### KS3 Physics

Energy
Motion and Forces
Waves
Electricity and Electromagnetism
Matter
Space Physics

# Earth’s Orbit and Seasons

## Earth’s Axis

The Earth has an imaginary axis that runs through its centre, connecting the North and South poles. This imaginary pole is called Earth’s axis. As you can see by the image below, the Earth’s axis is slightly tilted, at an angle of approximately 23.4 degrees.

The Earth’s spin is the rotation of Earth around its own axis, rotating counter-clockwise.

### Day and night

A ‘day’ is defined as the time it takes for a planet to complete one full spin on its axis. It takes Earth approximately 24 hours to make a complete rotation on its axis.

The Sun does not illuminate the whole Earth at once; it lights up one half at a time. This is why we have light and darkness, day and night. It will be daytime on the side of the Earth facing the Sun and night time for the side of the Earth facing away from the Sun.

During the year, we can have different lengths of daytime. In the summer, daytime is the longest, but in the winter, daytime is the shortest. In summer, the sun rises higher in the sky than it does in winter.

## Earth Orbiting the Sun

A year for a planet is the time it takes for it to go around the Sun completely. It takes Earth 365 days to complete an elliptical orbit around the Sun. Actually, it takes Earth around 365.25 days for Earth to orbit the Sun, which is why every 4 years there is a leap year. In a leap year, there is an extra day.

Different planets in our solar system take different amounts of time to orbit the Sun. For example

• Mercury takes 88 days to orbit the Sun
• Earth takes 365 days to orbit the Sun
• Mars takes 687 days to orbit the Sun

## Earth’s Seasons

We experience seasons because the Earth is tilted on its axis, and the Earth’s orbit is elliptical.

The angle at which the Sun’s rays strike Earth results in varying amounts of heat.

The four seasons are:

• Winter
• Spring
• Summer
• Autumn

December – Around this time, due to Earth’s axial tilt, the Northern Hemisphere is tilted away from the Sun, while the Southern Hemisphere is tilted towards the Sun. This results in winter in the Northern Hemisphere and summer in the Southern Hemisphere.

March – As Earth continues in its orbit, both hemispheres experience more equal exposure to the Sun, making it spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

June – Around this time, the Northern Hemisphere is tilted towards the Sun, while the Southern Hemisphere is tilted away from the Sun. This leads to summer in the Northern Hemisphere and winter in the Southern Hemisphere. Contrary to popular belief, Earth is actually furthest from the Sun in its orbit around this time.

September – As Earth moves further in its orbit, the Northern Hemisphere experiences autumn while the Southern Hemisphere experiences spring.

The equator is an imaginary line around the middle of the planet, midway between the North Pole and the South Pole. Wherever the Earth is in orbit, the Sun hits the equator at around the same angle each time. This means that the equator doesn’t experience distinct seasons.

### In the UK

• When the Northern hemisphere is tilted towards the Sun, it experiences summer.
• When the Northern Hemisphere is tilted away from the Sun, it experiences winter.
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