Solstice
On 21st June, the northern hemisphere is tilted towards the sun. The rays of the sun fall directly on the Tropic of Cancer. As a result, these areas receive more heat. The areas near the poles receive less heat as the rays of the sun are slanting.
- The North Pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months.
- Since a large portion of the northern hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21st June. This position of the earth is called the Summer Solstice.
- At this time in the southern hemisphere all these conditions are reversed. It is winter season there. The nights are longer than the days.
- On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (23½° s), a larger portion of the southern hemisphere gets light. Therefore, it is summer in the southern hemisphere with longer days and shorter nights. This position of the earth is called the Winter Solstice.
- The reverse happens in the northern hemisphere.
EQUINOX
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On 21st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.
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On 23rd September, it is autumn season [season after summer and before the beginning of winter] in the northern hemisphere and spring season [season after winter and before the beginning of summer] in the southern hemisphere. Thus it is termed as an Autumn Equinox.
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The opposite is the case on 21st March when it is spring in the northern hemisphere and autumn in the southern hemisphere. It is thus termed as Spring Equinox.
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Thus, days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.