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The axis of the Earth is an imaginary line on which the Earth rotates.

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It links up the two poles.

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Both the axis and the Earth are tilted at an angle of 23.5 degrees during a revolution.

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The tilting of the axis results in direct sunlight falling on different places during different seasons.

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This causes variations in the duration of days, nights, and seasons.

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Relationship between the location of the overhead sun and the seasons

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Similarly, the revolution of the Earth and the tilting of the axis

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results in different angle of the sun during different periods.

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When the sun is directly overhead, we call this the overhead sun.

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At this time, the Earth's surface and the midday sun forms a 90-degree angle.

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Different locations of the overhead sun results in variations in the amount of solar radiation received in different areas and at different periods.

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Spring Equinox

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On the 21st or 22nd of March, the overhead sun is over the equator.

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The equator receives the largest amount of solar radiation.

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At this time, the northern hemisphere is in the spring equinox,

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while the southern hemisphere is in the autumn equinox.

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The angle of the sun decreases towards the poles.

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On this day, the two hemispheres receive a similar amount of solar radiation,

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and the length of day and night is the same at all places on the Earth.

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After this day, it is spring in the northern hemisphere, where the day is longer than the night.

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In the southern hemisphere, it becomes autumn, when the day is shorter than the night.

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Summer solstice

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On the 21st or 22nd of June, the overhead sun is over the Tropic of Cancer.

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It receives the largest amount of solar radiation.

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At this time, the northern hemisphere is in the summer solstice,

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while the Southern Hemisphere is in the winter solstice.

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The angle of the sun decreases towards the poles.

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On this day, the length of daytime of the Northern Hemisphere is the longest in the year,

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while that of the Southern Hemisphere is the shortest in the year.

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Besides, there are 24 hours of daylight at the Arctic Circle

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and 24 hours of darkness at the Antarctic Circle.

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Autumn Equinox

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On the 22nd or 23rd of September, the overhead sun is over the equator again.

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The equator receives the largest amount of solar radiation.

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On this day, the northern hemisphere is in the autumn equinox,

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while the southern hemisphere is in the spring equinox.

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The angle of the sun decreases towards the poles.

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On this day, the two hemispheres receive a similar amount of solar radiation,

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and the length of day and night is the same at all places on the Earth.

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After this day, it is autumn in the Northern Hemisphere, where the day is shorter than the night.

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In the Southern Hemisphere, it becomes spring, when the day is longer than the night.

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Winter Solstice

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On the 21st and 22nd of December, the overhead sun is over the Tropic of Capricorn.

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It receives the largest amount of solar radiation.

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On this day, the northern hemisphere is in the winter solstice, while the southern hemisphere is in the summer solstice.

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The angle of the sun decreases towards the poles.

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On this day, the length of daytime in the northern hemisphere is the shortest in the year, while that of the southern hemisphere is the longest in the year.

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There are 24 hours of darkness at the Arctic Circle and 24 hours of daylight at the Antarctic Circle.