In the northern hemisphere the summer solstice occurs around the 22nd of June; and the winter solstice, around the 22nd of December. The former marks the beginning of summer, and the latter the start of winter. They occur when the sun reaches its point of largest (summer) or smallest declination. The declination is the angle to the celestial sphere.
The seasons are reversed in the southern hemisphere.
The summer solstice is also the longest day in the northern hemisphere (and the shortest in the southern); the winter solstice being the longest day in the southern hemisphere and the shortest one in the northern.
Notes and References
- Actually, on its elliptical orbit the earth is closest to the sun in the beginning of January (cf. perihelion, Black Sun). Yet, the main reason for the seasons is the earth's (permanent) axial tilt: around Christmas the North Pole looks away from the Sun - and the northern hemisphere gets less of the Sun's direct rays (i.e. less warmth); this is why it gets cold there in winter