The climatic sequence of spring, summer, autumn (fall), and winter into which the year is divided, based on the changing number of hours of daylight. Seasons occur because the Earth’s axis of rotation is tilted 23.5° to the plane of its solar orbit. As Earth orbits, therefore, first one and then the other hemisphere is tilted towards the Sun, the extreme positions being reached at the June and December solstices. At the winter solstice the Sun does not rise above the horizon and at the summer solstice it does not sink below the horizon in latitudes higher than the Arctic or Antarctic Circles. Midway between the solstices, the March and September equinoxes mark the position at which the noonday Sun appears directly overhead at the equator and day and night are of equal length worldwide.