A solstice occurs twice a year when the sun reaches its maximum or minimum declination relative to the celestial equator, resulting in the longest and shortest days of the year. The summer solstice (around 20–21 June in the Northern Hemisphere) marks the moment when the Earth's axial tilt points most directly toward the sun, producing the longest day and shortest night. The winter solstice (around 21–22 December) is the reverse — the shortest day and longest night.
The solstices are direct consequences of Earth's 23.44° axial tilt. At the summer solstice, the sun appears at its highest noon altitude and rises and sets at its most extreme northerly points on the horizon. At latitude 40°N (Madrid), the summer solstice provides about 15 hours of daylight versus roughly 9 hours at the winter solstice. At the Arctic Circle (66.5°N), the sun does not set at all on the summer solstice (midnight sun) nor rise on the winter solstice (polar night). At the equinoxes, day and night are approximately equal worldwide.
For meteorology and climate, the solstices mark the extremes of solar radiation input. However, the thermal lag of oceans and land means that the warmest and coldest periods follow the solstices by several weeks: in Spain, the hottest month is typically July or August (not June), and the coldest is January or February (not December). The summer solstice delivers about 3.5 times more solar energy per day at mid-latitudes than the winter solstice — this enormous difference in energy input is the fundamental driver of seasonal weather changes. Culturally, solstices have been celebrated since prehistory; Stonehenge and Newgrange are aligned with solstice sunrise.