When the Sun is directly overhead at its most northern or southern position, it is called the solstice. The Northern Hemisphere’s summer solstice occurs when the Sun is above the Tropic of Cancer — on 20, 21 or 22 June — and marks the beginning of summer. Its winter solstice (the Southern Hemisphere’s summer solstice) is on 21 or 22 December. The summer solstice is the longest day of the year; the winter equivalent is the shortest.
Solstice, either of the two moments in the year when the Sun’s apparent path is farthest north or south from Earth’s Equator. The situation is exactly the opposite in the Southern Hemisphere, where the seasons are reversed. At the winter solstice the day is the year’s shortest, and at the summer solstice it is the year’s longest. The term solstice also is used in reference to either of the two points of greatest deviation of the ecliptic (the Sun’s apparent annual path) from the celestial equator.
At the time of the summer solstice in the Northern Hemisphere, the North Pole is tilted about 23.4° (23°27´) toward the Sun. Because the Sun’s rays are shifted northward by the same amount, the vertical noon rays are directly overhead at the Tropic of Cancer (23°27´ N). Six months later the South Pole is inclined about 23.4° toward the Sun. On this day of the summer solstice in the Southern Hemisphere the Sun’s vertical overhead rays progress to their southernmost position, the Tropic of Capricorn (23°27´ S). Compare equinox. See also season.
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