • 23 September
  • 22 December
  • 23.5°
  • 66.5°
  • 90°
  • 66.5°
  • 23.5°
  • spring
  • summer
  • autumn
  • winter
  • Arctic Circle
  • Tropic of Cancer
  • Equator
  • Tropic of Capricorn
  • Antarctic Circle

On 22 December the Sun is directly overhead at noon at the Tropic of Capricorn. On this day astronomical summer starts in the Southern Hemisphere, while winter starts in the Northern Hemisphere. As the days pass, the Sun 'returns' towards the Equator.

When it is summer in the Southern Hemisphere, it warms up far more there than in the Northern Hemisphere.

Summer in the Southern Hemisphere (and winter in the Northern Hemisphere) ends on 21 March.

Important dates

  • spring
  • 21 March: vernal equinox
  • summer
  • 22 June: summer solstice
  • autumn
  • 23 September: autumnal equinox
  • winter
  • 22 December: winter solstice

Animation

  • spring
  • 21 March: vernal equinox
  • summer
  • 22 June: summer solstice
  • autumn
  • 23 September: autumnal equinox
  • winter
  • 22 December: winter solstice

Narration

The mean temperature at a given place on Earth is not constant throughout the year. This is attributed to the change of seasons which is the result of the Earth's orbiting around the Sun, the spherical shape of the Earth and the axial tilt of our planet.

The Earth revolves around the Sun on an elliptical orbit and it takes one year for it to complete one orbit. Since the Earth is spherical in shape, the Sun's rays hit the surface at different angles, resulting in differences in temperature.

Moving from the Equator towards the poles, the angle of the Sun's rays hitting the Earth gradually shrinks. While at the Equator the angle of the Sun's rays can be as high as 90°, that is, a right angle, at the poles this angle can be as low as 0°. The higher this angle is, the more strongly the rays heat up the surface and the air. As a result, the rate of warming decreases towards the poles and the temperature drops as well.

However, the Earth's orbit around the Sun and the spherical shape of the planet are not the only reasons why seasons change. The axial tilt of Earth also contributes to it. If the Earth's axis were perpendicular to the orbital plane, the incoming solar energy would be constant throughout the year at a given point at a certain latitude.

In reality, in December in the region between the Tropic of Cancer and the Arctic Circle, and in June in the region between the Tropic of Capricorn and the Antarctic Circle the angle of the Sun's rays is low, and thus the amount of solar energy received is also low. This is winter.

In June in the region between the Tropic of Cancer and the Arctic Circle, and in December in the region between the Tropic of Capricorn and the Antarctic Circle the angle of the Sun's rays approaches a right angle, and thus the amount of solar energy received is high. This is summer.
Between the two Tropics and near the polar regions, the angle of the Sun's rays is relatively constant, so there is no significant change in temperature throughout the year.

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