Summer is a warm season because the days are longer and the Sun is high in the sky, giving direct light to the ground. Winter is a cold season because the days are shorter and the Sun is low in the sky, giving indirect light to the ground. Both the changes in the length of the day and the height of the Sun at noon are caused by the tilt of the Earth's spin axis with respect to the plane of the Earth's path around the Sun. At any time, in any season, the northern and southern hemispheres (halves of the Earth) have opposite seasons.