The year, the length of time that the Earth takes to complete one orbit of the Sun, is divided into four approximately equal
parts called seasons: spring, summer, fall (autumn), and winter. In temperate climates, spring is a period of new plant growth, summer is the time of greatest average warmth, fall is the period when deciduous trees start shedding their leaves, and winter is the time of greatest average coldness and of frozen precipitation such as snow. Equatorial regions of the Earth remain warm and exhibit changes to a lesser degree or not at all, while the northern and
southern polar regions remain cold throughout the year.Seasonal changes exist because the Earth's axis of rotation is tilted to the plane of the Earth's orbit (see Earth, motions of). The direction of tilt does not change with respect to the celestial sphere (except very slowly and to a minor degree, over many thousands of years). This means that the Earth's axis of rotation remains pointed toward the same area of the heavens. This also means that the orientation of the Northern and Southern hemispheres changes with respect to the Sun throughout the year. This change causes the seasons, with winter taking place in the Northern
hemisphere as summer takes place in the Southern Hemisphere, and so on.In terms of the Gregorian calendar, the timing of the seasons in the Northern Hemisphere is as follows: spring runs from March 21 to June 22, summer from June 22 to September 23, fall from September 23 to December 22, and winter from December 22 to March 21. The Sun is directly over the equator at the start of spring (the vernal equinox) and at the start of fall (the autumnal equinox). At the start of summer the Sun is at its most northerly, and at the start of winter it is at its most southerly. These latter two points are known in the Northern Hemisphere as the summer solstice and the winter solstice, respectively.