Astronomy Answers
From the Astronomical Dictionary

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the solstice ⇒

solstice, plural solstices

A solstice is a moment when the declination of the Sun changes from increasing to decreasing, or from decreasing to increasing. There are two solstices each tropical year: the northern solstice and the southern solstice. The solstices mark midsummer's day and midwinter's day, and the beginning of the astronomical seasons of summer and winter.