The Northern) Solstice falls on Wednesday 21 June 2021 this year, the day on which the Sun rises highest in the sky and the longest day of the year in the Northern Hemisphere (where it is the Summer Solstice) and the day on which the Sun rises lowest in the sky and the shortest day in the Southern Hemisphere (where it is the Winter Solstice). Up until this date the days have been growing shorter in the Northern Hemisphere and longer in the Southern Hemisphere since the Southern Solstice in December last year (which is the Summer Solstice in the Northern Hemisphere and Winter Solstice in the Southern Hemisphere), but after it the situation will be reversed, with days growing steadily longer in the Northern Hemisphere and shorter in the Southern Hemisphere until the next Southern Solstice in December this year.
The solstices are entirely a product of variation in the Earth's rotation on its axis, which is at an angle of 23.5° to the plain of the Earth's orbit about the Sun. This means that in December the Earth's Southern Pole is tilted towards the Sun, while the Northern Pole is tilted away from it. This means that around the Southern Solstice the Southern Hemisphere is receiving radiation from the Sun over a longer part of the than the Northern, and at a steeper angle (so that it to pass through less atmosphere to reach the planet), creating the southern summer and northern winter.
The solstices are fairly noticeable astronomical events, and tied to the seasons which govern the life cycles of life on Earth, and they have been celebrated under different names by cultures across the globe, but most notably by those at higher latitudes, who are more profoundly affected by the changes of the seasons.
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