Many climate scientists define the start of summer in terms of meteorological phenomena and the calendar year. They claim that the meteorological summer season starts on June 1 and lasts three months, until September 1. Other people define the beginning of summer in academic terms—as the end of the school year. In astronomical terms, the start of summer can be defined very precisely: it begins on the summer solstice, which occurs on June 20 or 21 in the Northern Hemisphere and on December 21 or 22 in the Southern Hemisphere.
On the summer solstice in the Northern Hemisphere, the path of the Sun in the sky hits its northernmost point. The summer solstice occurs in the Southern Hemisphere when the Sun reaches its southernmost point. Either position maximizes the hours of daylight in the hemisphere where it occurs, making the summer solstice the longest day of the year. On the summer solstice in June, Earth reaches a point in its orbit around the Sun at which the North Pole tilts most directly toward the Sun, at an angle of about 23.4°. Likewise, on the summer solstice in December, the South Pole tilts most directly toward the Sun.
Astronomical summer lasts from the date of the summer solstice until the autumnal equinox, which occurs in the Northern Hemisphere on September 22 or 23 and in the Southern Hemisphere on March 20 or 21.