The same scene photographed in spring (1), summer (2), autumn (3) and winter (4)In many parts of the world, the weather changes according to the time of year. These different periods, spring, summer, autumn and winter, are called the seasons. They occur because as the Earth spins, it is not upright but tilted. Regions close to the Equator, the tropics, are hot all year round. To the north or south of the tropics are temperate lands. Here, the seasons are clearly marked.
Because the Earth is tilted, its northern half, the Northern Hemisphere, leans towards the Sun for a part of the year. The Sun is nearer, so it is warmer: this is summer. The solstice, the day when the Sun is closest of all, falls on 21st June. The Southern Hemisphere is further away from the Sun, so it has winter.
The Earth orbits the Sun at a speed of about 30 kilometres (just under 20 miles) per second.
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