A solstice is an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. The seasons of the year are determined by reference to both the solstices and the equinoxes. In the Southern Hemisphere, the winter solstice for 2018 will occcur at 8:07pm on the evening of June 21st.
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