June 21 is the 172nd day of the year (173rd in leap years) in the Gregorian calendar.
On non-leap years (until 2039), this day marks the summer solstice in the northern hemisphere and the winter solstice in the southern hemisphere, and is the day of the year with the most hours of daylight in the northern hemisphere and the least hours of daylight in the southern hemisphere.
Solstice is derived from the Latin words sol (sun) and sistere (to stand still).
The summer solstice occurs when the tilt of a planet's semi-axis, in either the northern or the southern hemisphere, is most inclined toward the star (sun) that it orbits. Earth's maximum axial tilt toward the sun is 23° 26'. This happens twice each year, at which times the sun reaches its highest position in the sky as seen from the north or the south pole.
The summer solstice occurs during a hemisphere's summer. This is northern solstice in the northern hemisphere and the southern solstice in the southern hemisphere. Depending on the shift of the calendar, the summer solstice occurs some time between December 20 and December 23 each year in the southern hemisphere and between June 20 and June 22 in the northern hemisphere in reference to UTC.
Though the summer solstice is an instant in time, the term is also colloquially used like midsummer to refer to the day on which it occurs. The summer solstice occurs on the day that has the longest period of daylight – except in the polar regions, where daylight is continuous, from a few days to six months around the summer solstice.
Worldwide, interpretation of the event has varied among cultures, but most recognize the event in some way with holidays, festivals, and rituals around that time with themes of religion or fertility.
Above diagram of the Earth's seasons as seen from the north. Far left: summer solstice for the Northern Hemisphere. Front right: summer solstice for the Southern Hemisphere.
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