The dates used for the first day of each season mark the beginning of the astronomical seasons. The beginning and end of the climatic seasons vary from these dates from place to place and from year to year.

The temperature and weather do not change instantly in response to the changing position of the earth in relation to the sun. The warmest and coldest weather generally occurs several weeks after the beginning of the summer and winter astronomical seasons. Heat that has been retained by the oceans plays an important role in producing this delayed response.

The seasons keep changing because the tilt of the earth's axis never changes while the earth circles the sun. One way to understand this is to picture which way the tilt of the axis causes the North Pole to slant at different times of the year.

When the North Pole slants toward the sun, the Northern Hemisphere receives the most sunlight and it is summer there. When the pole slants away from the sun, the Northern Hemisphere receives the least sunlight and it is winter. Spring begins when the pole starts to slant toward the sun, and autumn begins when the pole starts to slant away again.