The summer solstice is the beginning of summer, and the winter solstice marks the beginning of winter in the Northern Hemisphere. The summer solstice occurs on or about June 21 (depending on leap years) which is the longest day of the year. The winter solstice occurs on, or close to, December 22 and is the shortest day of the year. The term solstice is derived from Latin, meaning "sun" and "stand still" because the noonday sun appears to an observer here on earth to be at the same height for several days at the time of the solstice.
Like the seasons themselves, the solstice is a function of the tilt of Earth's axis, and to many ancient peoples meant seasonal change. Astronomically, the sun reaches its greatest angular distance north or south of the equator on the solstice. The North Pole is tilted toward the sun on the summer solstice at 23 and 1/2 degrees. This means that every place north of the Arctic Circle will have 24 hours of sunlight at the time of the summer solstice, and the length of day at all places north of the Equator will be more than 12 hours, increasing in length with increasing latitude. At latitude 40 degrees north (near Philadelphia and Denver, for example) the "longest day" will last about 15 hours and then the days gradually become shorter and shorter until the next solstice in December.
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