Also known as the Summer Solstice, the first day of summer is the longest day of the year, and usually occurs on June 20th or 21st. This means that the North Pole is titled towards the sun, which happens only once a year (the opposite happens on December 21st when the South Pole is titled towards the sun, called the Winter Solstice).

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In the United States, there will be about 14 1/2 hours of daylight today, so get out there and make the most of it!

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* Van H. English, "Solstice," in AccessScience@McGraw-Hill, http://www.accessscience.com.