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Weather and climate

Seasons

The same scene in different seasonsThe same scene in different seasons 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 SunA diagram showing the Earth's orbit around the Sun

As the year goes on, the Earth’s Northern Hemisphere starts to lean towards the Sun. The Sun is nearer and higher in the sky for longer each day so the north begins its summer. The southern half leans farther away from the Sun and days are shorter so it turns to winter. On 21st June the Sun shines directly overhead at the Tropic of Cancer. In the Northern Hemisphere, it is the summer solstice; in the Southern Hemisphere, the winter solstice.

On 23rd September, it is the equinox once more as the Sun again shines directly overhead at the Equator. In the Northern Hemisphere, autumn begins, while the Southern Hemisphere enjoys the onset of spring. This time, it is the Southern Hemisphere that starts to lean towards the Sun. The summer solstice arrives in the Southern Hemisphere on 22nd December when the Sun is overhead at the Tropic of Capricorn. In Australia, people can celebrate Christmas on the beach, while the Northern Hemisphere is in the grip of winter.

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

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