The primary cause of Earth's seasons is the change in the amount of sunlight reaching the surface …—© Merriam-Webster Inc.

Any of four divisions of the year according to consistent annual changes in the weather. In the Northern Hemisphere, winter formally begins on the winter solstice, December 21 or 22; spring on the vernal equinox, March 20 or 21; summer on the summer solstice, June 21 or 22; and fall (autumn) on the autumnal equinox, September 22 or 23. In the Southern Hemisphere, the dates of onset of summer and winter are reversed, as are those of spring and fall.

This entry comes from Encyclopædia Britannica Concise.
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