We know that the Earth revolves around the Sun and also rotates at its own axis. Days and nights are caused by the rotation of the Earth at its axis. The axis of the Earth makes an angle of 23½° with the vertical. It is this inclination which causes changes in seasons. Due to its inclined axis, as the Earth revolves round the Sun, the Sun’s rays hit the ground at different angles at the same place on the surface of the Earth at different times of the year. Due to this variation in angles, the distribution of the solar heat is not the same at the same place throughout the year. This uneven distribution of solar heat on the Earth contributes towards the making of summer or winter seasons. If we look at the picture, we see that in June, when the northern hemisphere is tilted towards the Sun, it is summer in Europe, Asia and North America (northern hemisphere) and winter in the southern hemisphere. Six months later, in December, the southern hemisphere is tilted towards the Sun, so it is summer in the southern hemisphere but winter in the northern hemisphere. On March 21 and September 23 of every year, the Sun is exactly over the equator. On these two days, the duration of the day and night is the same (12 hours) at every place on Earth. From March 21 to June 21, the Sun advances from the equator to the Tropic of Cancer. This results in hot season in the northern hemisphere, and days become longer and nights shorter. During this period, it is winter in the southern hemisphere. From June 21 to December 22, the Sun advances towards the Tropic of Capricorn. This causes the summer season in the southern hemisphere and winter in the northern hemisphere. In the northern hemisphere, the days are shorter and the nights are longer during this period. After December 22, the Sun again starts moving towards the north and reaches the equator again on March 21. During this period, the days in northern hemisphere start getting longer and the nights shorter. In March and September, the Sun is overhead at the equator, and the hemispheres experience either autumn or spring. Thus, the revolution of the Earth around the Sun and its rotation at its own inclined axis changes the seasons as well as the duration of the days and nights.