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The same scene photographed in spring (1), summer (2), autumn (3) and winter (4). In tropical regions it is hot all year round, but farther north or south the temperature varies through the year. These different periods, spring, summer, autumn and winter, are called the seasons. They occur because of the way the Earth orbits the Sun. The Earth’s orbit is not an exact circle around the Sun, but an oval-like ellipse. Also, the Earth spins each day around an imaginary line, or axis, going through the North and South poles. This axis is not at right angles to the Earth's orbit, but tilted at an angle of 23.5°. The combination of tilted axis and elliptical orbit produces the yearly cycle of seasons in northern and southern regions.


Equinox and solstice

The Earth takes just over 365 days to complete its orbit around the Sun. On 21st March it is the equinox. The Sun shines directly overhead at the Equator and everywhere on Earth there are 12 hours of daylight and 12 hours of darkness. In the Northern Hemisphere, spring begins; in the Southern Hemisphere, autumn begins.A diagram showing the Earth's orbit around the Sun, spinning on its tilted axis

The orbital speed of the Earth around the Sun averages about 30 km (just under 20 miles) per second.

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