Change of seasons (basic)
Due to the Earth´s tilted axis, the angle of the Sun's rays at given latitudes is continuously changing during the year.
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Orbit of the Earth
- 21 March: spring equinox
- 22 June: summer solstice
- 23 September: autumn equinox
- 22 December: winter solstice
The change of seasons is the result of the Earth orbiting with its tilted axis around the Sun. The angle at which the the Sun's rays hit the Earth's surface is different at different locations, resulting in them heating up at different rates. The Sun's rays can only reach the surface perpendicularly at noon between the tropics; beyond the tropics this is not possible.
Above the Equator, there are two days in the year when the Sun is directly overhead: the equinox days.
Above each tropic there is only one day every year when the Sun’s zenith is perpendicular: on the days of solstices. Then the tropics "turn the Sun back" towards the Equator, causing it to pass gradually lower in the sky. It takes one year for the Earth to orbit once around the Sun, that is why the same sequence of seasons is repeated 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’s rays hit the Equator perpendicularly. 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 two days in the year when the Sun reaches the highest or lowest point on its path across the sky over the tropics. On 22 June, the Sun is directly overhead at solar noon at the Tropic of Cancer – astronomical summer starts on this day in the Northern Hemisphere (on the Southern Hemisphere winter starts).
On 22 December, the the Sun is directly overhead at solar noon at the Tropic of Capricorn – astronomical summer starts on this day in the Southern Hemisphere (in the Northern Hemisphere winter starts).
- 21 March
- 22 June
- 23 September
- 22 December
- Arctic Circle
- Tropic of Cancer
- Tropic of Capricorn
- Antarctic Circle
Seasons change because the Earth orbits around the Sun. It takes one year for the Earth to travel around the Sun on an elliptical orbit. However, if it were not for our planet's tilted axis, things would be different. But since its axis is tilted relative to its orbital plane, the Sun's rays hit the surface at an oblique angle. When it is summer in the Northern Hemisphere, this hemisphere is tilted towards the Sun, so its rays hit the surface here at a relatively high angle. The higher this angle is, the more strongly the Sun's rays heat up the surface and the air. In the Southern Hemisphere, however, the angle of the Sun's rays hit the surface at the lowest possible angle so it is winter here. As the Earth moves along its orbit, the angle of the Sun's rays is constantly changing during the year at different locations. The Sun's rays can only hit the surface perpendicularly at noon between the tropics; beyond the tropics this is not possible. Twice a year, on the two equinoxes, the Sun's rays hit the surface at a right angle above the Equator. At these times the day and the night are exactly the same length. Above each tropic there is only one day every year when the Sun is directly overhead: this is called the solstice. Then the tropics 'turn the Sun back' causing it to pass gradually lower in the sky and return to the Equator.
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