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Revellers greet each other during the summer solstice celebrations at Stonehenge, June 2007.
Today is the shortest day of the year, which means things start getting a little lighter tomorrow. In astronomical terms, it's called the winter solstice, the natural mate of the summer solstice.
For a variety of spiritual and social cultures, a solstice is time to celebrate. NPR's Margot Adler explains the history of solstice celebrations and why they're still so important today.
On our blog: A scientist teaches Solstice 101.